bluuebunny/crossref_metadata_2025 huggingface dataset:
Tension as a function of sarcomere length and velocity of shortening in single skeletal muscle fibres of the frog.
Authors: Morgan D L , Claflin D R , Julian F J
Published: December 2014

1. Simple measurements of muscle tension at fixed fibre or segment length produce a range of length‐tension relationships, depending primarily on the duration of the interval between stimulation onset and tension measurement, in contradiction with the simple predictions of current models. This has been explained by non‐uniformity in sarcomere lengths, leading to internal motion and, in turn, to increasing tension because the force‐velocity relationship has a much greater slope for slow lengthening than for slow shortening. 2. Previous attempts to reduce the effect of internal motion have been focused on decreasing the initial extent of non‐uniformity and measuring tension early in a contraction, when non‐uniformities are at a minimum. An alternative approach that has not been attempted previously is to reduce the non‐linearity of the force‐velocity relationship by avoiding the discontinuity in slope at zero velocity. This is accomplished by imposing overall fibre shortening at velocities sufficient to ensure that all sarcomeres are shortening. 3. When the tension maintained during shortening was measured and plotted against sarcomere length for each release velocity used, linear length‐tension relationships resulted that extrapolated to a common sarcomere length intercept. This was true whether the release was applied early in the tetanus or near the end of the ‘creep phase’ of tension rise. These observations were duplicated by computer simulation using a multisarcomere model of a muscle fibre. 4. These results provide strong support for the view that cross‐bridges function as independent force generators and for the explanation of the creep phase of fibre or segment isometric tension as being due to internal motion. The results also imply that the force‐velocity relationship scales with sarcomere length without changing shape. 5. Using this novel method for obtaining length‐tension relationships, the sarcomere length at which active tension fell to zero was found, by extrapolation, to be 3.65 microns in semitendinosus fibres and 3.53 microns in tibialis anterior fibres from the frog (Rana temporaria).
The spatial distributions of odorant sensitivity and odorant‐induced currents in salamander olfactory receptor cells.
Authors: Lowe G , Gold G H
Published: December 2014

1. Suction electrode and whole‐cell recording were used to record membrane currents from defined regions of solitary olfactory receptor cells from Ambystoma tigrinum. 2. Under whole‐cell current clamp, stimulation of cells with odorants activated an inward current in the cilia, an outward current in the soma, and induced a membrane depolarization. Clamping the membrane potential at its resting value of ‐70 mV increased the inward ciliary current 5‐ to 10‐fold and abolished the outward somatic current. 3. Local odorant stimulation was accomplished by ejecting an odorant solution into a steady flow of Ringer solution. A suction electrode was used to immobilize a cell in the flow and to record the odorant‐induced somatic current. The amplitude of the odorant response increased approximately linearly with the length of cilia exposed to the stimulus, but was independent of the length of dendrite exposed to the stimulus, indicating that odorant sensitivity is predominantly localized to the cilia. 4. The latencies of responses recorded under flow did not vary with the region of the cilia which was exposed to the stimulus. Also, the magnitude of the inward ciliary current activated by odorants was equal to that of the whole‐cell current recorded under voltage clamp. These observations indicate that the odorant‐induced inward current is predominantly localized to the ciliary membrane. 5. Under whole‐cell current clamp, local application of a high‐K+ solution generated an outward somatic current when applied to the dendrite, but had no effect when applied to the cilia. This indicates that the density of the resting K+ conductance is lower in the ciliary membrane than in the dendritic membrane. 6. The results above are consistent with the hypothesis that all components of the transduction mechanism are uniformly distributed within the cilia, and that the cilia are electrotonically compact, even during an odorant‐induced conductance increase.
Extracellular alkalinization evoked by GABA and its relationship to activity‐dependent pH shifts in turtle cerebellum.
Authors: Chen J C , Chesler M
Published: December 2014

1. The effect of gamma‐aminobutyric acid (GABA) on extracellular pH (pHo) was investigated in the turtle cerebellum, in vitro, using double‐barrelled, H(+)‐selective microelectrodes. Responses evoked by GABA were compared with pHo shifts evoked by repetitive stimulation of the parallel fibres. 2. In media buffered with 35 mM‐HCO3‐ and 5% CO2, superfusion of GABA (1 mM) elicited an abrupt alkaline shift in the molecular layer, which averaged 0.05 +/‐ 0.02 pH units (+/‐ S.D., range 0.02‐0.12 pH units). pHo often recovered in the continued presence of GABA, and displayed a rebound acidification upon wash‐out. 3. The GABA‐evoked alkaline shift was blocked by picrotoxin and was mimicked by the GABAA agonists isoguvacine and muscimol. The GABAB agonist baclofen did not elicit an alkaline shift. Alkaline shifts evoked by stimulation of the parallel fibres were unaffected by picrotoxin. 4. In nominally HCO3(‐)‐free solutions, buffered with 35 mM‐HEPES, superfusion of GABA caused either no pHo change or a slow acid shift. In contrast, the alkaline shift evoked by stimulation of the parallel fibres became enhanced in HEPES‐buffered media. 5. The alkaline shift evoked by GABA was accompanied by an increase in extracellular K+ ([K+]o) which averaged 1.7 mM above baseline. Experimental elevation of [K+]o to a comparable level always caused a pure acid shift in the extracellular space. 6. The GABA‐evoked alkaline shift persisted when synaptic transmission was blocked using 4 mM‐kynurenic acid or saline prepared with nominally zero Ca2+ and 10 mM‐Mg2+. The alkaline shift evoked by repetitive stimulation of the parallel fibres was completely abolished in these media. 7. Although the GABA‐evoked alkaline shift was blocked in nominally HCO3(‐)‐free media, substitution of 35 mM‐formate for HCO3‐ restored the GABA response. Superfusion of 1 mM‐GABA in formate saline produced an alkaline shift of 0.040 +/‐ 0.034 pH units. 8. These results indicate that gating of GABAA channels in the vertebrate CNS gives rise to an HCO3‐ efflux which can significantly increase the pH of the brain microenvironment. However, this mechanism cannot account for the extracellular alkalinization caused by parallel fibre stimulation. Extracellular alkaline shifts capable of modulating local synaptic operations may therefore be a consequence of either excitatory or inhibitory synaptic transmission.
Force response to rapid length change during contraction and rigor in skinned smooth muscle of guinea‐pig taenia coli.
Authors: Arheden H , Hellstrand P
Published: December 2014

