bluuebunny/crossref_metadata_2025 huggingface dataset:
The calcium paradox in isolated guinea‐pig ventricular myocytes: effects of membrane potential and intracellular sodium.
Authors: Rodrigo G C , Chapman R A
Published: December 2014

1. Guinea‐pig ventricular myocytes, isolated enzymatically without the aid of special media, show a similar sensitivity to the calcium paradox as Langendorff‐perfused hearts. 2. Measurement of the intracellular activities of Na+ and Ca2+ ions, with a suction‐type ion‐sensitive microelectrode at rest, during calcium depletion and during inhibition of the Na+ pump (under both current and voltage clamp) yield values similar to those obtained from multicellular preparations and from isolated myocytes by other means. 3. In voltage‐clamped myocytes bathed by media free of divalent cations, an inward sodium current that flows through the L‐type Ca2+ channels, the rate of rise of aiNa and the strength of the contraction induced by return to normal Tyrode solution, show a similar bell‐shaped dependence on the membrane potential during the period of Ca2+ deprivation. 4. The rise in aiNa that occurs in Ca(2+)‐free, Mg(2+)‐free media, induces an outward current which is composed of currents due to activation of the Na+ pump and K+ channels. 5. On Ca2+ repletion the loading of the cells with Ca2+ does not generate an inward current and the contracture can be reduced, in a dose‐dependent way, by the introduction of BAPTA into the sarcoplasm from the solution in the voltage electrode. When [Ca2+]i is buffered by added BAPTA, the estimated amount of Ca2+ which can enter on Ca2+ repletion is sufficient to bind up to 10 mM of the BAPTA. This change in concentration is similar to that expected from the rise and fall in aiNa, seen on Ca2+ depletion and repletion, if a 3 Na+:1 Ca2+ exchange is responsible for the Ca2+ influx. 6. These data offer support for the so‐called intracellular sodium hypothesis for the origin of the calcium paradox in the heart. As the effects of Ca2+ repletion can be prevented by clamping the membrane potential so that aiNa does not rise, the contribution of the other effects of Ca2+ depletion to the initiation of the calcium paradox would seem to be less important.
Depression of a sustained calcium current by kainate in rat hippocampal neurones in vitro.
Authors: Nistri A , Cherubini E
Published: December 2014

1. High‐threshold, slow inactivating inward Ca2+ currents were studied in CA1 pyramidal neurones from rat hippocampal slices using the single‐electrode voltage clamp technique. 2. Kainate (50‐400 nM) induced a dose‐dependent depression of the amplitude of the slow Ca2+ current. At a dose of 200 nM the current amplitude was reduced from ‐0.63 +/‐ ‐0.06 to ‐0.32 +/‐ 0.06 nA. Such an effect of kainate was associated with the development of a small inward current (‐0.11 +/‐ 0.03 nA). Kynurenic acid (1 mM) or 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX; 20 microM) fully prevented these actions of kainate. 3. The structurally related kainate analogue alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA; 200 nM) depressed the slow Ca2+ current by 30 +/‐ 7%, an effect also blocked by CNQX. 4. In low‐Na+ medium slow Ca2+ currents were followed by sustained inward tail currents. Kainate reduced both the steady‐state Ca2+ current (from ‐0.98 +/‐ 0.14 to ‐0.63 +/‐ 0.15 nA) and the tail current (from ‐0.40 +/‐ 0.04 to ‐0.14 +/‐ 0.03 nA). 5. The inactivation process of the slow Ca2+ current was tested by a double‐pulse protocol and was found to be enhanced by kainate. 6. Equimolar replacement of Ca2+ by Ba2+ produced larger inward currents followed by prolonged tails. Kainate reduced the Ba2+ steady‐state current from ‐1.77 +/‐ 0.18 to ‐1.44 +/‐ 0.24 nA and the tail current from ‐0.47 +/‐ 0.15 to ‐0.17 +/‐ 0.05 nA. 7. In current clamp experiments Ca2+ action potentials were recorded from cells loaded with the Ca2+ chelator BAPTA. In these conditions kainate failed to reduce the Ca2+ action potential, while in the absence of BAPTA kainate shortened the Ca2+ action potentials by 30%. 8. It is suggested that low concentrations of kainate reduced the slow Ca2+ current by promoting its inactivation perhaps through a rise in free intracellular Ca2+.
Characterization of ionic currents of circular smooth muscle cells of the canine pyloric sphincter.
Authors: Vogalis F , Sanders K M
Published: December 2014

1. The ionic currents of circular muscle cells from canine pyloric sphincter were characterized using the whole‐cell patch clamp technique. 2. Subpopulations of circular muscle cells from the myenteric and submucosal halves of the circular layer were isolated and studied separately to determine whether differences in the currents expressed by these cells could explain differences in electrical behaviour observed in situ. 3. Resting potentials of isolated cells were about 20 mV positive to cells in intact muscles. Polarization under current clamp to the level of tissue resting potentials caused spontaneous discharge of action potentials in many cells. 4. Outward current measured under voltage clamp could be divided into a voltage‐dependent component and a voltage‐ and Ca(2+)‐dependent component. The latter was affected by manipulations of external [Ca2+], nifedipine and dialysis of cells with EGTA. 5. A few cells exhibited a channel that was activated with hyperpolarization. These channels produced inward current at potentials positive to the potassium reversal potential, EK, and reversed at ‐13 mV. 6. Inward currents, recorded from Cs(+)‐loaded cells, were characterized by a transient phase and a sustained phase that persisted throughout the test depolarization. The inward current was reduced by nifedipine but in some cells a nifedipine‐resistant component was observed. 7. There were no fundamental differences in the ionic currents recorded from circular muscle cells from the myenteric and submucosal regions, suggesting that the electrical activity of the tissue must be dependent upon structural characteristics (i.e. electrical coupling, fibre bundle dimensions, etc.) of the tissue. 8. The ionic conductance characterized can be related to many of the excitable events recorded from pyloric muscles.
Prolonged inhibition of a spinal reflex after intense stimulation of distant peripheral nerves in the decerebrated rabbit.
Authors: Taylor J S , Neal R I , Harris J , Ford T W , Clarke R W
Published: December 2014

