bluuebunny/crossref_metadata_2025 huggingface dataset:
Endothelin and endothelium‐derived relaxing factor control of basal renovascular tone in hydronephrotic rat kidneys.
Authors: Gulbins E , Hoffend J , Zou A P , Dietrich M S , Schlottmann K , Cavarape A , Steinhausen M
Published: December 2014

1. In order to investigate the control of renal vascular tone by endothelin (ET) and endothelium‐derived relaxing factor (EDRF) under basal conditions, we infused intravenously anti‐ET‐1/3 antibodies (a‐ET‐1/3) and NG‐nitro‐L‐arginine methyl ester (L‐NAME) in split hydronephrotic rat kidneys. 2. A 25 min I.V. infusion of a‐ET‐1/3 (4.0 x 10(‐13) mol kg‐1 min‐1) induced a time‐dependent vasodilatation of arcuate (16.5%) and interlobular arteries (18.6%) as well as an increase of glomerular blood flow (GBF) by 32%. 3. Inhibition of EDRF synthesis by L‐NAME produced a marked vasoconstriction of arcuate arteries (17.1%) and efferent (20.1%) arterioles and a decrease of GBF by 43%. 4. Co‐infusion of a‐ET‐1/3 and L‐NAME induced efferent vasoconstriction by 19.5%, whereas preglomerular vessel diameters remained unchanged. 5. The specificity of a‐ET‐1/3 effects was confirmed by simultaneous I.V. application of a‐ET‐1/3 and ET‐1 (160 ng I.V.) which produced no significant vascular effects. Injection of ET‐1 alone constricted arcuate arteries and decreased glomerular blood flow by 25%. 6. Experiments in normal rat kidneys with a‐ET‐1/3 I.V. revealed an increase of renal blood flow by 21%. 7. Our results demonstrate a physiological control of basal vascular tone in larger preglomerular arterioles by ET and EDRF. Efferent arteriolar tone is predominantly controlled by EDRF.
Ca2+ oscillations and Ca2+ influx in Xenopus oocytes expressing a novel 5‐hydroxytryptamine receptor.
Authors: Parekh A B , Foguet M , Lübbert H , Stühmer W
Published: December 2014

1. We expressed a novel 5‐hydroxytryptamine receptor (SRL) in Xenopus oocytes and monitored cytosolic Ca2+ through the endogenous Ca(2+)‐dependent Cl‐ channel activity using the double electrode voltage‐clamp technique. 2. 5‐Hydroxytryptamine (5‐HT; 200 nM) led to an initial rapid oscillatory current followed by a pronounced secondary one, which lasted long after 5‐HT wash‐out (20‐40 min) and was not affected by the receptor antagonist yohimbine. 3. Both phases of the current were abolished by heparin demonstrating a key role for IP3‐induced Ca2+ release. 4. Caffeine (10 mM) alone did not evoke a current but reduced both phases of the current evoked by 5‐HT. Ryanodine had no effect. No evidence for Ca(2+)‐induced Ca2+ release was found. 5. The secondary current activated by 5‐HT was sensitive to changes in extracellular Ca2+, suggesting it was evoked by Ca2+ influx. Reducing external Na+ did not affect this current, demonstrating that it was rather specific for Ca2+. 6. The Ca2+ influx pathway was much more sensitive to Cd2+ than other divalent ions (Co2+, Mn2+, Sr2+, Ba2+). It was insensitive to verapamil. 7. Injection of D‐myo‐inositol 1,4,5‐trisphosphate, 3‐deoxy‐3‐fluoro (IP3‐F; an analogue not metabolized to D‐myo‐inositol 1,3,4,5‐tetrakisphosphate (IP4)), evoked either an oscillatory current or a rapid current followed by a sustained secondary one. The latter was sensitive to external Ca2+ and was blocked by Cd2+. Heparin dramatically reduced the IP3‐F‐evoked current. 8. Perfusion in Ca(2+)‐free solution, once a secondary current had been generated, significantly decreased the amount of intracellular Ca2+ mobilized by 5‐HT, indicating that the Ca2+ influx pathway plays an important role in pool refilling. 9. Block of Ca2+ influx by Cd2+ in cells that were oscillating transiently increased the amplitude and then either abolished the oscillations or made them irregular. This effect was also elicited by increasing external Ca2+. 10. These results demonstrate that 5‐HT, acting via IP3, both releases Ca2+ from internal stores and evokes a pronounced Ca2+ influx. This last step is activated by pool depletion and is important for both refilling of the agonist‐sensitive stores and modifying the oscillatory pattern.
A dendritic gain control mechanism in axonless neurons of the locust, Schistocerca americana.
Authors: Laurent G
Published: December 2014

1. To understand the possible function of active dendritic currents in local neurons, the non‐linear electrical properties of locust axonless non‐spiking interneurons were considered in parallel with the properties of graded transmitter release from their dendrites. 2. The dendritic membrane of the non‐spiking interneurons was least responsive to an applied or synaptic current at potentials between ‐55 and ‐45 mV. This is because, at these potentials, the input resistance of the dendrites is reduced by the activation of voltage‐dependent K+ conductances. Conversely, the membrane of the non‐spiking interneurons was most responsive to an applied or synaptic current at potentials more negative than ‐55 mV (where the membrane behaves more or less passively), or more positive than ‐45 mV (where the activation of a Ca2+ current can boost depolarizing potentials). 3. The threshold for detectable release at the non‐spiking synapse was around ‐65 mV. The dynamic gain of the synape (slope of the synaptic transfer curve) was maximum around ‐50 mV. Saturation was observed around ‐40 mV. Synaptic transfer is therefore most efficient at presynaptic potentials where the non‐spiking dendritic membrane is least responsive to incoming signals. 4. The possible consequences of this matching of membrane and synaptic non‐linearities was studied theoretically, with computer‐assisted simulations, and experimentally, by recording simultaneously from the dendrites of synaptically connected non‐spiking interneurons and motoneurons. This precise matching of non‐linearities was found to have two important consequences: (i) it allowed the effective gain of polysynaptic pathways via non‐spiking dendrites to depend little on the state of the interposed interneuron (linearization) and (ii) it optimized coding by preventing undesired over‐amplification and synaptic saturation.
Voltage‐dependent block by internal Ca2+ ions of inwardly rectifying K+ channels in guinea‐pig ventricular cells.
Authors: Matsuda H , Cruz J dos S
Published: December 2014