1. Mechanical transients in fibre bundles of skinned smooth muscle of guinea‐pig taenia coli at 21‐22 degrees C were investigated by recording tension responses to length changes of up to 9%, complete within 0.3 ms. 2. The length‐force relationship, recorded continuously during rapid stretch of a Ca(2+)‐activated contracted muscle, was linear up to at least 2.5 times the isometric force, corresponding to a stretch of about 1%. The slope of the relationship (stiffness) increased with the velocity of stretch. 3. During rapid release (about 120 muscle lengths s‐1) the length‐force relationship was linear down to about 50% of the initial isometric force, reached at about 80 microseconds after the beginning of the release. At lower force the length‐force relationship was concave upwards. The linear portion extrapolated to zero force at about ‐0.008 muscle lengths. In large releases the length‐force plot approached the force baseline under an acute angle, and negative force was transiently exerted. 4. When the muscle was stretched back to the initial length after a shortening step, force transiently rose above the isometric force, but decayed back within a few milliseconds. Stiffness at the time of restretch was compared with that in the initial shortening step by plotting force vs. length, and was found to be decreased to 63% within 0.3 ms of a step to zero force. Stiffness decreased further with time at zero force, and after 256 ms was about 29% of the isometric value. 5. In rigor, caused by the introduction of ATP‐free solution during the plateau of isometric contraction, fibre tension decreased to about 30% of the active tension, whereas stiffness relative to force increased; 82% of the initial stiffness in rigor was detected in a restretch immediately after a shortening step, decreasing to 59% at 256 ms. When the fibre was activated at suboptimal [Ca2+] to cause the same force as in rigor, stiffness was lower than in rigor and decreased more after a release. 6. After completion of a release‐stretch cycle, stiffness was rapidly restored to the same value as in isometric contraction. Test stretches at different points in time after completion of the cycle revealed that most of the stiffness had been restored within 1 ms of the restretch, occurring concomitantly with a decay in force.(ABSTRACT TRUNCATED AT 400 WORDS)
Effects of age on physiological, immunohistochemical and biochemical properties of fast‐twitch single motor units in the rat.
Authors: Larsson L , Ansved T , Edström L , Gorza L , Schiaffino S
Published: December 2014

1. Physiological, enzyme‐histochemical, biochemical and morphometrical properties of fast‐twitch single motor units were compared between young (3‐6 months) and old rats (20‐24 months) using the glycogen depletion technique. Monoclonal antibodies (mAbs) were used to identify the myosin heavy chain (MHC) composition in the muscle fibres of the motor unit (motor unit fibres) in order to facilitate correlative physiological, histochemical, biochemical and morphometrical studies. 2. Earlier observations on effects of age on contractile properties of fast‐twitch motor units were confirmed and extended. That is, the duration of the isometric twitch, and the twitch and tetanus forces, were increased. Further, motor unit fibres were rearranged, occupying a larger territory and displaying an increased innervation ratio in old age, indicating a denervation‐reinnervation process. 3. Motor units with muscle fibres expressing the novel IIX myosin heavy chain (MHC) were observed in both young and old animals, and they constituted the predominant motor unity type identified in the old animals. In contrast to the type IIX MHC motor units in the young animals, the type IIX MHC units in old age often contained muscle fibres which expressed either the type IIA or type IIB MHC, although type IIX MHC fibres were in the majority (so called ‘IIX’ MHC motor units), but motor units containing all these three fibre types were never observed. There were also single fibres co‐expressing IIX and IIB MHCs in old age. 4. In the young animals the IIX MHC motor units had a higher (P less than 0.001) resistance to fatigue (fatigue ratio 0.45 +/‐ 0.11) than the type IIB MHC units (0.03 +/‐ 0.05), a succinate dehydrogenase (SDH) activity (0.62 +/‐ .007) intermediate (P less than 0.001) between those of type IIA muscle fibres classified according to myofibrillar ATPase activity after acid pre‐incubation, i.e. type IIA ATPase, (0.84 +/‐ 0.13) and type IIB MHC motor unit fibres (0.20 +/‐ 0.04), and cross‐sectional fibre areas (1650 +/‐ 320 microns 2) which were similar to those of type IIA ATPase muscle fibres (1460 +/‐ 150 microns 2) but smaller (P less than 0.001) than type IIB MHC motor unit fibres (4650 +/‐ 1180 microns 2).(ABSTRACT TRUNCATED AT 400 WORDS)
Relation between plasma K+ and ventilation during incremental exercise after glycogen depletion and repletion in man.
Authors: Busse M W , Maassen N , Konrad H
Published: December 2014