1. In decerebrated rabbits, repetitive stimulation of the high‐threshold afferents of the left common peroneal (CP) nerve evokes prolonged depression of the sural‐gastrocnemius medialis (GM) reflex recorded in the same limb. This inhibition is antagonized by co‐administration of the opioid antagonist naloxone with the alpha 2‐adrenoceptor antagonist idazoxan. The present study was designed to investigate whether such inhibition could be elicited from the contralateral hindlimb or the forelimbs. 2. The sural‐GM reflex of decerebrated rabbits was depressed for more than 15 min after stimulation of either ipsilateral or contralateral common peroneal (CP) or median nerves with 500 pulses of 20 V, 1 ms given at 5 Hz. The order of efficacy for generating this inhibition was ipsilateral CP greater than contralateral CP greater than or equal to ipsilateral median = contralateral median. In three of thirty‐nine rabbits, stimulation of the median nerves caused facilitation of the sural‐GM reflex. 3. Idazoxan (1‐2 mg/kg I.V.) did not significantly alter the depressant effect of ipsilateral CP stimulation but reduced that evoked by either median nerve and almost abolished the inhibition evoked from the contralateral CP nerve. 4. Naloxone (0.25 mg/kg I.V.) reduced the effects of ipsilateral CP stimulation, did not alter the inhibition evoked from contralateral CP, and had equivocal actions on the responses to median nerve stimulation. 5. When given together, the two antagonists almost abolished the effects of stimulating the median nerves and the contralateral CP nerve, and markedly reduced the inhibition evoked from the ipsilateral CP nerve. 6. These data show that prolonged inhibition of the sural‐GM reflex can be evoked by stimulation of nerves in all four limbs and that in each case the inhibition can be blocked or reduced by co‐administration of antagonists to opioid and alpha 2‐adrenergic receptors. Such persistent inhibition of reflexes may serve to inhibit withdrawal reflexes in situations where interruptions to normal movement would be disadvantageous.
Modulation of acetylcholine‐activated K+ channel function in rat atrial cells by phosphorylation.
Authors: Kim D
Published: December 2014

1. In voltage‐clamped whole cells dialysed with GTP, extracellular application of ACh elicits an inwardly rectifying K+ current which subsequently decreases to a steady‐state level well below the maximally induced current (desensitization). The mechanism of desensitization of the acetylcholine (ACh)‐activated K+ channel current was studied in rat neonatal atrial cells at the single‐channel level using the patch‐clamp technique. 2. In cell‐attached patches with ACh in the pipette, a similar pattern of K+ channel current desensitization was present. Single‐channel analyses revealed that the initial rapid decrease in channel activity was associated with progressive shortening of the mean open time (tau o) and prolongation of the mean closed time (tau c) of the K+ channel. 3. In excised, inside‐out patches with ACh in the pipette, GTP activated K+ channels with a tau o of approximately 1.0 ms. Addition of ATP to the cytosolic surface resulted in progressive increases in tau o (from 1 to 5 ms) and channel activity. These changes are similar but opposite in direction to those observed during the early phase of ACh‐induced channel desensitization in cell‐attached patches. 4. The effect of ATP on the channel kinetics was abolished in Mg(2+)‐free solution AMP‐PNP (adenylyl‐imidodiphosphate, a non‐hydrolysable analogue of ATP), ADP, CTP (cytidine triphosphate), ITP (inosine triphosphate) or UTP (uridine triphosphate) did not alter the channel kinetics, suggesting that the ATP effect on channel gating probably occurs via phosphorylation by a membrane‐bound kinase. H‐8 (an isoquinolinesulphonamide derivative which inhibits protein kinases A and C) failed to prevent the action of ATP on the channel. 5. The increases in tau o and channel activity produced by ATP could be completely reversed by an elevation of cytosolic [Ca2+] to 3 x 10(‐5) M or above. 6. The effect of Ca2+ on the ATP‐induced changes in channel kinetics was blocked by sodium vanadate, a general phosphatase inhibitor. Okadaic acid, an inhibitor of protein phosphatase 1 and 2A, did not block the Ca2+ effect. Calmodulin antagonists, N‐(6‐aminohexyl)‐5‐chloro‐1‐naphthalenesulphonamide (W‐7), trifluoroperazine, and calmidazolium, partially blocked the effect of Ca2+. 7. Alkaline phosphatase (20 units/ml) reversed the ATP‐induced increases in tau o and channel activity. These results suggest that the ACh‐activated K+ channel can be modulated by phosphorylation and dephosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)
Spontaneous and noradrenaline‐induced transient depolarizations in the smooth muscle of guinea‐pig mesenteric vein.
Authors: Van Helden D F
Published: December 2014

