bluuebunny/crossref_metadata_2025 huggingface dataset:
Amino acid receptor mediated excitatory synaptic transmission in the cat red nucleus.
Authors: Davies J , Miller A J , Sheardown M J
Published: December 2014

A study has been made of the effects of the selective N‐methyl‐D‐aspartate receptor antagonist, 2‐amino‐5‐phosphonovalerate (APV), and the broad spectrum excitatory amino acid antagonists, gamma‐D‐glutamylglycine (gamma‐DGG), gamma‐D‐glutamylaminomethylsulphonate (GAMS), 4(p‐chlorobenzoyl)‐cis‐piperazine‐2, 3‐dicarboxylate (pCB‐PzDA) and kynurenate, have been examined on excitation evoked on neurones in the magnocellular red nucleus (m.r.n.) of the anaesthetized cat by stimulation of the interpositus nucleus (i.p.n.) and sensorimotor cortex, and by ionophoresed excitant amino acid agonists. The profile of activity of the excitatory amino acid antagonists on m.r.n. neurones was similar to that described on neurones in other areas of the central nervous system. APV selectively depressed responses to N‐methyl‐D‐aspartate (NMDA), whereas the broader spectrum antagonists reduced responses to kainate and quisqualate as well as to NMDA. Neuronal responses to L‐glutamate and L‐aspartate were depressed by all the antagonists tested. I.p.n.‐evoked monosynaptic responses of m.r.n. neurones were reversibly reduced by the broad spectrum antagonists, but were unaffected by APV. Cortically evoked mono‐ and polysynaptic excitatory responses were reversibly depressed by APV and the broad spectrum antagonist, pCB‐PzDA. The action of APV corresponded with its ability to antagonize responses to NMDA. However, the cortically evoked responses appeared to be more sensitive to the actions of pCB‐PzDA than to those of APV, although the former is a less effective antagonist of NMDA‐induced excitation compared with APV. APV depressed excitation induced by cortical stimuli and L‐glutamate and L‐aspartate. However, there was no obvious correlation between the actions of the broad spectrum amino acid antagonists on synaptically evoked responses and those induced by L‐glutamate or L‐aspartate on the few neurones tested. These results are consistent with an amino acid being the transmitter in the interposito‐rubral and cortico‐rubral excitatory pathways which interacts with non‐NMDA and both NMDA and non‐NMDA receptors respectively. However, the identity of the transmitter acting at these receptors remains to be determined.
The degree of short‐term synchrony between alpha‐ and gamma‐motoneurones coactivated during the flexion reflex in the cat.
Authors: Connell L A , Davey N J , Ellaway P H
Published: December 2014

Cross‐correlation analysis of unitary neuronal discharges has been used to study the linkage between alpha‐ and gamma‐motoneurones coactivated during the flexion reflex of the semitendinosus muscle in the decerebrated spinal cat. A flexion reflex was elicited by firm grip or squeeze of the ipsilateral heel, shank or foot. The stimulus excited the discharges of both alpha‐ and gamma‐motoneurones and increased the frequency of discharge of those gamma‐motoneurones that had shown a background discharge prior to intentional stimulation. Short‐term synchrony was present between a high proportion of semitendinosus gamma‐motoneurones both for background discharges (sixteen out of nineteen pairs) and during the flexion reflex (thirteen out of fifteen pairs). All nineteen pairs of alpha‐motoneurones examined during the flexion reflex showed short‐term synchrony of discharge. Few alpha‐motoneurones displayed background discharges but synchrony was observed in the two instances studied. The degree of synchrony was measured as the ratio (kappa) of the total counts contributing to the peak of the correlogram over the number expected by chance alone. The ratio was higher when the average frequency of motoneurone discharge was low. Kappa was generally higher for alpha‐motoneurone pairs than for gamma‐motoneurone pairs. The higher degree of synchrony for alpha‐motoneurones reflected their lower discharge rates. During the flexion reflex the degree of synchrony between gamma‐motoneurones was greater than expected for that same discharge rate in the absence of intentional stimulation. Only twenty‐seven out of forty pairings of an alpha‐ with a gamma‐motoneurone showed a significant degree of synchrony of discharge. On average, the degree of synchrony for alpha/gamma pairs was lower than that for either alpha/alpha or gamma/gamma pairings at the equivalent discharge rate. The results support the conclusion that coactivation of alpha‐ and gamma‐motoneurones during the flexion reflex occurs largely through independent sets of interneurones. The possibility is discussed that those alpha‐motoneurones which showed short‐term synchrony with gamma‐motoneurones were skeleto‐fusimotor (beta‐motoneurones) in nature.
Effect of tetanus duration on the free calcium during the relaxation of frog skeletal muscle fibres.
Authors: Cannell M B
Published: December 2014