1. We have examined the relationship between expiratory ventilation (VE), plasma potassium concentration ([K+]P), blood lactate concentration ([Lac‐]B), and plasma pH (pHP) in five trained men before and after glycogen depletion and repletion in two successive incremental bicycle ergometer tests (tests A and B). 2. Though pHP was significantly higher after glycogen depletion (in relation to normal or repleted conditions) VE and [K+]P also tended to be higher. 3. There was no constant relation between the magnitude or the direction of change in lactic acidosis, or VE and [K+]P, respectively. Instead, a close temporal relationship between changes in VE and [K+]P was found. 4. A non‐linear increase in VE occurred independently of changes in pHP or [Lac‐]B, but could be well predicted from a non‐linear increase in [K+]P. 5. These findings indicate that lactic acidosis had no deciding effect on exercise ventilation in these experiments. They are consistent with the idea that the potassium increase may contribute to the ventilatory drive during exercise.
Characteristics of multiple Ca(2+)‐activated K+ channels in acutely dissociated chick ciliary‐ganglion neurones.
Authors: Dryer S E , Dourado M M , Wisgirda M E
Published: December 2014

1. Whole‐cell and single‐channel recordings were used to characterize Ca(2+)‐activated K+ channels (IK(Ca)) in acutely dissociated chick‐ganglion neurones. 2. Application of depolarizing voltage steps resulted in outward currents that could be separated according to their dependence on external Ca2+ and/or holding potential. IK(Ca) was the only outward current that could be evoked from holding potentials of ‐50 mV or less. IK(Ca) was eliminated by bath application of Ca(2+)‐free salines. A voltage‐dependent outward current (IK(V)) could be evoked from more negative holding potentials in Ca(2+)‐free salines. IK(V) was only partially blocked by as much as 30 mM‐tetraethylammonium (TEA). 3. Tail currents associated with IK(Ca) reversed close to the K+ equilibrium potential (EK). IK(Ca) tail currents appeared sigmoidal, but the falling phase of the tail currents could be fitted with exponential curves that decayed faster at more negative membrane potentials. 4. IK(Ca) was blocked completely and reversibly by 10 mM‐TEA. IK(Ca) was substantially reduced (80‐90%) by as little as 1 mM‐TEA. 5. Total IK(Ca) was reduced but not eliminated by saturating concentrations of apamin (200 nM). This blockade was not reversible with up to 30 min of washing. Application of 100 microM‐d‐tubocurare (dTC) also produced a partial blockade of total IK(Ca). 6. Whole‐cell current‐clamp recordings showed that IK(Ca) contributed to the late phases of spike repolarization and was the dominant current flowing during the spike after‐hyperpolarization (AHP). Application of 200 nM‐apamin caused a reduction in the duration of the AHP. This reduction was best seen when multiple spikes were evoked by prolonged (20‐50 ms) injections of depolarizing current. 7. Three distinct types of IK(Ca) channels could be observed in inside‐out patches in the presence of free Ca2+ concentrations of 2 x 10(‐7) M, but not in the presence of free Ca2+ at concentrations of less than 10(‐9) M. These had unitary chord conductances of 190 pS (i1), 110 pS (i2), and 45 pS (i3) with [K+]o = 150 mM and [K+]i = 75 mM. Each of these three channels had distinct kinetic properties. The 45 pS channel was most sensitive to activation by Ca2+ and could be detected at free Ca2+ concentrations as low as 10(‐8) M. 8. All three IK(Ca) channels could be observed in inside‐out patches held at membrane potentials where IK(V) was fully inactivated. Application of 10 mM‐TEA caused a complete block of IK(Ca) channels in outside‐out patches.(ABSTRACT TRUNCATED AT 400 WORDS)
Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus.
Authors: Zeuthen T
Published: December 2014

1. The interaction between Cl‐, K+ and H2O fluxes were studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion‐selective microelectrodes. The flux of H2O was measured by means of K+ electrodes as the dilution or concentration of intracellular choline ions, Ch+i. 2. In one series of experiments Cl‐ was readministered to the ventricular solution of tissues incubated in media with low Cl‐ concentrations. The resulting influx of Cl‐ was associated with an instantaneous influx of K+ and H2O. 3. Both the Cl‐ and the K+ influxes were reduced by the diuretic furosemide but were unaffected by inhibitors of Na+, K(+)‐ATPase or changes in membrane potentials induced by Ba2+. Since the influx of K+ proceeds against its electrochemical gradient and is unaffected by changes in membrane potentials, the membrane exhibits secondary active, electroneutral transport of K+. 4. The influx of water, initiated simultaneously with the influx of K+ and Cl‐, commenced before these ions had changed the osmolarity of the intracellular solution significantly. The influx of H2O could proceed against an osmotic gradient. The influx stopped when 100 mmol l‐1 of mannitol was added to the ventricular solution at the same time as the Cl‐ ions. The influx of H2O was inhibited by K+ removal, furosemide or high external Ba2+ (10 mmol l‐1), but not by strophanthidin, ouabain or low concentrations of Ba2+ (0.5 mmol l‐1). The influx could not continue with other permeable anions, NO3‐, acetate‐ or SCN‐, replacing Cl‐. 5. In another series of experiments Cl‐ was removed from the ventricular solution of tissues bathed in saline solutions with normal concentrations of Cl‐. The resulting efflux of Cl‐ was associated with an instantaneous efflux of K+ and H2O. This efflux of H2O could proceed against an osmotic gradient of up to 70 mosmol l‐1. This effect was inhibited by furosemide, in which case the water fluxes were entirely dependent on the osmotic gradients and the osmotic water permeability Lp of the ventricular membrane. 6. The data suggest that there is a coupling between the flux of KCl and of water in the ventricular membrane, which implies that the reflection coefficient sigma for KCl under the given circumstances is less than one. I suggest that the ability of leaky epithelia to transport against osmotic gradients depends on such a coupling, which derives from the properties of the proteins through which K+, Cl‐ and H2O leave the cell.
Sodium‐hydrogen exchange in guinea‐pig ventricular muscle during exposure to hyperosmolar solutions.
Authors: Whalley D W , Hemsworth P D , Rasmussen H H
Published: December 2014