1. Recordings of membrane current were made in the smooth muscle of short segments of mesenteric vein before or during stimulation with noradrenaline (NA). 2. Small veins (diameter less than 150 microns) when cut into short segments (of length less than 250 microns) had the passive electrical characteristics of short cables both before and during activation with NA. 3. Spontaneous transient depolarizations (STDs) or the underlying inward currents (STICs) were recorded in these preparations. STDs were of myogenic origin as they were not blocked by tetrodotoxin or antagonists to the alpha‐adrenoreceptor and persisted after either denervation or disruption of the endothelium. 4. STDs had time courses similar to the underlying currents and were generally slow compared to the membrane time constant of the short segments. 5. STDs and the underlying currents showed large variability in frequency and amplitude both within and between short segments. Currents were typically less than 0.3 nA, were characteristic in shape, had half‐durations normally in the range 0.1‐0.7 s and reversed at about ‐25 mV. 6. STDs persisted, but at markedly reduced frequencies, after exposure (3‐10 min) to a solution in which cobalt ions had been used to substitute for Ca2+. STDs were also substantially suppressed by exposure to low‐chloride solution. 7. Caffeine induced excitatory and inhibitory conductances. An initial component of the caffeine‐induced responses showed similar voltage dependence to STDs and was also suppressed by exposure to low‐chloride solution. 8. NA, through activation of alpha‐adrenoreceptors, caused a sustained depolarization or inward current (under voltage clamp) with considerable membrane potential or current noise often in the form of agonist‐induced spontaneous transient depolarizations (ASTDs) or currents (ASTICs). There were marked increases in amplitude and frequency of ASTDs with increase in NA concentrations. 9. ASTDs appeared to be generated within the smooth muscle as they were activated in preparations which had been denervated or in which the endothelium had been disrupted. 10. Except for the pathway of activation, ASTDs were indistinguishable from STDs having half‐durations in the same range (0.1‐2 s with the majority less than 0.7 s). The underlying currents again showed large variation in amplitude (typically less than 0.3 nA; maximum recorded 0.9 nA). They reversed at about ‐25 mV, could still be elicited in cobalt solution (but at reduced intensity for long exposures to this low‐Ca2+ solution) and were reduced by long term exposure to low‐chloride solution.(ABSTRACT TRUNCATED AT 400 WORDS)
Two types of calcium channels are expressed in adult bovine chromaffin cells.
Authors: Bossu J L , De Waard M , Feltz A
Published: December 2014

1. Calcium channel activity was recorded in chromaffin cells in the cell‐attached condition, using 110 mM‐Ba2+ as the permeant ion. 2. One type of calcium channel had a conductance of 16 pS, was completely inactivated at a holding potential of ‐20 mV and was insensitive to dihydropyridine agonists and antagonists. These characteristics correspond to a calcium channel of the N‐type. 3. A second type of calcium channel was active at holding potentials of ‐30 mV and above, had a channel conductance of 31 pS, and was sensitive to the dihydropyridine agonist, Bay K 8644. The channel opened along two dominant modes with characteristic time constants of 0.5 and 5 ms. The main effect of Bay K 8644 was to increase the probability of both short and long openings with no change in their relative proportions (6 to 1 respectively). These characteristics correspond to a calcium channel of the L‐type. 4. omega‐Conotoxin affected the activity of both N‐ and L‐type channels. It drastically reduced the number of N‐type channel openings and produced complex changes in L‐type channel activity. Long openings were less frequent and the conductance during short openings was slightly smaller than that measured in the presence of Bay K 8644. 5. The discussion focuses on modulation of L‐type channel activity. Openings of L‐type channels are rarely recorded in the cell‐attached configuration under control conditions. Addition of Bay K 8644 is needed to reveal the presence of L‐type channels. By contrast, L‐type currents recorded in the whole‐cell configuration are always observed and are insensitive to Bay K 8644. These results indicate that L‐type channels are normally inoperable in chromaffin cells.
Force‐velocity relations and myosin heavy chain isoform compositions of skinned fibres from rat skeletal muscle.
Authors: Bottinelli R , Schiaffino S , Reggiani C
Published: December 2014

1. This study was performed to assess whether muscle contractile properties are related to the presence of specific myosin heavy chain (MHC) isoforms. 2. Force‐velocity relations and MHC isoform composition were determined in seventy‐four single skinned muscle fibres from rat soleus, extensor digitorum longus and plantaris muscles. 3. Four groups of fibres were identified according to their MHC isoform composition determined by monoclonal antibodies: type 1 (slow), and types 2A, 2B and 2X (fast). 4. With respect to maximum velocity of shortening (V0), the fibres formed a continuum between 0.35 and 2.84 L/s (muscle lengths per second) at 12 degrees C. V0 in type 1 fibres (slow fibres) was between 0.35 and 0.95 L/s (0.639 +/‐ 0.038 L/s; mean +/‐ S.E. of mean). V0 in type 2 fibres (fast fibres) was consistently higher than 0.91 L/s. Ranges of V0 in the three fast fibre types mostly overlapped. Type 2A and 2X fibres had similar mean V0 values (1.396 +/‐ 0.084 and 1.451 +/‐ 0.066 L/s respectively); type 2B fibres showed a higher mean V0 value (1.800 +/‐ 0.109 L/s) than type 2A and 2X fibres. 5. Mean values of a/P0, an index of the curvature of force‐velocity relations, allowed us to identify two groups of fibres: a high curvature group comprised of type 1 (mean a/P0, 0.066 +/‐ 0.007) and 2A (0.066 +/‐ 0.024) fibres and a low curvature group comprised of type 2B (0.113 +/‐ 0.013) and 2X (0.132 +/‐ 0.008) fibres. 6. Maximal power output was lower in slow fibres than in fast fibres, and among fast fibres it was lower in type 2A fibres than in type 2X and 2B. 7. Force per unit cross‐sectional area was less in slow fibres than in fast fibres. There was no relation between fibre type and cross‐sectional area. 8. The results suggest that MHC composition is just one of the determinants of shortening velocity and of other muscle contractile properties.
Measurement of work done by ATP‐induced sliding between rabbit muscle myosin and algal cell actin cables in vitro.
Authors: Oiwa K , Chaen S , Sugi H
Published: December 2014