Single fibres were dissected from the tibialis anterior muscle of the frog and injected with the photoprotein aequorin. Tension and the light emission of the injected aequorin (a function of the free intracellular calcium concentration) were recorded both at rest and during tetanus relaxation. The level of light emission from resting single fibres corresponded to a free intracellular calcium concentration ([Ca2+]i) of 100 nM (+/‐ 40 nM, n = 4). The time course of the decline in light was examined during the three periods of muscle relaxation: period 1 during the slow phase of tension relaxation, period 2 during the exponential phase of relaxation and period 3 after the completion of force relaxation. The time course of the decline in light (after a correction for the kinetics of the aequorin reaction) showed that [Ca2+]i declines exponentially with a rate constant of 25 s‐1 (+/‐ 1.7, n = 3) after a single stimulus at 10 degrees C. With increasing tetanus duration, the rate of decline of [Ca2+]i decreased during period 1. It is suggested that this decrease in the rate of decline of [Ca2+]i results from an intracellular calcium buffer (which takes up calcium in parallel with the sarcoplasmic reticulum) becoming loaded with calcium during the tetanus. Throughout period 2 [Ca2+]i was elevated above resting levels. The level of [Ca2+]i during this period varied from fibre to fibre but could be as high as 1 microM. The mean level of [Ca2+]i during this period also depended on the tetanus duration. A long‐lasting elevation in [Ca2+]i was observed during period 3, [Ca2+]i returning towards resting levels with an approximately exponential time course. During this period the level of [Ca2+]i (at a given time after the last stimulus) depended on the tetanus duration. It is suggested that this long‐lasting elevation in [Ca2+]i reflects the release of calcium from the intracellular calcium buffer described above. The results suggest that the rate of decline of [Ca2+]i after a few seconds of tetanic stimulation can be explained by the rate of calcium sequestration by the sarcoplasmic reticulum. The increased rate of decline of [Ca2+]i after shorter periods of stimulation may be explained by the presence of a buffer that takes up calcium in parallel with the sarcoplasmic reticulum. The later release of calcium from this buffer gives rise to the long‐lasting elevation in [Ca2+]i during period 3. The slow kinetics of calcium binding and release by this buffer appear compatible with published data on the kinetic properties of parvalbumin.
Inorganic phosphate regulates the contraction‐relaxation cycle in skinned muscles of the rabbit mesenteric artery.
Authors: Itoh T , Kanmura Y , Kuriyama H
Published: December 2014

The effects were investigated of inorganic phosphate (Pi) on the Ca2+‐dependent and Ca2+‐independent contractions evoked in chemically skinned smooth muscles of the rabbit mesenteric artery. The relation between the concentration of Ca2+ and tension showed a sigmoidal curve in the range of pCa 7‐5.5. Pi (over 1 mM) inhibited the Ca2+‐induced contraction, shifted the pCa‐tension curve to the right and increased the Hill number from 2 to 3. Calmodulin did not change the Hill number and attenuated the inhibitory action of Pi as estimated from the shift of the curve, but this agent did not modify the increased Hill number in the presence of Pi. Pi consistently inhibited the Ca2+‐independent contractions provoked by application of trypsin‐treated myosin light chain kinase, of MgATP following adenosine‐5'‐o‐(3‐thiotriphosphate) (ATP gamma S) and Ca2+, and of a solution containing high Mg2+. These inhibitory actions of Pi were inversely proportional to the amplitude of the contraction. When Pi was applied simultaneously with ATP gamma S and Ca2+, there was no change in the amplitude of Ca2+‐independent contractions provoked by the application of MgATP. The amplitude of the rigor contraction evoked by ATP‐free solution was less than 7% of that of the 10 microM‐Ca2+‐induced contraction. When ATP was removed from the solution during the Ca2+ contraction, the rigor contraction was also generated. Pi did not inhibit either type of contraction. With a decrease in the concentration of Ca2+ from 10 microM to below 1 nM, the tissue relaxed at a slower rate than the rate of rise of the Ca2+‐induced contraction. The slow relaxation was not modified by a change in the concentration of EGTA or addition of 1 microM‐calmodulin. Pi reduced, and high Mg2+ prolonged the time required for the relaxation. This action of Pi was not prevented in the presence of calmodulin or of high Mg2+. The rates of rise and fall of the Ca2+‐induced contraction depended on the concentration of MgATP, and Pi consistently inhibited the Ca2+‐induced contraction in the presence of any given concentration of MgATP. We conclude that Pi may accelerate the detachment of cross‐bridges between the contractile proteins. Thus, the amplitude of Ca2+‐induced contraction is slightly inhibited and the relaxation is markedly facilitated. However, the site of action of Pi may differ from that of MgATP.(ABSTRACT TRUNCATED AT 400 WORDS)
Membrane currents of internally perfused neurones of the snail, Lymnaea stagnalis, at low intracellular pH.
Authors: Byerly L , Moody W J
Published: December 2014