1. The block of the inwardly rectifying K+ channel by intracellular Ca2+ was studied in guinea‐pig ventricular cells. 2. Single‐channel currents through the inwardly rectifying K+ channel were recorded in the inside‐out configuration at 150 mM external and internal K+. Internal Ca2+, at a concentration of 0.4‐10 microM, induced subconductance levels with one‐third and two‐thirds of the unitary amplitude in the outward currents without affecting the inward currents. 3. Occupancy at each sublevel was estimated from the amplitude histogram which showed four equally spaced peaks in the presence of internal Ca2+. At different degrees of blockade, the distribution of the current levels showed a reasonable agreement with the binomial theorem. 4. The outward mean open‐channel currents were measured at different Ca2+ concentrations and voltages. The current‐voltage relation rectified inwardly in the presence of internal Ca2+ in a concentration‐dependent manner. 5. The outward mean open‐channel currents were normalized to unitary amplitudes in the absence of Ca2+. The normalized current‐Ca2+ concentration curve was fitted by saturation kinetics with a Hill coefficient of 1 at each voltage. The voltage dependence of the dissociation constants gives the value for the fractional electrical distance of the Ca2+ binding site of 0.7. 6. The dwell times in each substrate were distributed exponentially. On the assumption that the inwardly rectifying K+ channel of cardiac cells is composed of three identical conducting subunits and each subunit is blocked by Ca2+ independently, the blocking (mu) and unblocking (lambda) rates were calculated. The value of mu increased with higher Ca2+ concentrations or larger depolarizations, while lambda was independent of Ca2+ and decreased with larger depolarization. 7. It is thus concluded that internal Ca2+ produces a voltage‐dependent block of the channel to cause inward rectification although the blocking effect is less potent than that of Mg2+. The substate behaviour seen with internal Ca2+ supports the triple‐barrelled structure of the cardiac inwardly rectifying K+ channel.
Axonal projections and synaptic connections of C5 segment expiratory interneurones in the cat.
Authors: Douse M A , Duffin J
Published: December 2014

1. Respiratory interneurones in the C4‐C6 segments of the spinal cord have only recently been described; until now their projections and connections were not known. We investigated expiratory interneurones in the C5 spinal segment, using antidromic mapping to trace their projections and spike‐triggered averaging to test their synaptic connections with phrenic motoneurones. 2. A total of seventy expiratory interneurones were recorded in nineteen cats anaesthetized with pentobarbitone, paralysed and ventilated. The interneurones were found scattered dorsomedial to the phrenic motor nucleus, with discharge patterns of a constant (66%), augmenting (24%) or decrementing (10%) type. 3. Interneurone axons were found in the ipsilateral ventrolateral funiculus using antidromic activation at thresholds < 20 microA. The axons of eighteen of thirty‐three interneurones tested (55%) were found to extend to the rostral part of the C6 segment, seventeen of thirty‐three (52%) to the caudal part of the C6 segment and ten of nineteen (53%) to the rostral part of the C7 segment. 4. Axon collaterals for thirteen of thirty‐three interneurones (39%) were found in the ipsilateral half of the C6 segment, with their endings near the phrenic motor nucleus. In three cases two collaterals were found. None of the interneurones had projections in the contralateral halves of the C5 or C6 segments. 5. In a separate group of thirty‐four expiratory interneurones, antidromic mapping was used to find an axon collateral in the C6 segment prior to spike‐triggered averaging. Eleven of these interneurones had collaterals (32%) and were subsequently tested for synaptic connections to thirty‐two phrenic motoneurones. In three separate instances (9%), inhibitory postsynaptic potentials were observed. Amplitudes, fall times and half‐amplitude widths of the inhibitory postsynaptic potentials were 6.7, 10.4 and 10.6 microV; 0.3, 0.5 and 0.7 ms and 0.6, 1.6 and 3.3 ms respectively. 6. We conclude: (i) there is a population of expiratory interneurones in the C5 segment, located predominantly dorsomedial to the phrenic motor nucleus; (ii) at least one‐half of these interneurones have ipsilateral intersegmental projections to the C6 segment and (iii) although synaptic connections from expiratory interneurones in the C5 segment to phrenic motoneurones in the C6 segment may be rare, the observed inhibitory postsynaptic potentials had fall times and latencies commensurate with monosynaptic connections.
Evidence for active maternal‐fetal transport of Na+ across the placenta of the anaesthetized rat.
Authors: Stulc J , Stulcová B , Sibley C P
Published: December 2014