1. The basic properties of the ATP‐dependent actin‐myosin interaction responsible for muscle contraction were studied using an in vitro force‐movement assay system, in which a glass microneedle coated with rabbit skeletal muscle myosin was made to slide on the actin filament arrays (actin cables) in the internodal cell of an alga Nitellopsis obtusa with ionophoretic application of ATP. 2. In response to an ATP current pulse (intensity, 5‐85 nA; duration, 0.5‐10 s), the myosin‐coated needle moved for a distance and eventually stopped, indicating reformation of rigor actin‐myosin linkages to prevent elastic recoil of the bent needle. A subsequent ATP current pulse again produced the needle movement starting from the baseline force attained by the preceding needle movement. 3. With a constant amount of ATP application, the amount of work done by the ATP‐induced actin‐myosin sliding first increased with increasing baseline force from zero to 0.4‐0.6P0, and then decreased with further increasing baseline force, thus giving a bell‐shaped work versus baseline force relation. 4. With increasing amount of ATP application, the amount of work done by the actin‐myosin sliding increased more steeply as the baseline force was increased from zero to 0.4‐0.6P0. 5. These results are discussed in connection with the basic properties of the actin‐myosin sliding in muscle contraction.
Extracellular K+ in the supraoptic nucleus of the rat during reflex bursting activity by oxytocin neurones.
Authors: Coles J A , Poulain D A
Published: December 2014

1. We have investigated changes in extracellular potassium concentration [K+]o in the supraoptic nucleus of lactating rats and in particular those that occur during the intense burst of firing by the oxytocin neurones involved in the milk ejection reflex. 2. Double‐barrelled K(+)‐selective microelectrodes containing a highly selective sensor based on valinomycin were lowered through the exposed cortex towards the supraoptic nucleus (SON) of female rats anaesthetized with urethane. The mean resting [K+]o in the hypothalami of five rats was 2.4 mM, S.D. = 0.3 mM. 3. Where the reference barrel recorded extracellular action potentials from an oxytocin cell, the reflex burst of firing (4 s, typical maximum 50 Hz) was accompanied by a mean increase in [K+]o (delta[K+]o) of 0.22 mM (S.E.M. = 0.02 mM, fifty‐seven bursts in eight cells in seven rats). The rise in [K+]o did not begin more than 0.1 s before the onset of the burst, and began to fall from its maximum during the burst. Slow field potentials, indicative of spatial buffering of K+, were undetectable (less than 50 microV). When the electrode was advanced in steps, the amplitudes of both delta[K+]o and the action potential declined steeply to about 10% over a distance of 20 microns: K+ from oxytocin cells appears to be prevented from dispersing freely through the extracellular space of the SON. 4. When the electrode recorded action potentials from a vasopressin cell, delta[K+]o during an oxytocin cell burst was very small: 0.021 mM (S.E.M. = 0.005 mM). At other sites in the SON, where antidromic stimulation evoked a field potential but no action potential, delta[K+]o was 0.047 +/‐ 0.005 mM. We conclude that the reason oxytocin bursts do not affect vasopressin cells is that [K+]o rises very little around vasopressin cells. A fortiori, since the increases in [K+]o were very small except where action potentials from oxytocin cells were recorded, they can make no significant contribution to synchronizing the onsets of bursts in oxytocin cells that are not contiguous. 5. A standard antidromic stimulation from the pituitary stalk, at 40 Hz for 4 s, which stimulated both oxytocin neurones and vasopressin neurones, caused a delta[K+]o of 0.17‐1.8 mM, the variation being mainly from rat to rat. The larger delta[K+]o values were accompanied by slow negative potentials of up to 1.5 mV, there was a gradient in delta[K+]o decreasing towards the pia at the inferior limit of the SON, and there was a slow increase in [K+] in the subarachnoid space.(ABSTRACT TRUNCATED AT 400 WORDS)
The morphology and electrical geometry of rat jaw‐elevator motoneurones.
Authors: Moore J A , Appenteng K
Published: December 2014

1. The aim of this work was to quantify both the morphology and electrical geometry of the dendritic trees of jaw‐elevator motoneurones. To do this we have made intracellular recordings from identified motoneurones in anaesthetized rats, determined their membrane properties and then filled them with horseradish peroxidase by ionophoretic ejection. Four neurones were subsequently fully reconstructed and the lengths and diameters of all the dendritic segments measured. 2. The mean soma diameter was 25 microns and values of mean dendritic length for individual cells ranged from 514 to 773 microns. Dendrites branched on average 9.1 times to produce 10.2 end‐terminations. Dendritic segments could be represented as constant diameter cylinders between branch points. Values of dendritic surface area ranged from 1.08 to 2.52 x 10(5) microns 2 and values of dendritic to total surface area from 98 to 99%. 3. At branch points the ratio of the summed diameters of the daughter dendrites to the 3/2 power against the parent dendrite to the 3/2 power was exactly 1.0. Therefore the individual branch points could be collapsed into a single cylinder. Furthermore for an individual dendrite the diameter of this cylinder remained constant with increasing electrical distance from the soma. Thus individual dendrites can be represented electrically as cylinders of constant diameter. 4. However dendrites of a given neurone terminated at different electrical distances from the soma. The equivalent‐cylinder diameter of the combined dendritic tree remained constant over the proximal half and then showed a pronounced reduction over the distal half. The reduction in equivalent diameter could be ascribed to the termination of dendrites at differing electrical distances from the soma. Therefore the complete dendritic tree of these motoneurones is best represented as a cylinder over the proximal half of their electrical length but as a cone over the distal half.
Differential effects of ryanodine and tetracaine on charge movement and calcium transients in frog skeletal muscle.
Authors: García J , Avila‐Sakar A J , Stefani E
Published: December 2014