The effects of low intracellular pH (pHi) on the membrane currents of snail neurone somata were studied using the internal perfusion and ion‐sensitive micro‐electrode techniques. Recordings with pH‐sensitive micro‐electrodes made while the pH of the perfusion solution was changed between 7.3 and 6.3 indicated that only with high buffer concentrations (100 mM) could pHi be changed effectively. H+ was slower to exchange into the cytoplasm than an unbuffered ion such as K+. When pHi was decreased to 5.9, large outward H+ currents could be recorded at voltages positive to ‐30 mV. The time course and amplitude of these currents were such that they did not affect the measurement of the peak amplitude of the fast transient K+ current (A‐current), but severely contaminated both Ca2+ and delayed K+ current measurements. Low pHi blocked the A‐current. The titration curve was consistent with the binding of two H ions to a site with a pK of 6.05 to block the channel. Low pHi appeared to block the slow inactivation of the delayed outward current without greatly changing its peak amplitude. However, when correction was made for the increase of H+ current at low pHi, the effect of internal H+ was found to be a block of the delayed K+ current with no consistent effect on inactivation. The Ca2+ current was also decreased at low pHi, but we were unable to determine whether this was a direct effect of pHi or secondary to a rise in internal free [Ca2+]. If no correction was made for H+ currents, the block of the Ca2+ current appeared greater and more reversible than it actually was. We conclude that under certain conditions, such as low pHi, the H+ current is a significant fraction of the total outward current in snail neurones, and may also be in a variety of other cells. The H+ currents must be accounted for under such conditions in order to study accurately the properties of K+ and Ca2+ currents.
Acetylcholine modulation of the conductance of intercellular junctions between rat lacrimal cells.
Authors: Neyton J , Trautmann A
Published: December 2014

The conductance of intercellular junctions between rat lacrimal cells was studied with the double whole‐cell tight‐seal recording technique. This conductance decreases spontaneously with time as a result of the double‐cell dialysis. The rate of this 'spontaneous' uncoupling is unaffected by changing the internal Ca concentration, [Ca]i, between 10(‐8) M and 10(‐6) M. This rate of uncoupling is greatly increased when [Ca]i is approximately 10(‐5) M, and this effect does not involve changes in the internal proton concentration. When [Ca]i is weakly buffered in one of the two cells, 1‐2 microM‐acetylcholine (ACh) both activates Ca‐dependent channels in that cell (Marty, Tan & Trautmann, 1984) and uncouples the two cells. The uncoupling is not synchronous with the increase in [Ca]i as reflected by the Ca‐dependent currents. When [Ca]i is strongly buffered in both cells, ACh fails to activate Ca‐dependent currents, but it can still uncouple the cells. This ACh‐induced uncoupling is often preceded by a transient enhancing of the coupling. In conclusion, ACh has several distinct effects on lacrimal cells: activation of Ca‐dependent channels in the plasma membrane, closure of junctional channels involving a Ca‐independent mechanism, and sometimes, an increase in the junctional coupling by a Ca‐independent mechanism.
Enteric neural regulation of slow waves in circular muscle of the canine proximal colon.
Authors: Sanders K M , Smith T K
Published: December 2014