1. In order to investigate mechanisms of Na+ transfer, the unidirectional maternal‐fetal clearance (Kmf) of 22Na+ and of 51Cr‐EDTA (a marker of paracellular diffusion) was measured across the intact or umbilically or dually perfused placenta of the anaesthetized rat. 2. The Kmf of 22Na+ in the intact preparation (18.5 +/‐ 2.7 microliters min‐1, mean +/‐ S.D., n = 105 placentas) exceeded that of 51Cr‐EDTA in the same experiments (1.4 +/‐ 0.3 microliters min‐1) by more than ten times, whereas the difference in their diffusion coefficients in water was only 2‐fold. In the perfused preparations the difference in the Kmf values was 6‐fold. 3. Assuming that a simple model of paracellular diffusion through wide pores was one component of transfer, the Kmf of 51Cr‐EDTA and the diffusion coefficients were used to calculate a component of 22Na+ clearance (Kmf,residual) and of Na+ flux (Jmf,residual) across the perfused placentas which could not be accounted for by transfer through the paracellular route. 4. Kmf,residual of 22Na+ across the dually perfused placenta was significantly lower when temperature was reduced, the temperature quotient (Q10) of the transfer being about 2. Kmf,residual was also significantly lower when 0.1 mM ouabain was perfused on the fetal side. Jmf,residual exhibited saturation kinetics characterized by an apparent Michaelis constant (Km) of 90 mM. Kmf,residual was not influenced by 0.5 mM frusemide, 0.5 mM amiloride or by 0.5 mM hydrochlorothiazide administered to the maternal side. It was significantly increased by 1 mM alanine on the maternal side suggesting that the coupled transfer of Na+ and amino acids may contribute significantly to the maternal‐fetal flux of Na+. 5. These observations suggest that most (80%) of the maternal‐fetal flux of Na+ across the rat placenta is effected by active transcellular transport. This transport involves passive entry of Na+ into the trophoblast from the maternal side by a largely unknown saturable mechanism and active extrusion of Na+ from trophoblast to the fetal side by Na(+)‐K(+)‐ATPase.
Evidence that distinct neural pathways mediate parasympathetic contractions and relaxations of guinea‐pig trachealis.
Authors: Canning B J , Undem B J
Published: December 2014

1. The guinea‐pig trachea was isolated with its extrinsic innervation intact and pinned to the bottom of a water‐jacketed dissecting dish filled with warmed, oxygenated Krebs solution. The trachea was not separated from the oesophagus. Isometric tension was measured in a segment of the rostral portion of the trachea. 2. Stimulation of the vagus nerves caudal to the nodose ganglia elicited contractions of the trachealis that were blocked by the muscarinic receptor antagonist atropine. Following addition of atropine and contraction of the trachealis with prostaglandin F2 alpha (PGF2 alpha), vagus nerve stimulation elicited non‐adrenergic, non‐cholinergic relaxations. Both responses elicited by stimulation of the vagi were abolished by cutting the recurrent laryngeal nerves and were considered parasympathetic in nature as they were sensitive to the autonomic ganglion blockers trimetaphan and hexamethonium. 3. Experiments were designed in which ganglionic blockers were added to the buffer bathing the entire preparation or, alternatively, added only to the buffer perfusing the tracheal lumen. When given equal access to the trachea and oesophagus, hexamethonium was 56‐fold more potent an inhibitor of vagally mediated relaxations of the trachealis than vagally mediated contractions. Selective administration of hexamethonium to the buffer perfusing the tracheal lumen did not decrease the potency of the ganglionic blocker versus vagally mediated contractions. By contrast, even at a concentration of 1 mM, intratracheally administered hexamethonium failed to inhibit vagally mediated relaxations by 50%. Comparable results were obtained using trimetaphan. 4. Consistent with previous observations, removing the portion of the oesophagus contiguous with the region of the trachea at which isometric tension was measured abolished parasympathetic relaxations of the trachealis. Oesophagus removal was without effect on parasympathetic nerve‐induced contractions. Removing the dorsal half of the oesophagus or the mucosa and submucosa of the oesophagus did not affect the parasympathetic relaxant innervation. 5. The compound action potential of guinea‐pig recurrent laryngeal nerves evoked by vagus nerve stimulation consisted of three distinct peaks representing populations of axons with fast, intermediate and slow conduction velocities. The voltage‐response characteristics of vagally mediated contractions were identical to those of the compound action potential peak representing fibres with intermediate (10 m/s) conduction velocities. By contrast, the voltage‐response characteristics of the vagally mediated relaxations were best correlated with the compound action potential peak representing fibres with slow (0.4‐3 m/s) conduction velocities.(ABSTRACT TRUNCATED AT 400 WORDS)
M1 muscarinic receptors increase calcium current and phosphoinositide turnover in guinea‐pig ventricular cardiocytes.
Authors: Gallo M P , Alloatti G , Eva C , Oberto A , Levi R C
Published: December 2014

1. Physiological and molecular evidence for the presence and functional role of M1 muscarinic cholinergic receptors (mAChRs) in adult guinea‐pig ventricular cells is presented. 2. Whole‐cell clamp measurements of the L‐type calcium current (ICa) in isolated myocytes were performed. Caesium was used to suppress potassium currents. ICa was increased by the muscarinic agonist carbachol in cells pretreated with pertussis toxin which blocked the M2 mAChR‐triggered cascade of intracellular signalling, while it was not changed in untreated cells. 3. If the M2‐mediated regulation of ICa was blocked by directly saturating the cell with cyclic adenosine monophosphate (cAMP) through the patch pipette, application of carbachol induced a further small increase of the current above the level reached after cAMP perfusion. This increase was more pronounced in cells pretreated with pertussis toxin. 4. The carbachol‐induced increase of ICa was blocked by the selective M1 mAChR antagonist pirenzepine. 5. The application of high concentrations of carbachol increased the accumulation of [3H]inositol monophosphate up to 240% above control levels. This increase was reduced by application of pirenzepine. 6. The expression of M1 receptor mRNA in ventricular cardiocytes was shown by reverse transcriptase‐polymerase chain reaction. 7. These results suggest that M1 mAChR regulation of ICa can be a component of the paradoxical positive inotropism induced by high concentrations of muscarinic agonists.
Renal sympathetic responses to conflicting baroreceptor inputs: rapid ventricular pacing in dogs.
Authors: Halliwill J R , Minisi A J , Smith M L , Eckberg D L
Published: December 2014