1. Charge movement and myoplasmic calcium transients were simultaneously recorded from frog skeletal muscle fibres by using the double‐seal Vaseline‐gap technique. Calcium transients were monitored with the fluorescent indicator Rhod‐2. 2. Ryanodine modified the kinetics and the total amount of charge moved during depolarizing pulses (Q(on)), while it did not significantly modify the charge after repolarization (Q(off)). The extracellular application of 100 microM‐ryanodine elicited a temporary initial increase of the delayed component of charge movement (Q gamma) and the calcium transient. Both phenomena were later blocked with the same temporal course and to the same extent. 3. The blockade of Q gamma and the calcium transient was also observed with ryanodine concentrations of 1‐10 microM. For membrane potentials positive to ‐10mV, the Qon measured was larger in the presence of ryanodine; Qoff was not modified. 4. Tetracaine (400‐500 microM) blocked a similar delayed component of Qon, identified as Q gamma, as well as the calcium transient monitored simultaneously. This effect was observed in the first minutes after the addition of tetracaine to the extracellular solution. 5. Tetracaine blocked a faster initial component of Qon for voltages positive to ‐10 mV, corresponding to the voltage range of activation of the calcium current. At these same membrane potentials, Qoff was also reduced to a similar extent to Qon. 6. Ryanodine and tetracaine showed different effects on calcium current. Whereas the slow calcium current was not modified upon the addition of ryanodine, it was completely blocked in the presence of tetracaine. The blockade of the slow calcium current made evident the fast calcium current. The effects of tetracaine on the charge movement, the calcium transient and the slow calcium current were reversible. 7. These results suggest that ryanodine and tetracaine may act at different sites. Ryanodine exerts its effect on the sarcoplasmic reticulum ryanodine receptor, blocking calcium release and Q gamma, while tetracaine at these concentrations may affect the release channel and the dihydropyridine receptor, causing a blockade of the charge movement, calcium transient and calcium current.
Membrane currents recorded from sexually dimorphic motoneurones of the bulbocavernosus muscle in neonatal rats.
Authors: Manabe T , Araki I , Takahashi T , Kuno M
Published: December 2014

1. The electrophysiological properties were compared between sexually dimorphic motoneurones in the spinal nucleus of the bulbocavernosus (SNB) and those innervating hindlimb muscles by the whole‐cell recording technique in thin slices of the neonatal rat spinal cord. 2. The mean duration of action potentials in SNB motoneurones was significantly longer than that in hindlimb motoneurones. 3. The spike duration of motoneurones was inversely related to the magnitude of transient K+ currents (IA), and in this relation, there was a continuous gradation between SNB and hindlimb motoneurones. 4. The mean duration of spike after‐hyperpolarization (AHP) in female SNB motoneurones was significantly longer than that in male SNB motoneurones. 5. In both SNB and hindlimb motoneurones, sustained, Ca(2+)‐dependent K+ currents (IAHP) appear to be responsible for the generation of AHP. When IAHP was recorded as a tail current following an identical depolarizing pulse, the magnitude and time course of IAHP were relatively uniform, regardless of the type of motoneurone. 6. In both SNB and hindlimb motoneurones, voltage‐gated Ca2+ currents showed an initial transient phase followed by a sustained phase. The mean magnitude of sustained Ca2+ currents was larger in female SNB motoneurones than in male SNB motoneurones, whereas the mean magnitude of transient Ca2+ currents showed no significant different between male and female SNB motoneurones. 7. It is concluded that SNB and hindlimb motoneurones cannot be classified into two distinct neurone types in terms of their electrophysiological properties. 8. It is suggested that the predominant occurrence of natural cell death in female SNB motoneurones during early development may be due in part to high densities of sustained Ca2+ channels in these neurones.
Ca2+ influx through ATP‐gated channels increments [Ca2+]i and inactivates ICa in myocytes from guinea‐pig urinary bladder.
Authors: Schneider P , Hopp H H , Isenberg G
Published: December 2014

1. Whole‐cell patch clamp was combined with microspectrofluometry (Indo‐1) to study the effects of bath applied ATP on membrane currents and cytoplasmic Ca2+ concentration ([Ca2+]i) in single smooth muscle cells of the guinea‐pig urinary bladder. Experiments were carried out at 22 degrees C and in 3.6 mM [Ca2+]o. Superimposed K+ currents were reduced by Cs+ dialysis from the patch electrode. 2. At ‐60 mV, ATP induced an inward current (Ins,ATP) that peaked within 0.4 s and then decayed. Ins,ATP was activated half‐maximally by 1.1 microM‐ATP and saturated at 50 microM‐ATP to ‐1.1 +/‐ 0.2 nA (mean +/‐ S.E.M.). At 3.6 mM [Ca2+]o, Ins,ATP had a reversal potential (Erev) of ‐5 +/‐ 2 mV. From the shifts in Erev during changes in [Na+]o or [Ca2+]o we estimated that approximately 7% of Ins,ATP is carried by Ca2+ ions. 3. ATP (50 microM) increased [Ca2+]i transiently from resting 130 +/‐ 40 nM to 730 +/‐ 100 nM. At 22 degrees C, [Ca2+]i rose at a rate proportional to the instantaneous current amplitude of Ins,ATP. This relation was lost, however, after warming to 36 degrees C which increased the peak Ins,ATP (Q10 = 1.25) but reduced the peak of the ATP induced [Ca2+]i transient (Q10 = 0.75). We suggest that warming to 36 degrees C stimulated Ca2+ sequestration and Ca2+ efflux to such a degree that peak [Ca2+]i was attenuated significantly. 4. The contribution of Ca2+ ions to Ins,ATP was evaluated from a comparison of the increments in [Ca2+]i due to Ins,ATP and due to L‐type Ca2+ channel current (ICa). For the same increment, Ins,ATP had to transport 19 times more charge than ICa. This number suggests that 5.8 +/‐ 0.8% of Ins,ATP is carried by Ca2+ ions which can be translated into a permeability ratio of PNa:PCa approximately 1:1. 5. During bath application of ATP, peak ICa was inhibited by 80 +/‐ 15%. Inhibition of ICa diminished to 20 +/‐ 8% after cell dialysis with 40 mM‐EGTA, and it was 19 +/‐ 7% when extracellular Ca2+ had been substituted by Ba2+. These results are in agreement with the hypothesis of ‘ICa inactivation by Ca2+’. Depletion of intracellular Ca2+ stores by pre‐treatment with 20 mM‐caffeine did not attenuate significantly the ATP‐induced rise in [Ca2+]i or the ATP‐induced inhibition of ICa. 6. The ATP‐induced [Ca2+]i transients and the reduction of peak ICa recovered along a similar time course.(ABSTRACT TRUNCATED AT 400 WORDS)
Cell volume regulation by trout erythrocytes: characteristics of the transport systems activated by hypotonic swelling.
Authors: Garcia‐Romeu F , Cossins A R , Motais R
Published: December 2014