1. The effect on intracellular pH (pHi) and intracellular Na+ activity (aNai) of exposure to hyperosmolar solutions was investigated in guinea‐pig ventricular muscle using ion‐sensitive microelectrodes. 2. Exposure of tissue to solution made hyperosmolar by the addition of 100 mM‐sucrose produced an intracellular alkalinization of 0.10 pH units and hyperpolarization of the membrane potential. 3. When extracellular Na+ was reduced to 15 mM by substitution of NaCl with choline chloride, exposure to hyperosmolar solutions caused a decrease in pHi. Identical experiments using LiCl as the sodium substitute resulted in an increase in pHi of a magnitude similar to that seen at physiological Na+ levels. 4. In the presence of 50 microM‐5‐(N,N‐dimethyl)amiloride (DMA), an inhibitor of Na(+)‐H+ exchange, pHi decreased upon exposure to hyperosmolar solution. 5. The recovery of pHi from an intracellular acidosis (induced by brief exposure to NH4Cl) was enhanced in hyperosmolar solution when compared to recovery in isosmolar solution. This enhancement was observed even when aNai was markedly elevated (greater than 25 mM) by inhibition of the Na(+)‐K+ pump. 6. There was an increase in aNai during exposure to hyperosmolar solutions. When the Na(+)‐K+ pump was inhibited with dihydro‐ouabain a component of this increase in aNai was sensitive to DMA. 7. We conclude that exposure of cardiac tissue to hyperosmolar solutions results in an intracellular alkalosis due to activation of the sarcolemmal Na(+)‐H+ exchanger. Such changes should be considered when exposure to hyperosmolar solutions is used in the study of excitation‐contraction coupling and cardiac muscle mechanics.
The peptide VIP is a neurotransmitter in rat adrenal medulla: physiological role in controlling catecholamine secretion.
Authors: Wakade T D , Blank M A , Malhotra R K , Pourcho R , Wakade A R
Published: December 2014

1. The perfused adrenal gland of the rat was used to establish the identity of a non‐cholinergic substance involved in splanchnic nerve‐mediated secretion of catecholamines. 2. The perfused adrenal medulla was rich in vasoactive intestinal polypeptide (VIP) content (28 pmol g‐1 of wet tissue). VIP‐immunoreactive nerve fibres were present in the adrenal medulla and the adrenal cortex. 3. Field stimulation (10 Hz for 15 min plus 1 Hz for 15 min) caused a large increase in the output of VIP in the perfusate over the spontaneous release of VIP. Secretion of catecholamines was also greatly elevated by field stimulation. Field stimulation‐evoked output of VIP and catecholamines was abolished after chronic denervation of the adrenal glands. 4. Infusion of acetylcholine (ACh) did not increase the output of VIP but caused a robust secretion of catecholamines. 5. The VIP output declined when the stimulation frequency was increased (8.6 x 10(‐3) fmol pulse‐1 at 1 Hz and 4.0 x 10(‐3) fmol pulse‐1 at 10 Hz). 6. In contrast, the output of 3H‐acetylcholine (3H‐ACh, expressed as a fraction of tissue 3H‐ACh content) increased from 7.0 x 10(‐2) pulse‐1 at 1 Hz to 16.3 x 10(‐2) pulse‐1 at 10 Hz. 7. Secretion of catecholamines evoked by low‐frequency stimulation (1 Hz) was reduced by 40% in the presence of cholinergic receptor antagonists (atropine plus hexamethonium). Inclusion of a VIP receptor antagonist ([Ac‐Tyr1, D‐Phe2]‐GRF 1‐29 amide) caused about 75% inhibition. 8. The VIP receptor antagonist inhibited VIP‐evoked secretion of catecholamines without affecting ACh‐evoked secretion. 9. In conclusion, VIP satisfies all the essential criteria to assume the role of a neurotransmitter in the rat adrenal medulla. The contribution of VIP to the secretion of adrenal medullary hormones is more prominent at low rates of neuronal activity whereas ACh is the major contributor at higher activity.
GABAB receptor‐mediated inhibition of Ca2+ currents and synaptic transmission in cultured rat hippocampal neurones.
Authors: Scholz K P , Miller R J
Published: December 2014