1. Ventricular tachycardia generates complex changes in baroreceptor input to the central nervous system: arterial baroreceptors are unloaded while cardiopulmonary receptors are stimulated. In humans with heart diseases, muscle sympathetic nerve activity increases during ventricular tachycardia. This suggests that arterial baroreceptor‐mediated sympathoexcitation overrides cardiopulmonary receptor‐mediated sympathoinhibition. However, the relative roles of each reflex are difficult to determine in humans. 2. We measured efferent renal sympathetic neural responses to simulated ventricular tachycardia, to determine what pathophysiological mechanisms are invoked when inputs from different baroreceptive areas change in opposite directions. In alpha‐chloralose anaesthetized, mechanically ventilated dogs, we recorded the electrocardiogram, mean left atrial and arterial pressures and multifibre efferent renal sympathetic nerve activity (RSNA) during 1 min of right ventricular pacing at 214 beats min‐1. Pacing was repeated after either sinoaortic or vagal cardiopulmonary denervation and again after both sinoaortic and cardiopulmonary denervation. 3. With all afferent baroreceptor pathways intact, right ventricular pacing elicited transient sympathoinhibition (delta RSNA, ‐19 +/‐ 10%, mean +/‐ S.E.M.). After sinoaortic denervation (cardiopulmonary receptors intact), right ventricular pacing elicited abrupt and sustained sympathoinhibition (delta RSNA, ‐53 +/‐ 8%, P < 0.05 vs. intact). After vagal cardiopulmonary denervation (sinoaortic receptors intact), right ventricular pacing elicited abrupt and sustained sympathoexcitation (delta RSNA, + 56 +/‐ 19%, P < 0.05 vs. intact). After both sinoaortic and vagal cardiopulmonary denervation, right ventricular pacing elicited a gradual increase in sympathetic outflow (delta RSNA, + 16 +/‐ 6%, P < 0.05 vs. intact). 4. We conclude that interactions between vagal cardiopulmonary and arterial baroreflexes determine renal sympathetic outflow during simulated ventricular tachycardia. In healthy anaesthetized dogs, the balance of the two opposing reflexes is weighted towards vagal cardiopulmonary‐mediated sympathoinhibition.
Evidence for all‐or‐none regulation of neurotransmitter release: implications for long‐term potentiation.
Authors: Perkel D J , Nicoll R A
Published: December 2014

1. We have used the whole‐cell patch‐clamp recording technique to examine the modulation of dual‐component excitatory postsynaptic currents (EPSCs) in CA1 pyramidal cells in guinea‐pig hippocampal slices. 2. The dramatic difference in the reported sensitivities of the N‐methyl‐D‐aspartate (NMDA) and non‐NMDA glutamate receptors to glutamate suggests that changes in transmitter concentration in the synaptic cleft would result in differential modulation of the two components of the EPSC. 3. To test whether presynaptic manipulations change transmitter concentration in the synaptic cleft, pharmacological modulation of transmitter release by the GABAB agonist baclofen or by the adenosine antagonist theophylline was used. These manipulations resulted in parallel changes of NMDA and non‐NMDA receptor‐mediated components of EPSCs over a sixteen‐fold range. 4. Stimuli that induce long‐term potentiation (LTP) did not cause a sustained enhancement of isolated NMDA receptor‐mediated EPSCs evoked in the presence of the non‐NMDA receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX). 5. To compare directly the effect of LTP on the components of the EPSC, dual‐component EPSCs were elicited while the postsynaptic membrane potential was held at +30 mV. Induction of long‐term potentiation by delivering low‐frequency synaptic stimulation in conjunction with such depolarization led to differential enhancement of the non‐NMDA receptor‐mediated component of the EPSC. 6. These data support the notion that synaptic transmission at individual boutons occurs in an all‐or‐none fashion, without changing peak transmitter concentration in the synaptic cleft. Long‐term potentiation could occur through a postsynaptic modification of receptors or through a presynaptic change involving increased transmitter concentration in the synaptic cleft, but is difficult to explain by a generalized increase in release probability.
A link between adenosine, ATP‐sensitive K+ channels, potassium and muscle vasodilatation in the rat in systemic hypoxia.
Authors: Marshall J M , Thomas T , Turner L
Published: December 2014

1. In anaesthetized rats, systemic hypoxia evoked hyperventilation, tachycardia, a fall in arterial pressure, vasodilatation in skeletal muscle and increases in K+ concentration measured in arterial plasma ([K+]a), venous efflux from muscle ([K+]v) and in right atrial plasma ([K+]at). The ATP‐sensitive potassium (K+ATP) channel inhibitor glibenclamide (10 or 20 mg kg‐1 i.v.) reduced the muscle vasodilatation and increase in [K+]v, but had no significant effect on the other changes. 2. The adenosine receptor antagonist, 8‐phenyltheophylline (8‐PT, 10 mg kg‐1 i.v.) had similar effects to glibenclamide. 3. Glibenclamide reduced the muscle vasodilatation evoked by the adenosine analogue, 2‐chloroadenosine given i.v. (30 micrograms kg‐1). 4. Infusion of adenosine (0.3 mg kg‐1 min‐1 for 5 min) into the hindlimb evoked muscle vasodilatation and an increase in [K+]v, both of which were abolished by 8‐PT. 5. We propose that during systemic hypoxia, part of the muscle vasodilatation that can be attributed to adenosine is due to the action of K+, which is released from skeletal muscle fibres through glibenclamide‐sensitive K+ channels (possibly K+ATP channels) that are activated by adenosine. This may be a general mechanism for the vasodilator influence of adenosine.
Parasympathetic non‐adrenergic, non‐cholinergic mechanisms in reflex secretion of parotid acinar granules in conscious rats.
Authors: Ekström J , Helander H F , Tobin G
Published: December 2014

