bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of ionic currents in the isolated vestibular hair cell of the chick.
Authors: Ohmori H
Published: December 2014

The ionic currents in enzymatically isolated vestibular hair cells of the chick were studied by a whole‐cell‐clamp variation of the patch voltage clamp, and by single channel recording. At membrane potentials more negative than ‐80 mV the hair cell showed anomalous rectification, and at potentials more positive than ‐40 mV large outward K currents were observed in normal saline. The outward K current decreased at large positive potentials, showing an N‐shaped I‐V relation. The outward K current was carried mostly through the Ca‐activated K channel. K currents through the anomalous rectifier channel showed a decay in normal saline. This decay was eliminated reversibly in Na‐free saline when the isotonic KCl‐EGTA solution was used as the internal medium. However, a fast decay was still observed in Na‐free high‐K external solution when isotonic CaCl‐EGTA was used as the internal medium. An increase in [K]o decreased the decay rate of the inward K current. The single‐channel conductance of the anomalous rectifier channel was 50 pS in 160 mM‐K saline and 23 pS in 40 mM‐K saline. In 100 mM‐Ca, ‐Sr and ‐Ba salines a large inward current was observed. At positive potentials the inward current carried by Ca and Sr ions showed significant decay; the current became outward at large positive potentials. Since the decay of the inward current was eliminated when 100 microM‐quinine was added to the bathing medium, it was probably due to the activation of some Ca‐activated K conductance which remained even with isotonic CsCl‐EGTA internal medium. The activation kinetics of the Ca channel were studied in 100 mM‐Ba solution at low temperatures (9‐13 degrees C). From a comparison of the time constants of activation with the time constants of the tail currents, it was concluded that the Ca channel follows Hodgkin‐Huxley‐type m2 kinetics. A slow component that deviated from m2 kinetics was frequently observed at relatively large positive potentials. The steady‐state fluctuations of Ba current showed a power density spectrum reasonably well fitted by a sum of two Lorentzian functions. The spectrum has a low‐frequency component which indicates kinetics close to the macroscopic activation process of the Ca channel and a high‐frequency component that indicates very fast flickering kinetics operating in the Ca channel.
Characteristics of a cationic amino acid transport system in the basolateral membrane of the cat salivary epithelium.
Authors: Mann G E , Wilson S M , Yudilevich D L
Published: December 2014

The specificity and kinetics of L‐lysine influx across the basolateral surface of the cat salivary epithelium have been investigated in the perfused cat submandibular gland using a high‐resolution, paired‐tracer dilution technique. L‐lysine influx was measured at several different perfusate concentrations (0.05‐2.5 mM) and was found to be saturable. A Michaelis‐Menten analysis based on a single entry site gave a Km of 0.49 +/‐ 0.08 mM and a Vmax of 231 +/‐ 20 nmol/min X g. The uptake of L‐lysine was highly stereospecific and markedly inhibited by L‐arginine (0.25‐2.5 mM). The inhibitor constant (Ki) was 0.23 mM, suggesting that the carrier had a greater affinity for L‐arginine than L‐lysine. When the inhibitory effects of L‐histidine (0.5‐10 mM) were examined the Ki, estimated at 10 mM, was 4.6 mM. Nine other neutral amino acids (L‐alanine, L‐serine, L‐cysteine, glycine, L‐proline, L‐homoserine, L‐leucine, L‐phenylalanine and L‐glutamine), and an acidic amino acid (L‐aspartate) were also tested at 10 mM and, although several caused inhibition, the Ki was always at least 20 times higher than the measured Km for L‐lysine. It is concluded the carrier is highly specific for the L‐form of the basic amino acids. The sodium dependence of L‐lysine influx was investigated over a range of L‐lysine concentrations (0.05‐1 mM), and total removal of sodium from the perfusate had no effect on L‐lysine influx. In the presence of sodium, L‐homoserine, an amino acid not normally present in animal tissues, inhibited L‐lysine influx (Ki = 13 mM). This inhibition was not observed in the absence of sodium, and contrasts with the observation that the inhibitory action of L‐histidine was sodium independent. The present data suggest that a specific cationic amino acid transport system is operative in the basolateral membrane of the cat salivary epithelium. The properties of this system appear to be similar to the system y+ which has been described in several other cell types.
Saturation of calcium channels and surface charge effects in skeletal muscle fibres of the frog.
Authors: Cota G , Stefani E
Published: December 2014