1. An osmolality reduction of the suspending medium leads to osmotic swelling of trout erythrocytes, which is followed by a volume readjustment towards the original level. The regulatory volume decrease (RVD) was not complete after 1 h. 2. During RVD the cells lost K+ and Cl‐ but gained Na+. This entry of Na+, which is about half the K+ loss, explains the incomplete volume recovery (it was complete when Na+ was replaced by impermeant N‐methyl‐D‐glucamine). The cells also lose large quantities of taurine, which accounts for about 53% of the volume recovery. In addition RVD is accompanied by the activation of a pathway allowing some large organic cations which are normally impermeant, such as choline or tetramethyl‐ammonium, to rapidly penetrate the cells. 3. The swelling‐activated K+ loss is not significantly affected by replacement of Cl‐ by NO3‐, indicating that K+ moves through a Cl(‐)‐independent K+ pathway. Furosemide, DIDS (4,4'‐diisothiocyanatostilbene‐2,2'‐disulphonic acid) and niflumic acid inhibit the K+ loss. From experiments performed in high‐K(+)‐containing media, it appears that these compounds block the K+ flux, not by inhibiting Cl‐ movements but by interfering with the K+ pathway. 4. All the volume‐activated pathways (K+, Na+, taurine, choline) are fully inhibited by furosemide and by inhibitors of the anion exchanger such as DIDS and niflumic acid. The concentration required for 50% inhibition (IC50) of both inorganic cations and taurine appears to be similar. It is proposed that DIDS interacts with a unique target which controls all the volume‐sensitive transport systems.
Low‐frequency oscillatory activities intrinsic to rat and cat thalamocortical cells.
Authors: Leresche N , Lightowler S , Soltesz I , Jassik‐Gerschenfeld D , Crunelli V
Published: December 2014

1. Low‐frequency membrane potential oscillations recorded intracellularly from thalamocortical (TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of their electrophysiological and pharmacological properties, four types of activity were distinguished and named: the pacemaker oscillations, the spindle‐like oscillations, the ‘very slow’ oscillations and the ‘N‐methyl‐D‐aspartate’ (NMDA) oscillations. 2. The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large‐amplitude (10‐30 mV) depolarizations which occurred at a frequency of 1.8 +/‐ 0.3 Hz (range, 0.5‐2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker oscillations were observed when the membrane potential was moved negative to ‐55 and positive to ‐80 mV, but in a given cell the upper and lower limits of this voltage range were separated by only 13.1 +/‐ 0.5 mV. Above ‐45 mV tonic firing consisting of single action potentials was seen in the cells showing this or the other types of low‐frequency oscillations. 3. The spindle‐like oscillations were observed in thirty‐nine (out of 173) TC cells and consisted of rhythmic (2.1 +/‐ 0.3 Hz), large‐amplitude depolarizations (and often associated burst firing) similar to the pacemaker oscillations but occurring in discrete periods every 5‐25 s and lasting for 1.5‐28 s. The spindle‐like oscillations were observed when the membrane potential was moved negative to ‐55 and positive to ‐80 mV and in two cells they were transformed into continuous pacemaker oscillations by depolarization of the membrane potential to ‐60 mV. 4. Pacemaker and spindle‐like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade of NMDA, non‐NMDA, GABAA, GABAB, nicotinic, muscarinic, alpha‐ and beta‐noradrenergic receptors. 5. The ‘very slow’ oscillations consisted of a TTX‐insensitive, slow hyperpolarization‐depolarization sequence (5‐15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN cells and in two VB cells. The pacemaker and the spindle‐like oscillations were recorded in one cell each which also showed the ‘very slow’ oscillations. 6. The ‘NMDA’ oscillations were observed only in a ‘Mg(2+)‐free’ medium (0 mM‐Mg2+, 2‐4 mM‐Ca2+; 64 out of 72 cells) and consisted of large‐amplitude (10‐25 mV) depolarizations that did not occur at regular intervals and were intermixed with smaller depolarizations present on the baseline and on the falling phase of the larger ones.(ABSTRACT TRUNCATED AT 400 WORDS)
Amiloride block of the mechanosensitive cation channel in Xenopus oocytes.
Authors: Lane J W , McBride D W , Hamill O P
Published: December 2014