The spontaneous electrical activities of circular muscle cells of the canine proximal colon were studied with intracellular micro‐electrodes. All circular muscle cells exhibited slow waves at frequencies ranging between 2.8 and 7.0 cycles/min. The slow waves consisted of an upstroke phase followed by a plateau phase of variable duration (2‐40 s). Many cells displayed a slow diastolic depolarization, or 'pre‐potential' between slow waves. Slow waves spontaneously varied in duration and frequency in most preparations, creating distinctive slow wave patterns. Atropine, 2 X 10(‐6) M, decreased the durations of slow waves in many preparations and often changed the pattern to a series of relatively uniform slow waves. A further reduction in mean slow wave duration was produced by additional treatment with tetrodotoxin, 10(‐6) M. These results suggested that slow wave duration and pattern were affected by spontaneous discharge from both cholinergic and non‐cholinergic excitatory nerves. Transmural nerve stimulation caused a short latency increase in slow wave duration (up to 38 s) that was abolished by atropine. In the presence of atropine, transmural stimulation evoked inhibitory junction potentials that reduced the amplitude and duration of the subsequent slow wave. The slow wave of reduced amplitude was followed by a slow wave of increased duration. The increase in duration of the slow wave did not appear to be related to the size of the preceding hyperpolarization, suggesting it was mediated by the release from non‐cholinergic excitatory nerves. All responses to transmural stimulation were blocked by tetrodotoxin. Microejection of acetylcholine on to the muscle adjacent to the micro‐electrode also produced an atropine‐sensitive increase in slow wave duration. Tissues that had been stored in the cold overnight to reduce intrinsic neural activity exhibited regular slow waves of short duration. It is proposed that the basic myogenic pattern of spontaneous slow wave activity consists of regularly occurring slow waves of short duration (2‐5 s). Intrinsic cholinergic and non‐cholinergic excitatory nerves appear to modulate slow wave activity in vitro, producing distinctive slow wave patterns of variable instantaneous frequency and duration.
Development of dendritic calcium currents in ganglion cells of the rat lower lumbar sympathetic chain.
Authors: Hirst G D , McLachlan E M
Published: December 2014

Post‐natal changes in the active membrane properties of ganglion cells of the rat lumbar sympathetic chain have been examined using current‐clamp and voltage‐clamp techniques. Ganglion cells from the youngest animals studied (3 days post‐natal) which received a synaptic input generated action potentials that were abolished by tetrodotoxin (TTX). In neurones from animals aged 7‐14 days, a TTX‐resistant component of the action potential was revealed which persisted in cells from older animals. Voltage‐clamp studies suggested that the TTX‐resistant component of the action potential resulted from calcium entry at a point that was electrically close to the somatic voltage clamp at early ages, but became electrically distant in older neurones. The site of the inward calcium current appeared to be restricted to specialized areas on one, or occasionally two, dendrites. Concurrent with these changes, the dendritic trees of the ganglion cells expanded and the nature of the synaptic input changed. Initially preganglionic stimuli initiated only subthreshold excitatory synaptic potentials, whereas in the more mature animals most cells also received at least one suprathreshold ('strong') input. The number of distinct sites of calcium current generation was correlated with the number of strong synaptic inputs that a given neurone received.
Correlation between long‐term potentiation and release of endogenous amino acids from dentate gyrus of anaesthetized rats.
Authors: Bliss T V , Douglas R M , Errington M L , Lynch M A
Published: December 2014

The relationship between long‐term potentiation (l.t.p.) and the release of endogenous amino acid transmitters has been investigated in the dentate gyrus of rats anaesthetized with urethane. The molecular layer was perfused with artificial cerebrospinal fluid using a push‐pull cannula. The perfusate was collected and analysed for glutamate, aspartate, glycine, glutamine and gamma‐aminobutyric acid (GABA) using high‐performance liquid chromatography (h.p.l.c.) with fluorometric detection. Recording electrodes were attached to the cannula to enable responses evoked by test stimuli to the perforant path to be monitored in the molecular and cell body layers. Perfusion was continued for 3 h while test stimuli were delivered to the perforant path at 30 s intervals. In the control group (n = 8), no further stimulation was given. In a second group (n = 8), a single high‐frequency train (250 Hz for 200 ms) was delivered at the end of the first hour to induce l.t.p. The average potentiation of the slope of the excitatory post‐synaptic potential (e.p.s.p.) 2 h later was 15%. In a third group (n = 8), the train to the perforant path was paired with a train to the commissural input to the dentate gyrus, a procedure which blocks the induction of l.t.p. In the potentiated group, there was an increase in the concentrations of glutamate and aspartate following the induction of l.t.p., relative to the decline seen in corresponding periods of the control group. This increase remained statistically significant for 1.5 h in the case of glutamate and for 45 min in the case of aspartate. There were no l.t.p.‐associated changes in the release of glutamine or glycine; there was an indication that l.t.p. may be associated with a decrease in the release of GABA. Increasing the frequency and intensity of perforant path activation resulted in enhanced concentrations of glutamate and aspartate in the perfusate; no such changes occurred when granule cells were activated antidromically. We discuss the origin of the relative increases in the concentration of glutamate and aspartate which are found in the perfusate following the induction of l.t.p. and conclude that the most likely source is a sustained increase in activity‐dependent release of these amino acids from perforant path terminals.(ABSTRACT TRUNCATED AT 400 WORDS)
A method for studying lymphatic pumping activity in conscious and anaesthetized sheep.
Authors: McHale N G , Thornbury K
Published: December 2014