Voltage‐clamp and current‐clamp experiments were performed to study Ca2+ and Ba2+ permeation through Ca channels in intact twitch skeletal muscle fibres of the frog. Surface charge effects were taken into consideration. Ca2+ (ICa) or Ba2+ (IBa) currents, or Ca2+ and Ba2+ action potentials were recorded in the presence of external tetraethylammonium (TEA+) ions and by replacing C1‐ for CH3SO3‐. To further block K+ outward currents, muscles were incubated in a K+‐free, TEA+ and Cs+‐containing solution prior to experiments. When 10 mM‐Ca2+ was replaced by 10 mM‐Ba2+, the I/V curve for the peak inward current shifted by 15‐20 mV to more negative potentials and the maximal peak inward current increased from ‐39 +/‐ 2 mA cm‐3 (5) to ‐51 +/‐ 3 mA cm‐3 (7). The decay of ICa and IBa followed a simple exponential time course and became faster for large depolarizations. The overshoot of the action potentials changed 29 +/‐ 3 mV or 32 +/‐ 3 mV for a 10‐fold change in the Ca2+ or Ba2+ concentrations respectively. Ca2+ action potentials were 15‐20 mV larger than Ba2+ action potentials. The maximum rate of rise Vmax and the Ca2+ or Ba2+ conductance GC2+ during the plateau tend to saturate as divalent cation concentration was increased. The Michaelis constant (Km) values obtained were respectively: 5.6 and 6.0 mM for Ca2+ and 12.5 and 8.0 mM for Ba2+. When Ca2+ or Ba2+ concentrations were increased, the effective threshold of the inward current Theff and the membrane potential E* at Vmax shifted to more positive potentials along the voltage axis. These shifts were similar for Theff and E* and were more pronounced for Ca2+ than for Ba2+. Voltage shifts could be adequately quantified by the Gouy‐Chapman theory with a density of surface charges near Ca channels of 0.20 e nm‐2 and including a specific binding constant for Ca2+ of 45 +/‐ 4 m‐1. The fractional increase of the Ca2+ and Ba2+ calculated concentrations at the membrane surface near the channel was smaller than the corresponding one in the bulk solution. This partially explained the reported saturation. Saturation was still present in the Vmax of GC2+ curves corrected for surface concentration. The corrected Km values for the Vmax data were 60 mM for Ca2+ and 350 mM for Ba2+.(ABSTRACT TRUNCATED AT 400 WORDS)
Effects of theophylline and dibutyryl cyclic adenosine monophosphate on the membrane potential of mouse pancreatic beta‐cells.
Authors: Henquin J C , Meissner H P
Published: December 2014

The effects of theophylline and dibutyryl cyclic AMP on the membrane potential of mouse beta‐cells were studied with micro‐electrodes. They were compared to their effects on insulin release by perifused mouse islets. In 3 mM‐glucose, theophylline (10 mM) depolarized the beta‐cell membrane and stimulated insulin release, but did not induce electrical activity. Dibutyryl cyclic AMP (1 mM) was without effect. In 7 mM‐glucose, theophylline (0.5‐2 mM) and dibutyryl cyclic AMP (1 mM) slightly depolarized the beta‐cell membrane, induced electrical activity in otherwise silent cells and increased insulin release. A higher concentration of theophylline (10 mM) hyperpolarized the beta‐cell membrane, did not induce electrical activity, but also stimulated insulin release. In 10 mM‐glucose, the membrane potential of beta‐cells exhibited repetitive slow waves with bursts of spikes on the plateau. Under steady state, these slow waves were differently affected by low or high concentrations of theophylline. At 0.5‐2 mM, theophylline shortened the intervals, lengthened the slow waves and slightly increased their frequency. On the other hand, 10 mM‐theophylline markedly decreased the duration of both intervals and slow waves, and increased their frequency. The effects of 1 mM‐dibutyryl cyclic AMP were similar to those of 2 mM‐theophylline. With 2‐10 mM‐theophylline, two other effects were also observed: a transient hyperpolarization with suppression of electrical activity immediately after addition of the methylxanthine and an increase in electrical activity upon its withdrawal. Theophylline and dibutyryl cyclic AMP markedly potentiated insulin release induced by 10 mM‐glucose. The magnitude of these changes did not correlate well with the importance of the changes in electrical activity. However, with 2‐10 mM‐theophylline the increase in release was also preceded by an initial transient inhibition, whereas withdrawal of the methylxanthine was accompanied by a further increase. When Ca influx was inhibited by D600, the slow waves were suppressed, the membrane was depolarized to the plateau level and only few spikes were present. Although theophylline markedly increased insulin release under these conditions, it did not affect the membrane potential. Several conclusions can be drawn from this study. Insulin release and electrical activity in beta‐cells can be dissociated when intracellular Ca is used to trigger exocytosis. High concentrations of theophylline produce effects unrelated to cyclic AMP.(ABSTRACT TRUNCATED AT 400 WORDS)
Effects of fast and slow patterns of tonic long‐term stimulation on contractile properties of fast muscle in the cat.
Authors: Eerbeek O , Kernell D , Verhey B A
Published: December 2014

Different physiological rates of 'tonic' long‐term electrical stimulation (rates 5‐40 Hz; activity greater than or equal to 50% total time) were delivered to the left‐side common peroneal nerve of the cat hind limb. The duration of treatment was 8 weeks, and the animals had previously been subjected to a left‐side hemispinalization and dorsal rhizotomy. In the absence of stimulation, these operations had no slowing or weakening effects on peroneal muscle contraction. The minimum two‐pulse interval that gave a summation of tension (neuromuscular refractory period) was longer for stimulated than for non‐stimulated muscles. Twitches of chronically stimulated muscles had become prolonged by more than 100%. Corresponding changes were found in the tension‐frequency relation and in the 'sag'‐behaviour of the stimulated muscles. There were no differences between the 'fast' (20 or 40 Hz pulse rates) and the 'slow' (5 or 10 Hz pulse rates) patterns of tonic stimulation with respect to their effects on speed‐related muscle properties. Furthermore, during the period of chronic stimulation, the prolongation of twitch contraction time occurred along the same time course for the fast and slow patterns of tonic treatment. All chronically stimulated muscles had become weaker than normal. In comparison to the slow patterns, the present fast patterns of long‐term activation caused (1) a smaller amount of decline in maximum muscle force, (2) a smaller twitch: tetanus ratio, and (3) the retention of a normal amount of post‐tetanic potentiation of twitch size (decreased by the slow patterns). When tested by a series of 40 Hz bursts, force was better maintained in chronically stimulated muscles than in normal ones. These effects on fatigue resistance were the same for the fast and slow patterns of long‐term activation. In peroneus longus muscles contralateral to the side of chronic activation, an evident impairment had commonly occurred in the capability to maintain force during tetani at the high rates needed for a maximum tetanic contraction. The results are discussed in relation to problems concerning the long‐term effects of motoneuronal activity patterns on the contractile properties of their muscle units.
Increased sodium conductance in the synaptic region of rat skeletal muscle fibres.
Authors: Betz W J , Caldwell J H , Kinnamon S C
Published: December 2014