1. Patch clamp recording techniques have been used to investigate the block by amiloride of the mechanosensitive cation‐selective channel in frog (Xenopus laevis) oocytes. 2. Cell‐attached and outside‐out patch recording configurations were employed to study the differences in block produced when amiloride was present at either the extracellular (external) or intracellular (internal) membrane face. 3. External amiloride causes a highly voltage‐dependent ‘flickery’ block of single mechanosensitive channel currents in which inward mechanosensitive current recorded at negative potentials is reduced in amplitude but outward mechanosensitive current recorded at positive potentials is almost unaffected. 4. At ‐100 mV the apparent dissociation constant (Kd) for external amiloride block is 0.5 mM. The extracellular concentration dependence of amiloride block yields a Hill coefficient equal to 2, inconsistent with a single site blocking stoichiometry. 5. The shapes of current‐voltage relationships measured in different external amiloride concentrations also indicate deviations from a simple channel plug model in which a single blocking cation is driven into the channel by the membrane potential. 6. Internal amiloride causes a voltage‐independent ‘flickery’ block of mechanosensitive channel currents which equally reduces both inward and outward mechanosensitive currents. 7. The present data indicate that a minimum of two amiloride binding sites are necessary to predict external amiloride block. A model involving a voltage‐dependent conformational change with subsequent voltage‐independent co‐operative binding of two amiloride molecules is found to explain the data. 8. The relevance of the present actions of amiloride on mechanosensitive channels is discussed in relation to reports of amiloride‐inhibitable cation flux pathways involved in a number of basic physiological functions including mechanosensitivity of sensory cells, volume regulation and fertilization.
Comparison of neurotransmission with nerve trunk and transmural field stimulation in guinea‐pig mesenteric artery.
Authors: Keef K D , Hottenstein O D , Meehan A G , Anthony T L , Kreulen D L
Published: December 2014

1. Intracellular electrical and contractile responses to sympathetic nerve trunk stimulation (NTS) and transmural electrical field stimulation (TMS) were compared in the guinea‐pig mesenteric artery in vitro. 2. Step increases in voltage with NTS gave rise to excitatory junctional potentials (EJPs) which reached a plateau amplitude of 5‐10 mV, whereas with TMS larger amplitude EJPs and sometimes action potentials were obtained. 3. EJPs of equal amplitude (1‐7 mV) elicited with TMS and NTS had the same rise time, duration and decay half‐time. 4. Slow depolarization obtained with repetitive stimulation was significantly greater in amplitude with TMS than with NTS. 5. Equal amplitude EJPs were obtained throughout the preparation with NTS. With TMS, the amplitude of responses declined substantially with distance from the stimulating electrodes. 6. Tetrodotoxin (TTX) completely blocked EJPs, slow depolarization and contraction with NTS; however, with TMS a TTX‐resistant component was observed. The TTX‐resistant response to TMS was abolished in the presence of a low‐Ca2+ superfusion solution but was not affected by endothelium removal. 7. Phentolamine or prazosin abolished slow depolarization but not EJPs with NTS or TMS. Prazosin abolished contraction with NTS and reduced but did not abolish contraction with TMS. 8. alpha, beta‐Methylene ATP abolished EJPs with NTS, whereas with TMS only EJPs obtained with low stimulus intensities were abolished. alpha, beta‐Methylene ATP did not block contraction with either NTS or TMS. 9. Combined TTX, prazosin and alpha, beta‐methylene ATP abolished EJPs initiated with TMS at all but the highest stimulus intensities (12‐20 V, 0.3 ms duration). 10. It is concluded that responses obtained with NTS can be reliably attributed to the release of transmitter by the conduction of action potentials in paravascular nerves, whereas activation by TMS is a more complex phenomenon dependent upon stimulus strength and probably involving multiple forms of activation.
Development of respiratory chemoreflexes in response to alternations of fractional inspired oxygen in the newborn infant.
Authors: Williams B A , Smyth J , Boon A W , Hanson M A , Kumar P , Blanco C E
Published: December 2014

1. We studied the reflex respiratory response to breath‐by‐breath alternations of fractional inspired oxygen (FI,O2) in full‐term human infants delivered either vaginally or by caesarian section at 3‐10 h (n = 6), 12‐24 h (n = 12), 24‐48 h (n = 18), 3‐4 days (n = 21) and 5‐8 days (n = 7) postnatally. 2. Respiration was measured by inductance plethysmography (Respitrace) and respiratory variables for each breath were calculated on‐line by a microcomputer. Test runs (with alternations of FI,O2 between 0.21 and 0.16) and control runs (with an FI,O2 of 0.21) of 50‐100 breaths were carried out during quiet sleep. For each respiratory variable the magnitude of the reflex breath‐by‐breath alternation was compared between control and test runs. 3. There was little respiratory response during control runs at any postnatal age. However, there was a significantly greater response to test runs in all infants studied and at all ages. 4. There were no significant differences in the degree of alternation during test runs between infants of similar postnatal ages delivered by caesarean section and those born vaginally. 5. In all infants the magnitude of the respiratory response increased with postnatal age, presumably reflecting postnatal increases in the hypoxic sensitivity of the peripheral arterial chemoreceptors. 6. The results indicate that in human infants the alternate breath method can be used to detect developmental changes in peripheral chemoreflexes between birth and postnatal day 8.
Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle.
Authors: Fedida D , Giles W R
Published: December 2014