1. The effects of activation of GABAB receptors on Ca2+ currents (ICa) were investigated by application of whole‐cell patch‐clamp techniques to pyramidal neurones and non‐pyramidal interneurones from the rat hippocampus grown in cell culture. 2. (+/‐)‐Baclofen (10 microM) reduced ICa evoked in pyramidal neurones at 0 mV from a holding potential of ‐80 mV by 33 +/‐ 3%. Inhibition could be observed at the peak of ICa with significant inhibition still present after 200 ms at 0 mV. When Ba2+ was used as the charge carrier (IBa) baclofen inhibited 28 +/‐ 3% of the current at ‐20 mV from a holding potential of ‐80 mV. The GABAB receptor antagonist 2‐OH‐saclofen (50‐200 microM) blocked the actions of baclofen. 3. The selective Ca2+ channel blocker, omega‐conotoxin fraction GVIA (omega‐CgTX), was used to characterize the Ca2+ currents inhibited by baclofen. omega‐CgTX (5 microM) blocked 24 +/‐ 3% of IBa. Following block of the omega‐CgTX‐sensitive current, baclofen inhibited significantly less current than under control conditions. 4. Addition of the dihydropyridine Ca2+ channel antagonist nimodipine (1 microM) inhibited 18 +/‐ 5% of ICa at 0 mV from a holding potential of ‐80 mV and 44 +/‐ 9% from a holding potential of ‐40 mV. In addition, nimodipine partially occluded subsequent responses to application of baclofen. 5. In the presence of both 5 microM‐omega‐CgTX and 200 nM‐nimodipine, responses to baclofen were almost completely blocked at depolarized holding potentials where the dihydropyridines are most effective. 6. Inclusion of 500 microM‐guanosine 5'‐O‐(3‐thiotriphosphate) (GTP‐gamma‐S) in the patch pipette enhanced the response to a subsaturating concentration of baclofen and rendered the response irreversible. Subsequent addition of the adenosine receptor agonist 2‐Cl‐adenosine (2‐CA) (1 microM; which also reduces ICa under control conditions) was without effect, suggesting that these two receptor‐effector pathways converge. 7. The actions of baclofen on ICa were blocked by pre‐treatment of the cultures with pertussis toxin (250 ng/ml). 8. Baclofen also inhibited ICa in non‐pyramidal neurones from the hippocampus, but was slightly less effective. 9. Baclofen reduced both excitatory‐ and inhibitory postsynaptic currents (EPSCs and IPSCs) recorded as a consequence of extracellular stimulation of presynaptic neurones. This action was blocked by 2‐OH‐saclofen (200 microM) and also by pretreatment of the cultures with pertussis toxin.(ABSTRACT TRUNCATED AT 400 WORDS)
Agonists at metabotropic glutamate receptors presynaptically inhibit EPSCs in neonatal rat hippocampus.
Authors: Baskys A , Malenka R C
Published: December 2014

1. The effects of metabotropic glutamate receptor agonists on excitatory synaptic transmission in the CA1 region of rat hippocampal slices (11‐30 days) were studied using extracellular and whole‐cell patch‐clamp recording techniques. 2. Trans‐1‐amino‐1,3‐cyclopentanedicarboxylic acid (trans‐ACPD; 25‐100 microM) reversibly depressed excitatory postsynaptic currents (EPSCs) without affecting presynaptic fibre excitability or EPSC reversal potential. 3. Ibotenate (25 microM) or L‐glutamate (250 microM), in the presence of the N‐methyl‐D‐aspartate (NMDA) receptor antagonist, D‐2‐amino‐5‐phosphonovaleric acid (APV, 50‐75 microM), depressed the EPSC amplitude while inducing no detectable inward current. L‐2‐Amino‐4‐phosphonobutyrate (L‐AP4, 25‐100 microM), the phosphonic derivative of glutamate, also depressed EPSC amplitude and caused no detectable inward current. 4. The NMDA receptor‐mediated component of the EPSC recorded in the presence of the non‐NMDA receptor antagonist, 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX, 20‐30 microM) was depressed by trans‐ACPD, L‐AP4, or quisqualate (1‐2 microM). 5. The response to ionophoretic application of alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionic acid (AMPA) was unaffected by trans‐ACPD or L‐AP4 although the simultaneously recorded EPSC was strongly depressed. In addition, paired‐pulse facilitation (50‐75 ms interstimulus interval) was reversibly enhanced by trans‐ACPD or L‐AP4. These results indicate that the depression of synaptic transmission likely was mediated by a presynaptic ‘autoreceptor’. 6. The effects of trans‐ACPD or L‐AP4 on synaptic transmission decreased significantly over ages 12‐30 days and were minimal in adult (greater than 80 days) slices. 7. The depression of synaptic transmission caused by trans‐ACPD or L‐AP4 was not altered following the induction of long‐term potentiation (LTP). 8. The results indicate that metabotropic glutamate receptor agonists suppress excitatory synaptic transmission in CA1 pyramidal cells by an action at a presynaptic site. This effect is developmentally regulated and is maximally expressed during the first postnatal month.
The role of acetylcholinesterase in denervation supersensitivity in the frog cardiac ganglion.
Authors: Streichert L C , Sargent P B
Published: December 2014

1. The sensitivity of normal and denervated cardiac ganglion cells to the cholinergic agonists acetylcholine and carbamylcholine (carbachol) were compared in the frog, Rana pipiens. Acetylcholine and carbachol bind to the same acetylcholine receptors, but, unlike acetylcholine, carbachol is resistant to hydrolysis by acetylcholinesterase. 2. Sensitivity was assessed by the peak depolarization elicited in response to a sustained pulse of ligand emitted from a pipette positioned 10 microns from the ganglion cell surface. This technique allows the sensitivity of the entire cell to be recorded with a single measurement. 3. The acetylcholine sensitivity of normal cardiac ganglion cells was increased by inhibiting extracellular acetylcholinesterase with echothiophate. 4. Denervation increased the sensitivity of cardiac ganglion cells to acetylcholine but not to carbachol. 5. Following the inhibition of extracellular acetylcholinesterase with echothiophate, sensitivity to acetylcholine was similar in normal and in denervated ganglion cells. 6. The increased sensitivity to acetylcholine of cardiac ganglion cells following denervation is caused by a reduction in the hydrolysis of the transmitter by acetylcholinesterase rather than by changes in the number and/or properties of acetylcholine receptors.
Excitatory and inhibitory synaptic currents and receptors in rat medial septal neurones.
Authors: Schneggenburger R , López‐Barneo J , Konnerth A
Published: December 2014