Differences in sodium conductance between end‐plate and extrajunctional regions of rat lumbrical muscle fibres were measured by comparing action potential maximum rate of rise (Vmax) in the two regions and by using a vibrating micro‐electrode to record steady inward current produced by application of veratridine. In normal Krebs solution, action potential Vmax was significantly greater (by 43%) in the end‐plate region than in extrajunctional regions of the fibres. When chloride conductance was greatly reduced by bathing muscles in solutions with low chloride concentration, Vmax was still significantly higher (by 28%) in the end‐plate region than in extrajunctional regions. The increased Vmax could be recorded only within a distance of about 150‐200 microns of the end‐plate. Steady inward current was recorded with a vibrating micro‐electrode at the end‐plate in response to veratridine; the current persisted when veratridine was introduced in low‐chloride Krebs solution, and it was rapidly reversed by tetrodotoxin. The current reflected a 5 mV difference in membrane potential between the end‐plate region and extrajunctional regions. The results suggest that sodium conductance is increased in the synaptic region relative to extrajunctional regions of the fibres.
Self‐generated electrical currents through Xenopus neurulae.
Authors: Robinson K R , Stump R F
Published: December 2014

An extracellular vibrating electrode has been used to investigate epithelial current production by Xenopus neurulae. Xenopus neurulae, in 5% DeBoer (DB) solution, stages 14‐22, generated an endogenous current which left the blastopores of the embryos, and entered all other locations. The inward current declined near the mid line of the neural groove in stage 15 embryos. The inward current was abolished or drastically reduced by exchange of 5% DB with Na+‐free (choline‐ or bis(2‐hydroxyethyl)dimethylammonium (BDAC)‐substituted) 5% DB, 5.5 mM‐KCl, 1 mM‐verapamil in 5% DB, 50 or 500 microM‐ouabain in 5% DB, or 1 microM‐amiloride in 5% DB. Verapamil (10 microM) in 5% DB, 5.5 mM‐NaCl and 1 mM‐CoCl2 in 5% DB had little or no effect on the magnitude observed. The current also was present in 5% DB containing Li+ instead of Na+; current carried by Li+ was abolished by amiloride. This is consistent with a Na+ current, similar to that of short‐circuited adult frog skin.
The effect of leakage on micro‐electrode measurements of intracellular sodium activity in crab muscle fibres.
Authors: Taylor P S , Thomas R C
Published: December 2014

The effect of lowering extracellular Na (Nao) on the intracellular Na activity has been measured in single muscle fibres from the crab Carcinus maenas using Na+‐sensitive glass micro‐electrodes. Measurements have been made with recessed‐tip micro‐electrodes inserted radially into intact fibres, and with axial electrodes in cannulated fibres. Reducing Nao to one‐tenth normal caused local contractions in intact fibres. The apparent steady‐state internal Na (Nai) and the fall in Nai when Nao was reduced were found to vary considerably not only between different fibres but also when measured with different electrodes in the same fibre. The steady‐state Nai, and the extent and rate of its decrease when Nao was reduced, could be reduced by pushing the Na+‐sensitive electrode deeper into the fibre. Cannulated fibres generally had higher internal Na activities than intact fibres, but at comparable levels of Nai the rate of fall recorded from cannulated fibres when Nao was reduced was much slower than with intact fibres. In both intact and cannulated fibres the decrease in Nai was reduced by ouabain. The level of Nai recorded in cannulated fibres was sensitive to depolarization. Depolarizations from ‐50 to ‐30 mV resulted in a rise in Nai while further depolarization to 0 mV resulted in a fall in Nai. We conclude that both our results and those of Vaughan‐Jones (1977) on undissected fibres are contaminated by leakage into the fibre round the micro‐electrode. The true internal Na activity is probably much lower than previously reported.
Cable analysis in quiescent and active sheep Purkinje fibres.
Authors: Pressler M L
Published: December 2014