1. Regional variations in the shape of early repolarization of the action potential have been correlated to differences in transient outward K+ current, I(t), in myocytes isolated from the epicardial surface, the endocardial trabeculae and the papillary muscles of rabbit left ventricles. Temperature was 35 degrees C during whole‐cell, and 22‐23 degrees C during cell‐attached experiments. 2. Membrane resting potentials were very similar regionally. At 0.1 Hz stimulation the action potential plateau amplitude in papillary muscle cells was significantly higher (104.7 mV) than in epicardial cells (96.47 mV). Exposure to 4‐aminopyridine or increases in the rate of stimulation from 0.1 Hz to 3.3 Hz increased plateau height and diminished the initial notch on repolarization. These effects were correlated to the magnitude of I(t) in these cells. At low rates of stimulation I(t) caused a 'spike and dome' morphology of the action potential. 3. Voltage clamp experiments confirmed a higher current density of I(t) in epicardial cells (7.66 pA/pF at +20 mV) than in endocardial (6.45 pA/pF) or papillary muscle cells (3.69 pA/pF). I(t) at 35 degrees C was faster and larger than previously reported and individual currents inactivated almost completely during 100 ms pulses to plateau potentials. No differences in the kinetics or voltage dependence of whole‐cell currents were found. Thus, the half‐inactivation potential was ‐37.8 mV in cells from all three regions. 4. Cell‐attached recordings from endocardial and epicardial cells showed very similar single‐channel amplitudes, burst open probabilities and ensemble averages. The peak channel open probability soon after the start of depolarizing voltage clamp pulses did not change between cell types (P approximately 0.8). The slope conductance of I(t) channels was 13.0 pS with an intercept near the resting potential of the cell. 5. We conclude that regional variations in the shape of initial repolarization in cells from rabbit left ventricle are caused by variations in the magnitude of the transient outward K+ current, I(t). Epicardial cells have the largest, and papillary muscle cells the smallest I(t). The differences are not explained by alterations in the whole‐cell kinetics or single‐channel kinetics and conductance. The most likely explanation for variations in whole‐cell current density is therefore a decrease in channel density in endocardium and papillary muscle compared with epicardial tissue. We estimate the density of I(t) channels per cell to be 1495 (one per 3‐4 micron2) in epicardium, 1175 (one per 4‐5 micron2) in endocardium, and 875 (one per 6 micron2) in papillary muscle cells.
Serotonin and noradrenaline excite GABAergic neurones of the guinea‐pig and cat nucleus reticularis thalami.
Authors: McCormick D A , Wang Z
Published: December 2014

1. The actions of serotonin (5‐HT) and noradrenaline (NA) in the cat perigeniculate nucleus (PGN) and the guinea‐pig nucleus reticularis thalami (NRT) were investigated with extracellular and intracellular recordings obtained from neurones in thalamic slices maintained in vitro. 2. Single, local application of either 5‐HT or NA resulted in pronounced (5‐50 Hz) and prolonged (2‐10 min) excitation associated with the occurrence of single‐spike activity. Serotoninergic excitation was specifically blocked by the 5‐HT2/5‐HT1C antagonists ketanserin and ritanserin, but not by the 5‐HT1A antagonist pindolol or the 5‐HT3 antagonist ICS 205‐930. Furthermore, the 5‐HT response was mimicked by alpha‐methyl‐5‐HT, but not by the 5‐HT1A agonist 8‐hydroxydipropylaminotetralin (8‐OHDPAT) or the 5‐HT3 agonist 2‐methyl‐5‐HT. Together, these results indicate that this excitatory response is mediated through 5‐HT2 receptors with the possible involvement of 5‐HT1C receptors. 3. Noradrenergic excitation was specifically blocked by the alpha 1‐antagonist prazosin, but not by the beta‐antagonist propranolol or the alpha 2‐antagonist yohimbine. Similarly, the response was mimicked by the alpha‐agonist phenylephrine, but not by the beta‐agonist isoprenaline. These results indicate that the noradrenergic excitation is mediated by alpha 1‐adrenoceptors. 4. Block of synaptic transmission either by lowering external calcium concentration ([Ca2+]o) to 0.5 mM and raising external magnesium concentration ([Mg2+]o) to 10 mM or by local application of tetrodotoxin failed to block the excitatory or depolarizing response to 5‐HT or NA indicating that these responses are direct and not mediated through the release of other neurotransmitters. 5. Intracellular recordings revealed that the 5‐HT‐ and NA‐induced excitations are mediated by a pronounced slow depolarization associated with an apparent decrease in input conductance and an increase in the membrane time constant. Current versus voltage plots obtained under voltage clamp before and during the presence of 5‐HT and NA revealed that these neurotransmitters induced an inward current which reversed to an outward current at ‐107 and ‐110 mV, respectively, in 2.5 mM external potassium concentration ([K+]o). This reversal potential was identical to that associated with an increase in potassium conductance activated by acetylcholine (‐110 mV) in the same neurones. Plots of the amplitude of the 5‐HT‐ or NA‐induced current versus membrane potential revealed a linear relationship in the voltage range from ‐140 to ‐60 mV.(ABSTRACT TRUNCATED AT 400 WORDS)
Breathing during prolonged exercise in humans.
Authors: Kearon M C , Summers E , Jones N L , Campbell E J , Killian K J
Published: December 2014

1. Six normal subjects cycled to endurance or for 60 min at four work rates (WR 1‐4): mean of 34% working capacity (93 watts for 60 min); 43% (120 watts for 56 min); 63% (177 watts for 37 min); and 84% (233 watts for 12 min), to determine how breathing pattern and dyspnoea change during prolonged activity. Four to six minutes were allowed to establish steady state and subsequent changes were considered to be endurance related. 2. Dyspnoea (Borg scale, 0‐10) increased with the duration of activity at all work rates. 3. Ventilation (VE) did not change at WR1; increased from 44 to 47 l min‐1 at WR2; from 60 to 88 l min‐1 at WR3; and from 111 to 132 l min‐1 at WR4. Dyspnoea was significantly and independently related to ventilation and duration of activity: dyspnoea = 0.004 VE1.36 time 0.25 (r = 0.81; partial F 202 and 26 respectively). 4. Inspiratory resistance did not increase at any work rate. Dynamic elastance remained constant during WR1, WR2 and WR3 but increased from 7.4 to 9.1 cmH2O l‐1 during WR4. 5. Peak inspiratory pressure did not increase, and the increase in VE was accomplished by an increased breathing frequency without change in duty cycle. 6. Duration of activity is an important contributor to dyspnoea independent of changes in respiratory muscle contractile activity.