1. A thin‐slice preparation was used to study the postsynaptic potentials and the underlying currents of visually identified rat medial septal (MS) neurones under tight‐seal voltage‐ and current‐clamp conditions. 2. Upon stimulation of the afferent fibres, all MS neurones exhibited a sequence of excitatory‐inhibitory postsynaptic potentials (EPSP‐IPSP). Under voltage clamp, with potassium glutamate as internal solution and at negative holding potentials (Vh), this synaptic pattern appeared as an initial inward current followed by a longer lasting outward current. 3. The inward postsynaptic current was completely abolished by 5 microM‐6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) whereas the outward current disappeared in the presence of 10 microM‐bicuculline. Thus the major excitatory and inhibitory synaptic inputs were identified as being due to activation of quisqualate/kainate glutamatergic and gamma‐aminobutyric acid (GABAA) receptors, respectively. 4. At positive Vh a CNQX‐resistant component of the excitatory postsynaptic current (EPSC) was revealed. This component was slower than the one mediated by the quisqualate receptor and was abolished by 3‐3(2‐carboxypiperazine‐4‐yl)propyl‐1‐phosphonate (CPP), indicating that N‐methyl‐D‐aspartate (NMDA) receptors are involved in excitatory synaptic transmission in MS cells. The existence of the two main subtypes (NMDA and non‐NMDA) of glutamatergic receptors in MS neurones was also confirmed by the responses of the neurones to bath application of the different agonists (glutamate, quisqualate, kainate and NMDA). 5. The CNQX‐sensitive EPSC had a reversal potential near 0 mV. The fast rise time (approximately 0.7 ms) indicates a somatic location of the excitatory synapses. The relaxation kinetics of the fast EPSC were fitted by a single exponential function with a time constant of 1.13 +/‐ 0.1 ms. This parameter was independent of Vh. Fast EPSCs were blocked by CNQX in a dose‐dependent manner (dissociation constant, KD = 0.2 microM). 6. Inhibitory postsynaptic currents (IPSCs) were studied in symmetrical chloride solutions after blockade of the excitatory receptors. The current‐voltage relation was linear and reversed at 0 mV. The IPSCs had a fast rise time and their decay was best fitted by the sum of two exponentials with time constant of approximately 20 and 50 ms (Vh = ‐60 mV). The IPSCs were abolished by bicuculline (KD = 1 microM), a selective antagonist of GABAA receptors. As expected, bath application of GABA produced large whole‐cell currents. 7. In many cells, in addition to the usual EPSP‐IPSP sequence, failures of either the EPSP or the IPSP were frequently observed during the experimental protocol.(ABSTRACT TRUNCATED AT 400 WORDS)
The effect of the benzothiazepine diltiazem on force and Ca2+ current in isolated frog skeletal muscle fibres.
Authors: Böhle T
Published: December 2014

1. The action of the D‐cis and L‐cis isomers of the benzothiazepine derivative diltiazem on isometric force and L‐type Ca2+ inward current in short muscle fibres from toe muscles of the frog (Rana esculenta and R. temporaria) was investigated under voltage clamp control with two internal microelectrodes. The experiments were performed at 10 degrees C in TEA‐sulphate solution, in which the concentration of ionized Ca2+ was about 4 mM. 2. In the presence of diltiazem (D‐cis, 0.25‐30 microM; L‐cis, 5‐100 microM) normal Ca2+ currents and phasic contractures could be elicited by a depolarizing voltage step. However, after long‐lasting depolarizations both the Ca2+ channel and the force controlling system (voltage sensor) remained refractory for minutes instead of seconds as under control conditions, i.e. they were ‘paralysed’. L‐cis diltiazem was about 20 times less effective in comparison with the D‐cis isomer. 3. Speed of restoration of force and Ca2+ current was retarded by up to two orders of magnitude, depending on the applied concentration of diltiazem. 4. The steady state potential dependence of force restoration (V0.5 = ‐49 mV; the potential at which half of the voltage sensors are restored) and that of Ca2+ current restoration (V0.5 = ‐57 mV) were not altered by diltiazem. 5. The time course of the transition to paralysis, which was quite slow under control conditions, could be accelerated by diltiazem in a concentration‐dependent manner. 6. Paralysis developed at the same speed when the fibre was depolarized from ‐90 mV to ‐30 mV or to +30 mV. 7. The voltage dependence of Ca2+ current inactivation (inactivation curve; conditioning pulses of 2 min duration) was shifted by about 14 mV to more negative potentials (V0.5 = ‐31 mV; V'0.5 = ‐45 mV) under the influence of 1 microM‐D‐cis diltiazem. It is assumed that no steady state was reached during the conditioning pulse. 8. The voltage dependence of force activation (V0.5 = ‐45 mV) was not altered by diltiazem. 9. Since the effects of both isomers of diltiazem could only be disclosed during the adjustment of a new steady state and since the drugs acted in a similar way on the inactivation kinetics of force and Ca2+ current, it is concluded that, in depolarized fibres, the drug facilitates the transition to a stabilized inactivated form of the voltage sensor/Ca2+ channel molecule.
Parvalbumin, labile heat and slowing of relaxation in mouse soleus and extensor digitorum longus muscles.
Authors: Berquin A , Lebacq J
Published: December 2014

1. Parvalbumin content, heat rate and rate of relaxation were measured in two mouse muscles: the slow‐twitch soleus and the fast‐twitch extensor digitorum longus (EDL). 2. No trace of parvalbumin was found in the soleus; EDL contained a mean of 4.86 mg of this protein per gram of fresh muscle (S.D. = 1.25). 3. Heat rate during 7 s isometric tetani in isolated soleus muscle at 20 degrees C can be described by the sum of an exponentially decaying term and a constant term. The exponential term is reduced by 67% in a second tetanus performed 1 s after a first one; its repriming is complete after a resting period of about 1 min. The exponential term has therefore the properties of labile heat. 4. Relaxation rate measured during 15 s of isometric interrupted tetani at 20 degrees C is nearly constant in the soleus, but decreases continuously with increasing tetanus duration in the EDL. In the latter, isometric tension also decreases continuously. 5. Therefore, parvalbumin can account neither for the labile heat production in mouse soleus nor for the slowing of relaxation associated with muscle fatigue observed after a few seconds of tetanus in EDL. The role of parvalbumin in striated muscles is thus reassessed, and other possible causes of labile heat production and slowing of relaxation are discussed.
Calcium current variation between acutely isolated adult rat dorsal root ganglion neurons of different size.
Authors: Scroggs R S , Fox A P
Published: December 2014