Cable properties of sheep cardiac Purkinje fibres were studied under resting and paced conditions. Standard micro‐electrode techniques were used to apply intracellular current pulses and record the resultant voltage changes at various distances from the current input. In a parallel set of experiments, fibre dimensions were measured after freezing and serial sectioning. Fibres selected on the basis of a cylindrical appearance had approximately uniform cross‐sectional diameters which varied +/‐ 12% along their length. Electrotonic potentials recorded at rest and in diastole (under conditions that minimized diastolic depolarization) adhered quite closely to the behaviour expected for a unidimensional cable provided voltages were recorded greater than or equal to one fibre diameter from the current source. The unidimensional space constant, input resistance, and membrane time constant were significantly larger during quiescence than in diastole. These differences were accounted for by a 90% increase in membrane resistance at rest. There was no significant change in internal longitudinal resistance nor membrane capacitance associated with activity. The voltage distribution close to the current input (i.e. within one fibre diameter) strongly deviated from the theoretical three‐dimensional voltage decay expected for a homogeneous cylinder. This finding suggests that the transverse resistance to current flow is much greater than the longitudinal resistance. The anisotropic behaviour within the cardiac Purkinje fibre may explain several previous observations: (i) the lack of a relationship between conduction velocity and fibre diameter; and (ii) the much shorter liminal length for excitation in Purkinje fibres than for point‐stimulated squid axons.
Relaxin inhibits the plateau component of the action potential in the circular myometrium of the rat.
Authors: Chamley W A , Parkington H C
Published: December 2014

The effects of relaxin on contractility and membrane potential of the longitudinal and circular smooth muscle layers of the uterus have been studied in vitro using oestrogen‐treated, non‐pregnant rats and pregnant rats. Relaxin decreased the amplitude of contractions induced by electrical stimulation of longitudinal myometrium by decreasing the duration of the bursts of action potentials. This effect was transient and tachyphylaxis always developed and was observed following injection of steroids and up to day 17 of pregnancy. There was no inhibition of tissues from rats from day 18 of pregnancy to term. The peptide had no effect on resting membrane potential, space constant or time constant. Action potentials recorded from circular myometrium of non‐pregnant rats pre‐treated with oestrogen consisted of an initial spike or short burst of spikes followed by a prolonged plateau of depolarization. Spontaneous action potentials and associated contractions were abolished within 2 min of exposure to relaxin (10(‐8) g/ml) while contractions of much smaller amplitude could be evoked with depolarizing current pulses. This effect was associated with depression of the plateau component of the action potential whereas the spike component was left intact. Relaxin had no effect on passive membrane properties. The action potentials of circular myometrium of rats up to day 21 of pregnancy were qualitatively similar to those recorded in the same muscle layer from oestrogen‐treated, non‐pregnant rats and the plateau component was also blocked by relaxin in these tissues. Bursts of spikes were observed in circular strips 24‐36 h before parturition, and the effect of the peptide on these was a transient inhibition similar to that observed in longitudinal myometrium. Oxytocin increased the amplitude of the spike and the amplitude and duration of the plateau. Relaxin abolished the plateau in the presence of 10(‐11) and 10(‐10) M‐oxytocin but was ineffective when the concentration of the spasmogen was increased further. Prostaglandin F2 alpha increased the amplitude and duration of the plateau. Relaxin abolished the responses to 10(‐10) and 10(‐9) M‐prostaglandin F2 alpha. The results of this study demonstrate that relaxin specifically inhibits contractions in the circular layer of the myometrium by abolishing the plateau component of the action potential. This action appears to be different from that of other smooth muscle relaxants tested in these experiments (isoprenaline, papaverine and verapamil). All of these abolished simultaneously both the spike and plateau components of the action potential.
The action of cholinomimetic substances on impulse conduction in the habenulointerpeduncular pathway of the rat in vitro.
Authors: Brown D A , Docherty R J , Halliwell J V
Published: December 2014

The effects of some cholinomimetic substances and their antagonists on the peak height of compound action potentials recorded from the terminal region of the habenulointerpeduncular pathway have been studied using a rat brain slice preparation. Carbachol and acetylcholine (ACh) depressed the peak height of the compound action potential and increased the latency to peak. The nicotinic agonists nicotine and dimethylphenylpiperazinium depressed the peak height of the compound action potential while muscarine and glutamate had no effect. The depressant effect of carbachol was blocked by the nicotinic antagonists hexamethonium, mecamylamine and d‐tubocurarine but not by atropine. Physostigmine enhanced the effects of ACh and, to a lesser extent, carbachol. In the presence of physostigmine, carbachol or ACh initiated a spontaneous oscillation of the amplitude of the compound action potential which was Ca2+ dependent and was blocked by mecamylamine. It is concluded that depression of the amplitude of the compound action potential is due to activation of presynaptic nicotinic receptors. The results are discussed with reference to possible cholinergic mechanisms in the habenulointerpeduncular pathway.
Intrinsic inhibition in magnocellular neuroendocrine cells of rat hypothalamus.
Authors: Andrew R D , Dudek F E
Published: December 2014

Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with intracellular recordings in the rat hypothalamic slice preparation. Hyperpolarizing after‐potentials (duration up to 125 ms) followed single action potentials and after‐hyperpolarizations (a.h.p.s) lasting hundreds of milliseconds followed brief evoked spike trains. The amplitude and duration of the a.h.p. increased after spike trains of longer duration or higher frequency. The a.h.p. appears endogenous, rather than synaptically mediated from recurrent inhibition, because it persisted after pharmacological blockade of axonal conduction or of chemical synaptic transmission. The reversal potential of the a.h.p. was at least 20 mV more negative than that of inhibitory post‐synaptic potentials. Cl‐ ionophoresis did not alter the a.h.p. Chelation of intracellular Ca2+ with EGTA injection eliminated the a.h.p. A Ca2+‐activated K+ conductance, rather than recurrent synaptic inhibition, apparently causes the a.h.p. and is at least partly responsible for the inhibition after single spikes in magnocellular neurones. During hormone release, this endogenous mechanism may contribute to the post‐burst silent period in putative oxytocinergic cells and to the interburst interval in phasic neurones, which are known to fire repetitive bursts associated with vasopressin release.
Phasic modulation of trunk muscle efferents during fictive spinal locomotion in cats.
Authors: Koehler W J , Schomburg E D , Steffens H
Published: December 2014