A 20 cm length of the main mesenteric lymph duct in the sheep was cannulated at both ends and measurements were made in both conscious and anaesthetized animals of the ability of the duct to pump saline from an inflow reservoir to an outflow at the same height. Fluid was propelled only when the lymphatic contracted and this was unaffected by movements of the animal round the cage or by fluctuations in abdominal pressure or by respiratory movements. It was confirmed that the mesenteric duct was 'isolated' from the rest of the lymphatic system by closing the inflow tap whereupon fluid propulsion ceased. Raising transmural pressure by varying inflow and outflow by the same amount had the effect of increasing fluid output; this was achieved by an increase in both the frequency and force of lymphatic contractions. Lymphatic frequency of contraction and fluid output increased when the animals were frightened. Intravenous infusions of noradrenaline increased the frequency of lymphatic contraction and increased fluid propulsion, while isoprenaline infusions depressed flow. This preparation demonstrates that it is possible to study the control of lymphatic pumping in conscious sheep without the complication of changing rates of lymph formation.
Effects of fluctuating skin temperature on thermoregulatory responses in man.
Authors: Keatinge W R , Mason A C , Millard C E , Newstead C G
Published: December 2014

Fluctuations in the skin temperature of volunteers in water, of up to +/‐ 1.5 degrees C, were induced by circulating water through a tube suit at a fluctuating temperature for 80 min. Fluctuations with a cycle length of 2.5, 5.0 or 10.0 min all slowed the fall in core temperature that otherwise occurred in water at 29 degrees C, and reduced thermal comfort. Metabolic rate increased. Symmetrical fluctuations, or ramp fluctuations with either slow cooling and rapid warming phases or slow warming and rapid cooling phases, were all effective. Similar but less clear effects on core temperature were usually produced by fluctuating skin temperature in water at 25 and 33 degrees C; metabolic rate tended to increase during fluctuation in the 25 degrees C experiments, but not in the 33 degrees C experiments. Stability of the temperature of cutaneous receptors appears to be factor in insidious hypothermia produced by immersion in mildly cool water.
Intracellular free magnesium in frog skeletal muscle fibres measured with ion‐selective micro‐electrodes.
Authors: Alvarez-Leefmans F J , Gamiño S M , Giraldez F , González-Serratos H
Published: December 2014

Intracellular free Mg2+ concentration [( Mg2+]i) was measured in frog skeletal muscle fibres, using Mg2+‐selective micro‐electrodes based on the neutral ligand ETH‐1117. In calibration solutions the electrodes showed significant interference from K+, and to a lesser extent from Na+, at concentrations found intracellularly. Therefore, in order to calibrate the electrodes properly, it was necessary first to obtain an accurate value for intracellular free Na+ and K+ concentrations ([Na+]i and [K+]i), using the appropriate liquid ion exchanger micro‐electrodes. In fibres from muscles maintained in Ringer solution, the mean value for [Na+]i was 6.2 +/‐ 0.4 mM (S.E. of mean; n = 20 fibres in five muscles), while [K+]i was 104 +/‐ 1.7 mM (range 83‐122 mM; n = 25 fibres in eight muscles). Due to the substantial variability found for [K+]i, not only between fibres from different muscles, but also between fibres belonging to the same muscle, it was necessary to measure [Mg2+]i and [K+]i simultaneously in the same fibre to determine as accurately as possible the degree of K+ interference on Mg2+‐selective micro‐electrode response. In nineteen fibres from six muscles maintained in Ringer solution, the mean [K+]i was 91.7 +/‐ 2.7 mM (range 71‐110 mM), while the mean [Mg2+]i was 0.80 +/‐ 0.07 mM (range 0.2‐1.2 mM). The mean resting potential was ‐79.3 +/‐ 0.4 mV (S.E. of mean). In fifteen fibres from four muscles equilibrated in Ringer solution containing 0.5 mM‐Mg2+, the mean [K+]i was 115.5 +/‐ 0.1 mM (range 97‐129 mM) and the mean [Mg2+]i measured simultaneously in the same fibres was 1.69 +/‐ 0.21 mM (range 0.2‐2.7 mM). The mean resting potential was ‐83 +/‐ 0.7 mV. The mean [K+]i and [Mg2+]i found in these fibres was significantly higher (P less than 0.0001) than those measured in fibres from muscles maintained in standard Ringer solution (i.e. without external Mg2+). Possible explanations for this finding are discussed. Whether in the presence (0.5 mM) or in the absence of external Mg2+, our values for [Mg2+]i are distinctly lower than those previously reported by others, using the same type of Mg2+‐selective micro‐electrodes but calibrated simply from assumptions about the actual level of K+ and Na+ interference on Mg2+‐selective micro‐electrode response.(ABSTRACT TRUNCATED AT 400 WORDS)
Innervation of the iris by individual parasympathetic axons in the adult mouse.
Authors: Jackson P C
Published: December 2014