1. The distribution of pharmacologically and/or biophysically unique Ca2+ current subtypes was studied in different diameter rat dorsal root ganglion (DRG) neuron cell bodies. DRG cells which fell into three diameter ranges, small (20‐27 microns), medium (33‐38 microns) and large (45‐51 microns), were studied. T‐type Ca2+ current was defined as low‐threshold, rapidly inactivating current evoked by a weak test depolarization (‐50 mV) from negative holding potentials (‐80 to ‐100 mV), and which was sensitive to changes in holding potential. L‐type Ca2+ current was defined as peak high‐threshold Ca2+ current evoked from a holding potential of ‐60 mV and sensitive to blockade by 2 microM‐nimodipine. N‐type Ca2+ current was defined as peak high‐threshold Ca2+ current evoked from a holding potential of ‐60 mV and sensitive to blockade by 0.9 microM‐omega‐conotoxin GVIA. 2. T‐type Ca2+ currents were observed in small and medium diameter, but not in large diameter, DRG cell bodies. Large diameter DRG cell bodies had a small amount of low‐threshold Ca2+ current but this current did not inactivate and was insensitive to a change in holding potential from ‐80 to ‐90 mV, and thus did not appear to be conducted through T‐type Ca2+ channels. The T‐type Ca2+ currents observed in medium diameter DRG cell bodies were considerably larger in amplitude (1‐6 nA) than those observed in small diameter DRG cell bodies (100 pA‐1 nA). This difference could not be accounted for by the difference in membrane surface area of small versus medium diameter DRG cell bodies. 3. The T‐type Ca2+ currents observed in medium diameter DRG cells were sensitive to blockade by amiloride. Amiloride (500 microM) blocked 79.4 +/‐ 0.9% (mean +/‐ S.E.M.) of T‐type Ca2+ current amplitude in six medium diameter DRG cell bodies which were held at ‐80 mV and depolarized to ‐50 or ‐40 mV. Amiloride (500 microM) failed to block high‐threshold current in five medium diameter DRG cell bodies, indicating that it was specific for T‐type Ca2+ current in these cells. 4. The percentage of peak whole‐cell L‐type Ca2+ current was significantly larger in small diameter DRG cell bodies (52.9 +/‐ 4.7% of total whole‐cell Ca2+ current) than in medium diameter DRG cell bodies (6.6 +/‐ 3.9% of total whole‐cell Ca2+ current) or large diameter DRG cell bodies (19.4 +/‐ 5.7% of total whole‐cell Ca2+ current).(ABSTRACT TRUNCATED AT 400 WORDS)
Cytosolic calcium and myofilaments in single rat cardiac myocytes achieve a dynamic equilibrium during twitch relaxation.
Authors: Spurgeon H A , duBell W H , Stern M D , Sollott S J , Ziman B D , Silverman H S , Capogrossi M C , Talo A , Lakatta E G
Published: December 2014

1. Single isolated rat cardiac myocytes were loaded with either the pentapotassium salt form or the acetoxymethyl ester (AM) form of the calcium‐sensitive fluorescent probe, Indo‐1. The relationship of the Indo‐1 fluorescence transient, an index of the change in cytosolic calcium [Ca2+]i concentration, to the simultaneously measured cell length during the electrically stimulated twitch originating from slack length at 23 degrees C was evaluated. It was demonstrated that even if the Ca2+ dissociation rate from Indo‐1 was assumed to be as slow as 10 s‐1, the descending limb (‘relaxation phase’) of the Indo‐1 fluorescence transient induced by excitation under these conditions is in equilibrium with the [Ca2+]i transient. Additionally, the extent of Indo‐1 loading employed did not substantially alter the twitch characteristics. 2. A unique relationship between the fluorescence transient and cell length was observed during relaxation of contractions that varied in amplitude. This was manifest as a common trajectory in the cell length vs. [Ca2+]i phase‐plane diagrams beginning at the time of cell relengthening. The common trajectory could also be demonstrated in Indo‐1 AM‐loaded cells. The Indo‐1 fluorescence‐length relation defined by this common trajectory is steeper than that described by the relation of peak contraction amplitude and peak fluorescence during the twitch contractions. 3. The trajectory of the [Ca2+]i‐length relation elicited via an abrupt, rapid, brief (200 ms) pulse of caffeine directly onto the cell surface or by ‘tetanization’ of cells in the presence of ryanodine is identical to the common [Ca2+]i‐length trajectory formed by electrically stimulated contractions of different magnitudes. As the [Ca2+]i and length transients induced by caffeine application or during tetanization in the presence of ryanodine develop with a much slower time course than those elicited by electrical stimulation, the common trajectory is not fortuitous, i.e. it cannot be attributed to equivalent rate‐limiting steps for the decrease of [Ca2+]i and cell relengthening. 4. The [Ca2+]i‐length relation defined by the common trajectory shifts appropriately in response to perturbations that have previously been demonstrated to alter the steady‐state myofilament Ca2+ sensitivity in skinned cardiac fibres. Specifically, the trajectory shifts leftward in response to an acute increase in pH or following the addition of novel myofilament calcium‐sensitizing thiadiazinone derivatives; a rightward shift occurs in response to an acute reduction in pH or following the addition of butanedione monoxime.(ABSTRACT TRUNCATED AT 400 WORDS)
Directional sensitivity of human periodontal mechanoreceptive afferents to forces applied to the teeth.
Authors: Trulsson M , Johansson R S , Olsson K A
Published: December 2014