In high spinal paralysed cats electromyograms were recorded from nerves supplying lumbar back muscles (longissimus dorsi) and abdominal muscles (obliquus abdominis externus) during fictive locomotion induced by I.V. injection of nialamide and L‐DOPA. Activity in nerves to hind‐limb muscles was also recorded. During periods of stable 'locomotor' activity in the hind‐limb nerves the efferents to the back and abdominal trunk muscles were generally also rhythmically active. Three different patterns of activity were observed. The predominant rhythmic pattern showed a synchronous activation of the efferents to the back and abdominal muscles of one side together with an activation of the hind‐limb flexors of that side, alternating with activation of the efferents to the corresponding contralateral muscles. This pattern was very stable and could last for about 3 h. Such a pattern of activity would be expected during the alternate stepping characteristic of walking and trotting. The second type of rhythmic locomotor activity was characterized by a synchronous bilateral activation of the efferents to the back muscles, alternating with activation of the abdominal muscles on both sides. This pattern occurred only for short periods and appears to correspond to the activity during in‐phase stepping such as occurs during a gallop. Beside these well co‐ordinated patterns less well co‐ordinated rhythmic activities were also observed. These included regular rhythmic activity which occurred independently in different muscle groups as well as irregular rhythmic activity with unstable phase relations between different muscle groups. The rhythmic locomotor activity in efferents to trunk and limb muscles could be modulated by afferent nerve stimulation and by hypoxia. The results reveal that the spinal cord deprived of its supraspinal and peripheral control may generate a variety of different locomotor patterns, which incorporate the trunk muscles in an apparently meaningful way.
The effect of long‐term high‐frequency stimulation on capillary density and fibre types in rabbit fast muscles.
Authors: Hudlicka O , Tyler K R
Published: December 2014

Rabbit fast muscles (tibialis anterior, t.a.; extensor digitorum longus, e.d.l.; and peroneal muscles) were stimulated for up to 28 days by electrodes implanted in the vicinity of the lateral popliteal (peroneal) nerve for 8 h/day, using either intermittent high‐frequency (three trains at 40 Hz/min, each 5 s duration), or continuous stimulation at 10 Hz. This did not result in muscle hypertrophy even after 28 days. Capillary density (number of capillaries/mm2) was increased in e.d.l. from 251 +/‐ 3 to 366 +/‐ 6 after 14 days of stimulation and from 251 +/‐ 3 to 514 +/‐ 13 after 28 days of stimulation at 40 Hz. In t.a., capillary density increased from 373 +/‐ 5 to 583 +/‐ 10 after 14 days of stimulation at 40 Hz. The capillary/fibre ratio increased in e.d.l. from 1.25 +/‐ 0.02 to 1.86 +/‐ 0.04 at 14 days and to 2.07 +/‐ 0.06 at 28 days. In t.a., capillary/fibre ratio increased from 1.40 +/‐ 0.03 to 1.83 +/‐ 0.05 at 14 days. All these changes were significant (P less than 0.0005). Analysis of capillary density, capillary/fibre ratio, fibre areas and proportion of different fibre types in muscles stimulated for shorter periods showed no changes in capillary density, capillary/fibre ratio or fibre areas in e.d.l. or t.a. stimulated for 4 days; there was a decrease in the proportion of fast glycolytic fibres from 42 to 32% (P less than 0.0025) and increase in fast oxidative from 37.6 to 41.2% in e.d.l. Muscles stimulated for 7 days showed increases in capillary density and capillary/fibre ratio in fast predominantly glycolytic fibres in e.d.l., and a decrease in capillary density in fast and slow oxidative fibres in t.a. This was partly due to the increase in fibre areas in these groups (capillary/fibre ratio in t.a. was not significantly changed). No changes were observed in fibre areas in e.d.l. Stimulation at 10 Hz produced increase in capillary/fibre ratio in the vicinity of glycolytic fibres after only 4 days. High‐frequency intermittent stimulation leads to a massive capillary growth which starts first in the muscle with a higher proportion of glycolytic fibres (e.d.l.), has a later onset than continuous low‐frequency stimulation, and may be due to a combination of high blood flow and metabolic factors.
Release of acetylcholine from rat brain synaptosomes by various agents in the absence of external calcium ions.
Authors: Adam-Vizi V , Ligeti E
Published: December 2014