1. Female adult rats were subjected to sympathetic denervation of the parotid glands by bilateral removal of the superior cervical ganglion 10‐12 days before acute experiments. The sympathectomy was in some of the experimental groups combined with either bilateral adrenal medullectomy, treatment with the sensory neurotoxin capsaicin or parasympathetic denervation of the gland by cutting the auriculotemporal nerve. 2. Food but not water was withheld for 29‐32 h before acute experiments. All animals were given an intraperitoneal injection of phentolamine (2 mg kg‐1) and propranolol (1 mg kg‐1) and, when appropriate, also atropine (1 mg kg‐1). Then the experimental animals were fed their ordinary food of hard chow for 60‐90 min. Thereafter, these animals and their non‐fed controls were killed, and the parotid glands were removed and used for either morphometric assessment or measurement of amylase activity. 3. In the atropinized rats subjected to sympathectomy alone, eating reduced the numerical density of acinar secretory granules by 50% and the total activity of amylase by 55%; the corresponding figures were, when sympathectomy was combined with adrenal medullectomy, 51 and 63%. Also, in atropinized animals subjected to sympathectomy and capsaicin pretreatment, eating reduced the numerical density of acinar granules and the total amylase activity, in this case by 45 and 35%, respectively. 4. In the atropinized rats subjected to sympathectomy and parasympathectomy, eating caused no change in the numerical density of acinar granules but reduced the total amylase activity by 35%. 5. In the non‐atropinized rats subjected to sympathectomy alone, eating reduced the numerical density of acinar granules by 22%, while there was no change in the total amylase activity. 6. In conclusion, eating evoked a reflex activation of the sympathectomized parotid gland that engaged non‐adrenergic non‐cholinergic receptors of the acinar cells. The present results give weight to a physiological role for non‐adrenergic non‐cholinergic parasympathetic mechanisms in salivary secretion under reflex conditions.
Role of sodium‐calcium exchange in activation of contraction in rat ventricle.
Authors: Bouchard R A , Clark R B , Giles W R
Published: December 2014

1. The functional role of reverse Na(+)‐Ca2+ exchange in the activation of contraction of rat ventricular myocytes has been studied. Mechanical activity of single cells, measured as unloaded cell shortening, was recorded simultaneously with membrane current and voltage using a single microelectrode voltage clamp and a video edge detection device. 2. The voltage dependence of contraction was studied by applying trains of depolarizations. At test potentials between +20 and +80 mV (under conditions where large outward currents were activated) a plateau on the shortening vs. voltage (S‐V) relationship was observed. Significant cell shortening also occurred at test potentials between ‐70 and ‐40 mV; and these contractions were accompanied by large inward Na+ currents. We have investigated the ionic mechanisms for three components of the S‐V relation in rat ventricle: (i) shortening which occurs between ‐70 and ‐40 mV and is thought to be dependent on the sodium current; (ii) phasic contractions in the voltage range ‐40 to +40 mV where the L‐type Ca2+ current is present; (iii) the plateau of the S‐V relation at strongly depolarized voltages where reverse Na(+)‐Ca2+ exchange may occur. 3. Experiments in which two independent microelectrode impalements were made in a single myocyte showed that during activation of contraction at test potentials between ‐70 and ‐40 mV, and during very large depolarizations (+20 to +80 mV), there were significant deviations of the measured membrane potential from the applied voltages. Activation of cell shortening in these voltage ranges could be eliminated by electronic series resistance compensation, which significantly reduced these voltage errors. Consistent with these findings, when tetrodotoxin (TTX) and 4‐aminopyridine (4‐AP) were used to block inward Na+ and transient outward K+ currents, respectively, no significant voltage errors were present and a bell‐shaped shortening‐voltage (S‐V) relationship was obtained. 4. When Na+ and K+ currents were blocked, depolarizations from holding potentials of either ‐80 or ‐50 mV demonstrated that the threshold for activation of contraction was about ‐30 mV, and that the voltage dependence of peak shortening was very similar to that of the L‐type Ca2+ current (ICa,L). These contractions were suppressed completely by either Cd2+ or ryanodine, showing that activation of cell shortening was due to Ca2+ influx through L‐type channels which induced release of Ca2+ from the sarcoplasmic reticulum (SR). No T‐type calcium currents were observed.(ABSTRACT TRUNCATED AT 400 WORDS)
Control of frequency during swimming in Xenopus embryos: a study on interneuronal recruitment in a spinal rhythm generator.
Authors: Sillar K T , Roberts A
Published: December 2014