1. Single‐unit impulse activity from thirty‐eight mechanoreceptive afferent fibres was recorded in the human inferior alveolar nerve using tungsten microelectrodes. All afferents responded to mechanical stimulation of the teeth and most likely supplied periodontal mechanoreceptors. 2. All afferents showed their highest sensitivity to forces applied to a particular tooth (the lower incisors, the canine or the first premolar). Forces with ‘ramp‐and‐hold’ shaped profiles of similar magnitudes were applied to that tooth in the following six directions: lingual, labial, mesial and distal in the horizontal plane, and up and down in the axial direction of the tooth. Both static and dynamic response components were analysed. 3. All afferents were 'slowly adapting’ since they discharged continuously in response to static forces in at least one stimulation direction. Twenty‐five afferents (66%) were spontaneously active in the sense that they showed an on‐going discharge in the absence of external stimulation. 4. Diverse receptive fields were observed. Most afferents (74%) responded to static forces in two or three of the four horizontal directions. Likewise, all units showed excitatory responses to axial loading with a majority (74%) responding in one of the two axial directions and the remainder in both axial directions. Spontaneously active afferents generally decreased their discharge rate when stimulated in directions opposite to the directions exciting the afferent. With regard to population responses, approximately half of the afferents showed excitatory responses to each stimulus direction except for downwards, in which 86% responded. 5. Twenty‐three afferents (61%) exhibited the strongest response to forces in one of the horizontal directions. Of those, a majority were most responsive to the lingual direction (52%) and some to the labial direction (30%). Accordingly, the discharge rates during force application averaged over the whole afferent sample were highest in these directions. Of the remaining afferents, most responsive to one of the axial directions, 60% showed their strongest responses to forces in the downward direction. 6. Forty‐five per cent of the afferents showed wider receptive fields to the dynamic component of the force stimulation than to the static. The direction of maximal sensitivity, however, remained the same with few exceptions. 7. It was demonstrated that even though individual periodontal mechanoreceptive afferents provide ambiguous information regarding the direction of a force applied to a tooth, populations of such afferents are well suited to give detailed directional information. It is suggested that such information may play an important role for the control of mastication.
A capsaicin‐sensitive inhibitory reflex from the colon to mesenteric arteries in the guinea‐pig.
Authors: Meehan A G , Kreulen D L
Published: December 2014

1. The present in vitro study examined the effect of distension of the distal colon on membrane potential in the inferior mesenteric artery of the guinea‐pig. 2. Distension of the distal colon up to an intraluminal pressure of 25 cmH2O induced a hyperpolarization in the inferior mesenteric artery. The average amplitude of hyperpolarizations induced by 2 min distensions of the colon was 3 mV and their average duration was 268 s. 3. Distension‐induced hyperpolarizations (DIHs) were abolished in the presence of tetrodotoxin or a low‐Ca2+ (0.5 mM) superfusion solution. 4. Superfusion of capsaicin (10 microM) induced slow hyperpolarizing responses in mesenteric arteries. Following application of capsaicin (10 microM), DIHs were abolished. 5. These findings provide strong evidence that mesenteric arteries receive an inhibitory, capsaicin‐sensitive sensory innervation from the distal colon which is activated during periods of colon distension to induce hyperpolarization of the arterial smooth muscle. This extramural inhibitory reflex pathway may play a physiological role in co‐ordinating mesenteric blood flow with changes in gut motility.
Electrical coupling between cells of the insect Aedes albopictus.
Authors: Bukauskas F , Kempf C , Weingart R
Published: December 2014

1. Cell pairs of an insect cell line (Aedes albopictus, clone C6/36) were used to study the electrical properties of intercellular junctions. A double voltage‐clamp approach was adopted to control the voltage gradient between the cells and measure the intracellular current flow. 2. Determinations of junctional conductance (gj) revealed two types of intercellular contacts, gap junctions and cytoplasmic bridges. Identification occurred by means of functional criteria, i.e. the dependency of gj on (i) junctional membrane potential, (ii) non‐junctional membrane potential, and (iii) heptanol. 3. In cell pairs with putative gap junctions, gj was dependent on the junctional membrane potential (Vj). When determined at the beginning of voltage pulses, gj was insensitive to Vj; when determined at the end of 15 s pulses, it depended on Vj in a bell‐shaped manner (70% decrease for a change in Vj of +/‐ 75 mV). 4. These cell pairs also showed a dependency of gj on the non‐junctional membrane potential (Vm). When determined immediately after changing the non‐junctional membrane potential in both cells, gj was not affected by Vm; when determined 30 s later, gj was modified by Vm in a S‐shaped fashion (100% decrease when Vm was depolarized to +50 mV). 5. Exposure to 3 mM‐heptanol gave rise to complete and reversible block of gj in cell pairs with putative gap junctions. 6. Cell pairs susceptible to uncoupling by heptanol revealed junctional currents indicative of the operation of gap junction channels. The single‐channel conductance, determined at a Vm of ‐50 to ‐70 mV, was 133 pS. 7. In the case of putative cytoplasmic bridges, gj was insensitive to the junctional and non‐junctional membrane potential. In addition, it was not affected by 3 mM‐heptanol. 8. While most cell pairs showed functional properties characteristic of gap junctions or cytoplasmic bridges, few cell pairs exhibited junctional currents compatible with the co‐existence of both junctional structures.