The relationship between 86Rb+ distribution across synaptosomal membrane and [14C]acetylcholine (ACh) release have been studied in a rat brain cortex synaptosomal preparation using K+, ouabain and veratridine depolarization. Decrease in membrane potential, approximated from the 86Rb+ distribution, is accompanied by an increase in [14C]ACh release, but the extent of the increase at a certain depolarization is dependent on how the depolarization is induced. A substantial depolarization by K+ is necessary to enhance ACh release, as compared to ouabain and veratridine where only a slight depolarization is accompanied by an increase in ACh release. In Ca2+‐free, EGTA‐containing medium ouabain and veratridine can also increase [14C]ACh release. The relationship between membrane potential and ACh release is very similar in the presence of ouabain and veratridine both in Ca2+‐containing and Ca2+‐free medium. The effect of ouabain and veratridine on the Na‐K exchange pump is different; ouabain can completely abolish Na‐K‐ATPase activity and 86Rb+ uptake of synaptosomes, whereas veratridine does not seem to influence the activity of the pump. m‐Chloro‐carbonylcianid phenyl hydrazon (50‐500 nM) increases [14C]ACh release in a concentration‐dependent manner without a considerable change of membrane potential or Na‐K pump activity. The Ca2+ ionophore A 23187 induces a substantial increase in [14C]ACh release in the absence of external Ca2+. In this case neither Na‐K pump activity nor membrane potential of synaptosomes is changed. A possible role of intracellular Ca2+ mobilization as a consequence of increased intracellular Na+ concentration in some depolarization‐induced transmitter release is discussed.
3',5'‐cyclic adenosine monophosphate and adenylate cyclase in phototransduction by limulus ventral photoreceptors.
Authors: Brown J E , Kaupp U B , Malbon C C
Published: December 2014

Biochemical and electrophysiological measurements were made on photoreceptor cells from Limulus ventral eyes to investigate the possible role of cyclic AMP and adenylate cyclase in the visual transduction mechanism. Cyclic AMP content in a photoreceptor‐enriched fraction (the end organs) of Limulus ventral eyes was approximately 15 pmol/mg protein. The cyclic AMP content was increased by bathing eyes in 1‐methyl‐3‐isobutyl xanthine or forskolin and was increased almost 100‐fold when bathed in both. Illumination did not change cyclic AMP content significantly in any of these conditions. Discrete events that can be recorded electrophysiologically occur spontaneously in darkness. An increase in the frequency of discrete events is evoked by dim illumination. The discrete events are a sign of excitation of Limulus photoreceptor cells. Drug‐induced changes in the rate of occurrence of discrete events recorded electrophysiologically in darkness were not correlated with changes in cyclic AMP content. Adenylate cyclase activity measured from a small number of pooled photoreceptor clusters was stimulated by fluoride and vanadate ions, hydrolysis‐resistant analogues of GTP, cholera toxin and forskolin. The Limulus enzyme is similar pharmacologically to mammalian and avian adenylate cyclases. Activation of adenylate cyclase by drugs was not correlated with changes in the rate of occurrence of discrete events recorded electrophysiologically in darkness. A heat‐treated Lubrol extract of membranes from Limulus ventral eyes reconstituted the adenylate cyclase activity of membranes from S49 mouse lymphoma cyc‐ mutant cells which lack a functional regulatory protein. These findings suggest that Limulus ventral eye photoreceptors contain a regulatory protein that mediates the activation of adenylate cyclase by guanine nucleotides, fluoride or cholera toxin. This regulatory protein is homologous with that found in mammalian and avian adenylate cyclases. Our findings suggest that neither cyclic AMP nor adenylate cyclase activation is a necessary or obligatory component of the excitation mechanism in Limulus ventral photoreceptors.
A non‐selective cation conductance in frog muscle membrane blocked by micromolar external calcium ions.
Authors: Almers W , McCleskey E W , Palade P T
Published: December 2014

Membrane currents were recorded from voltage‐clamped, EGTA‐loaded muscle fibres under conditions where currents through ordinary Na+, K+ and Cl‐ channels were prevented by drugs or by absence of permeant ions (K+, and Cl‐). At 10 mM‐external [Ca2+], substitution of Na+ for the large and presumably impermeant organic cations tetramethyl‐ (TMA+) or tetraethylammonium (TEA+) failed to increase peak inward current. Hence the Ca2+ channel was not significantly permeable to Na+ under these conditions. When external [Ca2+] was reduced to levels below 1 microM in the presence of external Na+, step depolarizations to negative potentials produced tetrodotoxin‐resistant inward currents. At ‐20 mV, they rose to a peak of 30‐200 microA/cm2 within 150 ms and declined thereafter. Ca2+ and several other divalent cations reversibly blocked this inward current. The sequence of blocking potencies was Ca2+ greater than Sr2+ greater than or equal to Co2+ greater than Mn2+ congruent to Cd2+ greater than Ni2+ congruent to Mg2+. Large inward currents may be carried by Li+, Na+, K+, Rb+ and Cs+ but not by TMA+ and TEA+. The effect of external Ca2+ ([Ca2+]o) was explored over a 10(8)‐fold range in concentrations. Na+ was present at a fixed concentration. When [Ca2+]o was gradually increased from 10(‐10) to 10(‐2) M, inward current first diminished 10‐fold, reached a minimum at [Ca2+]o = 60 microM and then increased again as [Ca2+]o was increased further and Ca2+ itself became a current carrier. Block of inward current at [Ca2+]o less than 10(‐5) M could be described by binding of a single Ca2+ to a site, with a dissociation constant of the order of 0.7 microM at ‐20 mV.
The uptake and release of glutamate at the crayfish neuromuscular junction.
Authors: Kawagoe R , Onodera K , Takeuchi A
Published: December 2014