Ciliary neurones were identified electrophysiologically in isolated ganglia and the projection of individual axons to the intrinsic muscles of the eye determined by labelling identified cells with horseradish peroxidase (HRP). The ciliary ganglion of the adult mouse contains 80‐100 parasympathetic neurones clustered at the point where the oculomotor nerve branches to supply the medial and inferior rectus. These nerve cells show varying degrees of dendritic complexity (zero to six primary dendrites) and differing degrees of innervation by preganglionic axons (one to seven inputs). Cells with more complex dendritic arborization generally receive synaptic input from commensurately larger numbers of preganglionic axons. The axons of individually labelled cells give rise to varicose terminals in the iris and ciliary body; in most cases a single parent axon branches only after entering the iris, producing a network of terminals in the region of both the sphincter and the antagonist dilator muscle. Cells with both simple and complex dendritic geometry show this pattern of axonal arborization. The usual net effect of stimulating a single ciliary ganglion neurone through an intracellular electrode is constriction of the pupil. Since the terminals of individual ciliary ganglion neurones are commonly distributed to both the sphincter and dilator regions of the iris, impulses invading the terminals of a single ciliary neurone cause a contraction of the sphincter muscle, perhaps accompanied by a simultaneous relaxation of the dilator. Direct examination of individually labelled neurone has revealed details of the pattern of parasympathetic axon distribution to the iris, thus accounting for the origin of cholinergic terminals in the dilator, classically considered to be primarily a target of adrenergic sympathetic neurones.
Noradrenaline modulates calcium channels in avian dorsal root ganglion cells through tight receptor‐channel coupling.
Authors: Forscher P , Oxford G S , Schulz D
Published: December 2014

Averaged ensemble Ba currents were recorded from tissue cultured embryonic chick dorsal root ganglion (d.r.g.) cells using the cell‐attached patch‐clamp technique. Noradrenaline (NA) applied to extrapatch membrane had no clear consistent effect on drug‐free patch currents. This finding supports a previous suggestion that second messengers may not be involved in NA‐mediated decreases in Ca currents in sensory neurones (Forscher & Oxford, 1985). Cell‐attached patch currents sometimes increased slowly after extrapatch application of NA, but were not reversibly decreased by drug treatment. Large patch currents were used to trigger cellular action potentials. NA reversibly decreased action potential duration as reflected in extracellularly recorded patch action currents. Simultaneously recorded inward patch currents were not affected. D.r.g. cell adenylate cyclase activity was assayed. NA did not affect intracellular cyclic AMP levels at concentrations which cause 30‐70% decreases in gCa in dialysed cells (Forscher & Oxford, 1985). Treatment with forskolin (50 microM) or isoprenaline (10 microM) resulted in 60‐ and 2‐fold increases respectively in adenylate cyclase activity over basal levels. These results suggest that NA decreases Ca currents by direct NA interactions with the Ca channel or a molecule tightly coupled to channel function in d.r.g. cells.
Properties of rat medial septal neurones recorded in vitro.
Authors: Segal M
Published: December 2014