1. In Xenopus embryos, the frequency of natural and fictive swimming usually drops slowly as swimming continues but can increase following stimulation of the skin or dimming of the illumination. We have investigated whether such increases are associated with an increase in the number of neurones active at higher frequencies. 2. Recordings from ventral presumed motoneurones show that these were reliably active at all swimming frequencies. 3. Recordings from more dorsal presumed interneurones showed that in the majority of these firing probability decreased as a function of swimming frequency. Dye‐filled microelectrodes were used to show that some of these neurones had the anatomy of known classes of excitatory and inhibitory premotor interneurones. 4. If skin stimulation is given at appropriate phases of the swimming cycle, it can lead to a transient increase in frequency. Recordings from silent premotor interneurones during such stimulation show that they can be recruited to fire during the post‐stimulus frequency increases. 5. It was possible that spike failure in the interneurones could have been due to damage by the recording microelectrodes. We therefore measured the amplitudes and probability of occurrence of rhythmic ‘on‐cycle’ IPSPs which occur in sensory interneurones and ‘on‐cycle’ IPSPs which sometimes occur in motoneurones during fictive swimming. Both decreased in amplitude and could fail as frequency dropped, providing further evidence that the number of inhibitory interneurones firing on each cycle of swimming is a function of frequency. 6. We conclude that premotor rhythm‐generating interneurones are not active on all cycles of swimming and that their probability of firing action potentials increases with swimming frequency. This suggests that swimming frequency is determined in part by the number of premotor interneurones which are active.
The spatial distribution of calcium signals in squid presynaptic terminals.
Authors: Smith S J , Buchanan J , Osses L R , Charlton M P , Augustine G J
Published: December 2014

1. The fluorescent Ca2+ indicator dye, fura‐2, was used to examine the spatial distribution of intracellular Ca2+ signals in giant presynaptic terminals of squid. Brief trains of presynaptic action potentials were evoked to open Ca2+ channels within the giant presynaptic terminals and elevate presynaptic Ca2+ concentration. 2. Electrical stimulation produced pronounced rises in presynaptic Ca2+ concentration. These rises were much larger in the terminal region than in the adjacent axonal region of the presynaptic neuron, suggesting that Ca2+ channels are most abundant in the terminal. 3. Stimulation also produced gradients in Ca2+ concentration across the width of the presynaptic terminal. During stimulation, Ca2+ concentration was highest in the compartment of the presynaptic terminal closest to the postsynaptic neuron. This suggests that the Ca2+ channels are localized to this region of the presynaptic terminal. 4. Following the end of action potential trains, the rises in Ca2+ concentration became uniform across the width of the terminal. The redistribution of Ca2+ presumably is due to diffusion of Ca2+ throughout the presynaptic cytoplasm. Stimulus‐evoked rises in Ca2+ declined slowly over several tens of seconds. 5. Histological examination of a giant presynaptic terminal used for imaging experiments revealed that the spatial compartments where stimulus‐induced rises in Ca2+ concentration were highest were also enriched in active zones, the presynaptic sites of transmitter secretion. The co‐localization of Ca2+ transients and active zones strongly suggests that neurons cluster Ca2+ channels selectively at active zones and that they do so to enhance the magnitude of Ca2+ signals in the vicinity of the active zone. 6. Longitudinal gradients in Ca2+ concentration also occur within presynaptic terminals and can be quantitatively accounted for by gradients in surface/volume ratio and density of active zones along the length of the presynaptic terminal.
Reduction of resting K+ current by metabotropic glutamate and muscarinic receptors in rat CA3 cells: mediation by G‐proteins.
Authors: Guérineau N C , Gähwiler B H , Gerber U
Published: December 2014

1. Effects of 1S,3R‐1‐aminocyclopentane‐1,3‐dicarboxylate (1S,3R‐ACPD) acting at metabotropic glutamate receptors (mGluRs), and methacholine (MCh), acting at cholinergic muscarinic receptors, were investigated in CA3 neurones in hippocampal slice cultures using the patch‐clamp technique. 2. Both 1S,3R‐ACPD (10 microM) and MCh (0.5 microM) activated an inward current associated with a decrease in membrane conductance. The current was observed when the slow calcium‐dependent after‐hyperpolarizing current (IAHP) and the voltage‐dependent current (IM) were not activated, reversed close to the reversal potential for K+ (EK) (Erev = ‐92.8 +/‐ 10.7 and ‐89.2 +/‐ 8.6 mV for 1S,3R‐ACPD and MCh, respectively), varied linearly with membrane potential, and thus corresponds to a leak K+ current. 3. The decrease in K+ conductance elicited with 1S,3R‐ACPD (50 microM) was substantially reduced (> 70%) with bath application of (RS)‐alpha‐methyl‐4‐carboxyphenylglycine (MCPG, 1 mM), a selective mGluR antagonist and was not mimicked by the enantiomer 1R,3S‐ACPD (100 microM). 4. The effects of 1S,3R‐ACPD and MCh were mediated by activation of G‐proteins since no inward current could be elicited in GDP beta S‐loaded cells (500 microM). When cells were dialysed with GTP (100 microM) or GTP gamma S (250 microM), however, the amplitude of the current was significantly enhanced. 5. These findings provide evidence that G‐proteins couple the activation of mGluRs and muscarinic receptors to a decrease in leak K+ conductance.
The mechanism of zinc uptake by cultured rat liver cells.
Authors: Taylor J A , Simons T J
Published: December 2014

1. The initial rate of 65Zn uptake into cultured rat hepatocytes has been measured over a range of Zn2+ concentrations from 3 x 10(‐10) M to 5 x 10(‐6) M. Histidine and albumin were used to buffer Zn2+ ions at concentrations below 1 x 10(‐6) M. 2. The results suggest there are two mechanisms for Zn2+ uptake; a high‐affinity, saturable pathway, with a maximum velocity (Vmax) of 20‐30 pmol (mg protein)‐1 min‐1 and a Michaelis‐Menten constant (Km) of about 2 x 10(‐9) M Zn2+ (with histidine), and a low‐affinity, linear pathway, that only makes a significant contribution to Zn2+ uptake at Zn2+ concentrations above 1 x 10(‐6) M. 3. Transport via the high‐affinity pathway is dependent on the concentration of Zn2+ ions and not on the concentrations of Zn(2+)‐ligand complexes, suggesting that Zn2+ is the transported species. 4. The affinity of the saturable pathway for Zn2+ is slightly lower in the presence of albumin, with a Km of about 1.3 x 10(‐8) M. The reason for this is uncertain.
The metabolic consequences of an increase in the frequency of stimulation in isolated ferret hearts.
Authors: Elliott A C , Smith G L , Allen D G
Published: December 2014