The abdominal slow flexor muscle of the crayfish (Cambarus clarkii) was pre‐incubated in deuterium‐labelled glutamate (glutamate‐d5) solution, and the release of glutamate, glutamate‐d5 and aspartate in the bath solution was measured by mass fragmentography. The glutamate‐d5 was taken up into the preparation and released together with endogenous glutamate. The resting release of glutamate‐d5 was 8.2 +/‐ 0.96 pmol/10 min after the incubation with 0.5 mM‐glutamate‐d5, and those of endogenous glutamate and aspartate were 7.4 +/‐ 1.19 and 2.8 +/‐ 0.81 pmol/10 min, respectively. The release of glutamate‐d5 was not significantly increased by nerve stimulation, while that of endogenous glutamate was increased by about 9.7 pmol/10 min above the resting release. The application of high‐K solution induced a net increase of 1.9 pmol/10 min in glutamate‐d5 release, 6.1 pmol/10 min in endogenous glutamate release and 7.9 pmol/10 min in aspartate release. The relative amount of glutamate‐d5 accumulated into the preparation during pre‐incubation with 0.5 mM‐glutamate‐d5 was a few per cent of the endogenous glutamate in the preparation. It is concluded that the exogenous glutamate is taken up mainly into a compartment which differs from that related to nerve evoked transmitter release and that high‐K solution has effects on the amino acid release which differ from those of nerve stimulation.
Intracellular pH changes induced by injection of cyclic nucleotides into gastropod neurones.
Authors: Connor J A , Hockberger P
Published: December 2014

Injections of cyclic AMP (cAMP) or cyclic GMP (cGMP) into identifiable gastropod neurones under voltage clamp induced reversible cytoplasmic pH changes which were measured using the indicator dye Arsenazo III and pH micro‐electrodes. Similar injections of 5' AMP or 5' GMP did not induce such effects. In about one‐half of the population of neurones examined, cAMP injections (0.1‐10 mM) induced pH decreases with latencies of 1‐5 min and maximum response times 12‐25 min post‐injection. Comparable injections of cGMP into these cells resulted in decreases with latencies less than 1 min and maximum response times 5‐10 min. The amplitude and duration of these pHi decreases were dose dependent. In the other half of the population of neurones tested, cAMP injection induced an immediate alkalinization lasting 5‐10 min followed by an acidification which displayed a maximum response time within the same range as those in the first group. cGMP injection into these cells induced only acidifications with faster peak responses than with cAMP. Since cGMP did not elicit alkalinizing responses, the slower response time of the first group of cells with cAMP may reflect an always present underlying alkalinization. The nucleotide‐induced acidification was potentiated in neurones bathed in the phosphodiesterase inhibitor IBMX. Injections of non‐hydrolysable cAMP analogues also induced acidifications which were longer lasting than comparable doses of cyclic AMP. These results indicate that the elevated [H+]i was not simply due to hydrolysis of the injected cyclic nucleotide. The pHi changes were invariably of much longer duration than a nucleotide‐induced inward current (Connor & Hockberger, 1984) and persisted in Na‐free salines which eliminated the current response. These results support the notion that cyclic nucleotide elevation can affect cellular metabolic processes distinct from effects on ion transport mechanisms.
Exchange area and surface properties of the microvasculature of the rabbit submandibular gland following duct ligation.
Authors: Clough G , Smaje L H
Published: December 2014

The exchange area of the submandibular salivary gland microvasculature has been measured to allow the value of microvascular permeability (P) to hydrophilic solutes to be calculated from previous measurements of permeability‐surface area (PS) products. Glands whose ducts had been ligated for 2 weeks and the contralateral control glands were perfusion‐fixed with a modified Karnovsky's fixative after perfusion with a solution containing cationized ferritin, and examined with transmission electron microscopy. Stereological techniques were used to estimate the surface area of the exchange vessels on random thin sections from four control and four duct‐ligated glands. The mean exchange surface area in control glands was 512 cm2 g‐1 and 336 cm2 g‐1 in duct‐ligated glands. The fenestral density was calculated to be 0.57% of the exchange surface in control glands and 0.30% in duct‐ligated tissue. Molecules of cationized ferritin appeared bound to the luminal surface of the microvascular endothelium, including the surface of the fenestrae to a depth of about 25 nm in both control and ligated glands. These experiments have shown that the exchange surface area of the fenestrated endothelium of the submandibular salivary gland is comparable to that in cardiac muscle but the permeability (P) to small solutes is about 10 times greater. Following ligation of the salivary gland duct, solute permeability falls and an explanation of this, based on the reduced surface area and the nature of the permeability‐flow relationship for small solutes is offered.
Initiation of active contraction by photogeneration of adenosine‐5'‐triphosphate in rabbit psoas muscle fibres.
Authors: Goldman Y E , Hibberd M G , Trentham D R
Published: December 2014