Activity of neurones of the rat medial septal nucleus (m.s.) was recorded in in vitro slice preparations. The recorded population could be divided into low (less than 30 M omega)‐ and high‐input‐resistance (greater than 30 M omega) neurones. The high‐resistance neurones tended to fire spontaneous action potentials and post‐synaptic potentials. Some of the spontaneously active cells fired rhythmically at rates of 2‐10 Hz. The rhythmicity disappeared following hyperpolarization of the recorded cell. The cells could fire repetitive Ca2+ spikes in the presence of tetrodotoxin (TTX) and intracellular Cs+. Cd2+ blocked this rhythmicity. Most of the m.s. cells had a non‐linear voltage‐current relation in both the hyperpolarizing and depolarizing directions. Hyperpolarizing rectification was selectively blocked by extracellular Cs+ whereas depolarizing rectification could be blocked by TTX. A recovery from hyperpolarization was associated in many cells with a transient depolarization (anodal break (a.b.) potential). A 20 ms 15 mV hyperpolarization could trigger an a.b. potential. The a.b. potential was reduced by TTX and Cs+ but not by Cd2+ or Mn2+. Depolarization of quiescent neurones triggered action potential discharges. A common pattern of discharge was a burst of two spikes which kept a fairly constant interspike interval. The second spike in a doublet could not follow a rate of 10 Hz depolarizing current pulses. It was also sensitive to topical application of Cd2+. It is therefore suggested that Ca2+ might be involved in the generation of the doublet. Long depolarizing current pulses produced trains of action potentials, showing little accommodation and little after‐hyperpolarization, indicating that these cells possess little Ca2+‐dependent K+ current. Many cells emitted spontaneous post‐synaptic potentials at high rates. These could be blocked by picrotoxin. Stimulation of the lateral septal (l.s.) nucleus produced a Cl‐dependent i.p.s.p. The i.p.s.p. was blocked by picrotoxin. Topical application of gamma‐aminobutyric acid (GABA) produced a marked Cl(‐)‐dependent increase in conductance. It is suggested that l.s. projects a GABA‐mediated inhibitory connexion to the m.s. Acetylcholine (ACh) depolarized m.s. neurones and caused an increase in input resistance. The response was present in TTX or Cd2+‐containing medium. Atropine blocked responses to ACh. 5‐Hydroxytryptamine (5‐HT) hyperpolarized m.s. neurones in a manner consistent with an increase in K+ conductance. The effects of 5‐HT were seen in TTX‐ and Cd2+‐treated m.s. slices.(ABSTRACT TRUNCATED AT 400 WORDS)
Activation of the contractile apparatus of skinned fibres of frog by the divalent cations barium, cadmium and nickel.
Authors: Stephenson D G , Thieleczek R
Published: December 2014

The contractile apparatus of mechanically skinned muscle fibres of frog can be reversibly activated by Ba2+ and Cd2+. The maximum force induced by both Ba2+ and Cd2+ is the same as that induced by Ca2+ and Sr2+. The ionic concentrations of the divalent cations required to induce 50% of the maximum activated force at 1 mM‐Mg2+, pH 7.10, 22 degrees C and 250 mM ionic strength are about 8 X 10(‐7) M for Ca2+, 5 X 10(‐6) M for Cd2+, 2.6 X 10(‐5) M for Sr2+ and 7 X 10(‐4) M for Ba2+. Exposure of the skinned fibre to relatively low Ni2+ concentrations (between 10(‐6) and 10(‐5) M) resulted in a transient force response accompanied by an irreversible change in the ability of the preparation to develop force. The Ba2+‐ and Cd2+‐activation curves are considerably flatter than the corresponding curves for Ca2+ and Sr2+. An increase in Mg2+ concentration from 1 to 3 mM decreased the sensitivity of the contractile apparatus to Ba2+ by a factor of about 1.5 without affecting the maximum force response. The Ca2+‐activation curve was modified in the presence of subthreshold concentrations of Ba2+ and the results indicate that Ba2+ could have both an activating and an inhibitory action on the Ca2+‐activated force. A kinetic model which can quantitatively explain the results for activation of contraction by Ba2+ and Ca2+, is described.
Ionic basis for the electroresponsiveness of guinea‐pig ventromedial hypothalamic neurones in vitro.
Authors: Minami T , Oomura Y , Sugimori M
Published: December 2014