1. The metabolic consequences of an increase in the frequency of stimulation were examined in isolated ferret hearts. Intracellular pH (pHi) and the intracellular concentrations of phosphocreatine ([PCr]i), inorganic phosphate ([Pi]i) and ATP were measured by 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Increasing the stimulus rate from 0.1‐0.7 to 2 Hz caused an increase in [Pi]i and a decrease recovery of both [PCr]i and [Pi]i during continued stimulation. There was no change in [ATP]i during stimulation at 2 Hz. Increasing the stimulus rate caused an intracellular acidosis of around 0.1 pH units. 3. Increasing the stimulus rate generally caused an initial increase in developed pressure, followed by a decrease over 1‐2 min to a steady level slightly lower than developed pressure at the low (control) stimulus rate. The increase in stimulus rate caused a 4‐ to 6‐fold increase in time‐averaged muscle activity. 4. Both oxygen uptake and production of lactate increased on 2 Hz stimulation. Lactate production accounted for less than 5% of ATP production at low or high stimulus rates, suggesting that significant anoxia was not occurring during stimulation. The observed lactate production was, however, sufficient to explain most of the intracellular acidosis observed when the stimulus rate was raised. When glycolysis was prevented by removal of glucose and depletion of glycogen stores, 2 Hz stimulation was accompanied by an intracellular alkalosis rather than an acidosis, suggesting that lactate production by glycolysis was the cause of the intracellular acidosis. 5. Reducing the rate of glycolysis increased the size of changes in [PCr]i and [Pi]i evoked by stimulation at 2 Hz. Furthermore, there was now no partial reversal of the changes in [PCr]i and [Pi]i during 2 Hz stimulation. 6. When oxidative phosphorylation was inhibited by replacing O2 with N2, increasing the rate of stimulation from 0.1‐0.7 to 1‐2 Hz caused an initial increase followed by a large fall in developed pressure, which declined to a level well below that at the control stimulus rate. The increase in stimulus rate was accompanied by a large fall in [PCr]i, an increase in [Pi]i, and an intracellular acidosis of 0.1‐0.3 pH units. The fall in developed pressure was consistent with the known effects of the rise in [Pi]i and the fall in pHi on the contractile apparatus.
Gating, selectivity and blockage of single channels activated by cyclic GMP in retinal rods of the tiger salamander.
Authors: Sesti F , Straforini M , Lamb T D , Torre V
Published: December 2014

1. Patches in the inside‐out configuration were excised from the membrane of outer and inner segments of the larval tiger salamander, Ambystoma tigrinum. The current flowing through single channels opened by cyclic GMP was studied with the voltage clamp technique. 2. Amplitude histograms of current recordings from patches containing only one flickering channel, excised from the inner segment and in the presence of 100 microM cyclic GMP, could be fitted by a theoretical scheme in which the single channel conductance was at least 55 pS at +40 mV and at least 45 pS at ‐40 mV. The mean open time was no longer than the time constant of our recording system, about 35 microseconds. Similar results were obtained by analysis of the amplitude histograms of patches from the outer segment containing many channels, and in the presence of 1‐5 microM cyclic GMP. 3. In membrane patches excised from the outer segment, reducing the temperature from 24 to 8 degrees C did not reduce the flickering, but changed the amplitude histograms of current fluctuations activated by 1 microM cyclic GMP in a way consistent with a decrease of 50% in the single channel conductance and a decrease of 50% in the open probability. 4. In the presence of 1 microM cyclic GMP at +60 mV, when Na+ was replaced by NH4+ or K+, brief outward current transients flowing through single channels were observed. When Na+ was replaced with Li+, Rb+ or Cs+, current transients were very small. 5. The shape of the power spectrum of current fluctuations induced by 1 microM cyclic GMP at +60 mV did not change when the permeating ion was Na+, K+ or NH4+. Analysis of the amplitude histogram did not show any effect of the tested monovalent cations on the open probability or on channel gating. At +60 mV, the estimated single channel currents were at least 4, 2.8 and 2 pA for NH4+, Na+ and K+ respectively. 6. The addition of 0.5 or 1 mM Ca2+ to the medium bathing the cytoplasmic side of the membrane greatly reduced the frequency of openings, but single channel activity could still be observed. The blocking effect of 1 mM Ca2+ on the channel activity induced by 2 microM cyclic GMP could be counterbalanced by increasing the cyclic GMP concentration. The addition of 0.5 or 1 mM Ca2+ did not change the shape of power spectra obtained at membrane voltages between ‐100 and +100 mV.(ABSTRACT TRUNCATED AT 400 WORDS)
A comparison of non‐NMDA receptor channels in type‐2 astrocytes and granule cells from rat cerebellum.
Authors: Wyllie D J , Cull‐Candy S G
Published: December 2014