Mechanical and biochemical descriptions of the muscle cross‐bridge cycle have been correlated. Skinned muscle fibres of rabbit psoas muscle in rigor were incubated in solutions containing approximately equal to 30 microM‐Ca2+ ions and P3‐1‐(2‐nitro)phenylethyladenosine‐5'‐triphosphate, 'caged ATP', an inert photolabile precursor of ATP. ATP was liberated from caged ATP within the fibres by pulses of 347 nm radiation from a frequency‐doubled ruby laser. The mechanical responses of muscle fibres to the rapid increase of ATP concentration were monitored. Tension dropped briefly and then rose above the rigor value to the level characteristic of a steady active contraction. Liberation of ATP decreased in‐phase stiffness (measured at 500 Hz) from the rigor level to a maintained value intermediate between rigor and relaxed values. Out‐of‐phase stiffness increased to a maintained level indicating a phase lead of tension with respect to imposed length oscillations. Rigor tension was varied prior to photolysis by slight alterations of fibre length. Tension traces starting at different rigor tensions converged to a common tension level at the same rate, whether or not Ca2+ was included in the medium. These data suggest that the rate of cross‐bridge detachment by ATP from the rigor state is not influenced by Ca2+. Analysis of the tension records, in terms of sequential detachment and reattachment reactions, provided a measure of cross‐bridge reattachment rate and an alternate measure of the detachment rate. Detachment from the rigor state was approximately proportional to the ATP concentration, with a second‐order rate constant of at least 5 X 10(5) M‐1 S‐1. Reattachment with force generation had no detectable dependence on the concentration of ATP liberated by photolysis. A simple kinetic model of the cross‐bridge cycle in terms of chemically defined intermediates was compatible with most of the experimental data. The ATP dependence of cross‐bridge detachment, the kinetics of maintained cross‐bridge reattachment in the presence of Ca2+, and transient reattachment and final relaxation in the absence of Ca2+ were explained. In this model, reversibility of cross‐bridge attachment and the steps leading to force production allow the relatively high observed detachment rate to be accommodated with other data relating to active contraction. These data include the steady ATPase rate of active muscle fibres and the fewer attached cross‐bridges in active contractions compared to rigor.
Effects of vasopressin on electrolyte transport across isolated colon from normal and dexamethasone‐treated rats.
Authors: Bridges R J , Rummel W , Wollenberg P
Published: December 2014

Vasopressin enhanced the absorption of Na+ and Cl‐ across the short‐circuited colon descendens from normal rats. This effect of vasopressin results from an increase in the mucosal to serosal movement of Na+ and Cl‐ and a decrease in the serosal to mucosal movement of Cl‐ and was accompanied with a decrease in the short‐circuit current (ISC). Neither the base‐line absorption of Na+ and Cl‐, the vasopressin‐induced increase in Na+ and Cl‐ absorption nor the decrease in ISC were inhibited by amiloride in the colon from normal rats. Colon descendens from rats treated for 3 days with dexamethasone had remarkably higher transmural potential difference (p.d.), tissue conductance (Gt) and ISC. The absorption of Na+ across the short‐circuited colon descendens from dexamethasone‐treated rats was increased 3‐fold when compared to colon from normal rats. The absorption of Cl‐ in normal rats was reversed to Cl‐ secretion in treated rats. Amiloride rapidly and reversibly decreased the p.d., Gt and ISC in colon from dexamethasone‐treated rats. The transport of Na+ was nearly completely inhibited by amiloride in treated rats. In contrast to its enhancing effects on Na+ absorption in colon from normal rats vasopressin did not enhance Na+ absorption in colon from dexamethasone‐treated rats. This enhancement of Cl‐ absorption by vasopressin was retained in colon from treated rats. This enhancement of Cl‐ transport was due solely to a decrease in the serosal to mucosal movement of Cl‐ and was accompanied with a decrease in ISC and Gt. The results support the hypothesis that vasopressin causes inhibition of the electrogenic secretion of Cl‐ in colon from dexamethasone‐treated rats. Furthermore, the results suggest that the increase in the mucosal to serosal movement of Na+ and Cl‐ and the decrease in the serosal to mucosal movement of Cl‐ in colon from normal rats are caused by independent effects of vasopressin.
Evidence for two voltage‐dependent calcium currents in the membrane of the ciliate Stylonychia.
Authors: Deitmer J W
Published: December 2014

Action potentials and voltage‐dependent membrane currents have been investigated in the fresh‐water hypotrich ciliate Stylonychia mytilus, using two intracellular micro‐electrodes. The inward current‐voltage (Iin‐V) relationship has two maxima, the first around ‐45 mV, and the second around ‐17 mV (resting and holding membrane potential being ‐50 mV). The shape of the Iin‐V relationship is virtually unaltered in the presence of the K‐channel blockers tetraethylammonium, 4‐aminopyridine or internal Cs. The inward currents exhibit a differential sensitivity to both external CO and Cd; the inward current activated at potentials greater than or equal to ‐40 mV is more sensitive to these divalent cations than the inward current activated at around ‐45 mV. This suggests the presence of two different types of Ca inward currents. Both types of inward currents are present when Ca is replaced by Ba (or Sr). The small inward current recorded between ‐48 and ‐40 mV relaxes similarly in Ca and in Ba solutions. The larger inward current, recorded at ‐30 or ‐20 mV, relaxes rapidly in Ca solution but only slowly and incompletely in Ba solution. A two‐pulse protocol revealed that for both types of inward currents inactivation may depend partially upon the influx and/or intracellular accumulation of the charge‐carrying divalent cation. There appears to be a significant difference in the degree of inactivation of the two types of inward currents, however, when Ba is the charge carrier. When the cell spontaneously released, or was induced to release its membranellar band (row of compound cilia), the second, "all‐or‐none' component of the action potential, and the maximum of the Iin‐V relationship at ‐45 mV disappeared. The first, graded peak of the action potential and the larger maximum of the Iin‐V relationship remained essentially unaltered. The smaller Ca current and the action potential shoulder also disappeared when the anterior half of the cell (with most of the membranellar band) was severed, but not when the posterior half was cut off. When recording from a membranellar band vesicle both types of inward currents were present. The results suggest that the two components of the action potential may correspond to the two types of Ca currents. These Ca currents are separable by their localization in the membrane. The smaller Ca current appears to be restricted to the membranellar band.(ABSTRACT TRUNCATED AT 400 WORDS)