1. Patch‐clamp recording methods have been used to compare the pharmacological properties and single‐channel characteristics of non‐NMDA receptor channels in cerebellar type‐2 astrocytes and granule cells. 2. In type‐2 astrocytes whole‐cell concentration‐response curves for glutamate, quisqualate, AMPA and kainate gave EC50 values of 5.8, 3.8, 7.6 and 160 microM and Hill slopes of 1.65, 1.18, 1.64 and 1.65, respectively, resembling estimates for granule cell receptors. 3. The non‐NMDA receptor antagonists CNQX and diCl‐HQC (see Methods) inhibited whole‐cell kainate currents in both cell types. The IC50 for CNQX antagonism of the kainate response was 536 nM in type‐2 astrocytes, and 500 nM in granule cells. The IC50 for diCl‐HQC was 3.5 microM in astrocytes and 3.7 microM in granule cells. 4. CNQX acted as a competitive antagonist of whole‐cell kainate responses in type‐2 astrocytes and granule cells giving Schild plots with a slope near 1. The equilibrium constant, K, for CNQX binding was 524 nM in astrocytes and 489 nM in granule cells. 5. Quisqualate and AMPA responses showed rapid desensitization in type‐2 astrocytes with a ratio of steady‐state to peak response of 0.09. Concanavalin A reduced this desensitization. 6. Non‐NMDA channels in type‐2 astrocytes and granule cells showed a low permeability to Ca2+ ions with a reversal potential, for kainate‐activated whole‐cell currents in isotonic Ca2+, of approximately ‐25 mV for astrocytes and ‐45 mV for granule cells. 7. Outside‐out patches from type‐2 astrocytes exhibited a range of single‐channel conductances that were superficially similar to the glutamate‐activated conductances in granule cells. However, the type‐2 astrocytes were devoid of NMDA receptors, hence all of these conductances originated from non‐NMDA channels. Their slope conductances were approximately 11, 21, 32, 42 and 52 pS. Amplitudes were verified with mean low‐variance plots and single‐channel current‐voltage curves, which were linear. 8. There was also evidence of lower conductance kainate‐activated channels in astrocyte patches. From noise analysis their estimated mean conductance was 1.9 pS, as described for the ‘low‐conductance’ type kainate responses in cerebellar neurones. 9. Apparent open times, shut times and burst lengths of AMPA‐activated (3‐10 microM) channels were examined in patches from type‐2 astrocytes, and kinetic properties of the 40 and 50 pS levels were compared with the lower levels. 10. Our results indicate some marked pharmacological similarities between non‐NMDA receptor channels in type‐2 astrocytes and granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)
Force responses to rapid length changes in single intact cells from frog heart.
Authors: Colomo F , Poggesi C , Tesi C
Published: December 2014

1. Force transients in response to step perturbations in length were recorded in intact atrial cells from frog heart at various temperatures (6‐15 degrees C). Length changes of various sizes and in either direction, complete in 0.5 ms, were applied to single myocytes near slack length (initial sarcomere length 2.1‐2.2 microns) just before the peak of an isometric twitch. The frequency response of the force transducers used was 2‐4 kHz in Ringer solution. 2. An early quick force recovery phase was clearly observed after the elastic force response to the length step and before the start of much slower recovery processes. The quick recovery phase became progressively faster with larger shortening steps and was almost as fast as that originally described in intact frog skeletal muscle fibres (rate constants above 1000 s‐1 in large releases at 10 degrees C). 3. The force‐extension relation determined at the end of the length change (T1 curve) indicates that an instantaneous shortening of 0.5‐0.6% of the initial cell length (L0) brings the force to zero. The force‐‐extension relation determined at the end of the quick recovery phase (T2 curve) showed that the early recovery leads to an almost complete restoration of the original force with small stretches and releases (up to 0.3% L0) and that it becomes negligible in shortening steps of about 1.4% L0. 4. The results suggest that the mechanical properties of attached cross‐bridges and the rate of transitions between attached cross‐bridge states are approximately the same in frog atrial cells and fast skeletal muscle fibres.(ABSTRACT TRUNCATED AT 250 WORDS)
Evidence for corticospinal excitation of presumed propriospinal neurones in man.
Authors: Gracies J M , Meunier S , Pierrot‐Deseilligny E
Published: December 2014

1. The possibility that stimulation of the motor cortex facilitates transmission in the pathway mediating non‐monosynaptic (‘propriospinal’) excitation from low‐threshold afferents to upper limb motoneurones was investigated. 2. Convergence between peripheral afferent volleys (from the ulnar or musculo‐cutaneous nerve) and corticospinal volleys (evoked by magnetic stimulation of the motor cortex) was investigated using the spatial facilitation technique. Thus the effects of these volleys on the flexor carpi radialis H reflex were compared when applied separately and together. When cortical stimulation was optimal for the muscle from which the conditioning volley originated the facilitation of the reflex on combined stimulation was significantly larger than the algebraic sum of the effects of separate stimuli. 3. The extra facilitation on combined stimulation had all the characteristics of ‘propriospinal’ excitation (low threshold, long central delay, brief duration and depression when the afferent input was increased), and it is suggested that this reflects corticospinal excitation of ‘propriospinal’ neurones. 4. When varying the time interval between cortical and test stimulations, it was shown that extra facilitation on combined stimulation began 1 ms later than the onset of the control reflex facilitation. Assuming that the latter onset reflects the arrival of the monosynaptic corticospinal volley at the motoneurone pool, this 1 ms delay suggests a disynaptic pathway for the cortical excitation of motoneurones through ‘propriospinal’ neurones. 5. As at the onset of voluntary movement, the pattern of the cortical excitation of ‘propriospinal’ neurones was quite specific: extra facilitation of the reflex on combined stimulation only occurred when the cortical volley was preferentially directed to motoneurones supplying the muscle from which the afferents used for the peripheral volley originated. 6. It is concluded that corticospinal axons activate human ‘propriospinal’ neurones and thereby produce disynaptic excitation of the motoneurone pool. Given temporal summation with the monosynaptic excitation, this ‘propriospinally mediated’ disynaptic excitation might make a significant contribution to the evoked EMG potential.