The difference between the ionic bases for electroresponsiveness in the three types of neurone in the ventromedial hypothalamic nucleus (v.m.h.) were studied in in vitro brain slice preparations of the guinea‐pig. The current‐voltage relationships in all three types of neurone showed anomalous rectification. Application of tetrodotoxin (TTX) abolished the fast action potentials in all three types of cell. In type A cells, injections of outward current pulses did not evoke spikes when the cells were perfused with TTX alone, but the addition of tetraethylammonium chloride elicited broad spikes. In type B and C cells, broad spikes could be evoked with TTX alone. These results suggest the presence of a minimal high‐threshold Ca2+ current in type A neurones, and a more prominent one in type B and C neurones. In type B neurones, Ca2+ conductance blockage with Mn2+ or Co2+, or replacement of Ca2+ by Mg2+, abolished the low‐threshold response (l.t.r.). Substitution with Ba2+ did not increase the duration of the l.t.r. significantly, suggesting that under normal conditions the falling phase of the response was caused by inactivation of Ca2+ conductance. In type B and C neurones, the amplitude and duration of the after‐hyperpolarization (a.h.p.) following direct activation by long outward current pulses were markedly reduced in Ca2+‐free solution. These findings indicate that a large component of this response was generated by the Ca2+‐dependent K+ conductance increase. In type A cells the a.h.p. amplitude was originally small and was not affected by the above treatment, suggesting that the participation of this conductance was minimal in this type. In type C neurones, the membrane potential following an inward current pulse showed a delayed return to the base line. This delay was produced by transient K+ conductance, since it was reduced by 4‐aminopyridine. The frequency‐current (f‐I) relation of the first interval in type A and C cells was scarcely affected in Ca2+‐free solution, while the slope of the initial firing f‐I curves in type B neurones which had the l.t.r. became flatter. Furthermore, the f‐I curves of the third interval in type C cells became steeper in Ca2+‐free solution. The data indicate that the distinct membrane characteristics related to the heterogeneity among cells in the v.m.h. can be attributed to their specific ionic mechanisms, with the type A neurones showing a minimal high‐threshold Ca2+ current, the type B having l.t.r. and the type C having a transient K+ (IA) current.
The influence of free calcium on the maximum speed of shortening in skinned frog muscle fibres.
Authors: Julian F J , Rome L C , Stephenson D G , Striz S
Published: December 2014

The influence of [Ca2+] on the maximum velocity of shortening (Vmax) was examined in mechanically skinned Rana pipiens and Rana temporaria fibres using improved force clamps and the slack test techniques. All measurements were made at 7.5 degrees C. At low relative loads (P/P0 less than 0.1), maximally activated R. pipiens fibres shortened more rapidly than did submaximally activated fibres. At higher relative loads, however, little difference in the speed of shortening was observed. Vmax (determined by the slack test) of R. pipiens fibres increased as the level of activation increased. Over sarcomere lengths 1.8‐2.1 microns it was 2.28 muscle lengths/s (m.l./s) (S.E. of mean +/‐ 0.25, n = 5) at 20‐35% activation, 2.89 m.l./s (+/‐ 0.22, n = 7) at 40‐60% activation, and 4.18 m.l./s (+/‐ 0.25, n = 6) at 100% activation. At longer sarcomere lengths (2.2‐2.6 microns), higher Vmax values were observed at all levels of activation, but the influence of Ca2+ on Vmax persisted. Vmax was 3.54 m.l./s (+/‐ 0.41, n = 4) at 20‐30% activation and 5.15 m.l./s (+/‐ 0.22, n = 5) at 100% activation. In R. temporaria fibres, Vmax (determined by force clamps over sarcomere lengths 1.8‐2.1 micron) also increased as the level of activation increased, from 3.47 m.l./s (+/‐ 0.06, n = 6) at 13‐29% activation to 5.62 m.l./s (+/‐ 0.17, n = 6) at 100% activation. Vmax was also determined (using the slack test) in mechanically and chemically skinned rabbit soleus fibres. Vmax at 15 degrees C (1.05 m.l./s, +/‐ 0.11, n = 5) at full activation decreased by more than 3‐fold as the level of activation was reduced to 10%. We conclude that the level of activation influences the Vmax of skinned skeletal muscle fibres. This has now been demonstrated in three different preparations and by a variety of techniques. This effect is most pronounced at low relative loads, and might not be observed if there are experimental limitations which prevent making velocity measurements at low relative loads.
The effect of small changes in arterial carbon dioxide tension on carotid chemoreceptor activity in the cat.
Authors: Cross B A , Leaver K D , Semple S J , Stidwill R P
Published: December 2014

The hypothesis that the fluctuations in carotid chemoreceptor discharge represent more than a simple proportional response to respiratory oscillations of Pa, CO2 has been examined. Simultaneous recordings of the activity in few‐ or multi‐fibre chemoreceptor preparations of the cut carotid sinus nerve and oscillations of carotid arterial pH have been made in four anaesthetized cats which were paralysed and artificially ventilated at constant frequency. Chemoreceptor activity was averaged over a minimum of twenty consecutive pH cycles and the amplitude of the fluctuations in discharge frequency determined. Using the mean Pa, CO2 obtained from blood‐gas analysis and the slope of the relationship between log Pa, CO2 and pH obtained in each cat, the amplitude of the pH oscillations was converted to Pa, CO2 amplitude. This amplitude relative to the mean Pa, CO2 has been compared with the amplitude of the corresponding fluctuation in the chemoreceptor discharge frequency recorded from the carotid sinus nerve (e.s.n.d. amplitude) relative to its mean level. Whereas the Pa, CO2 amplitude was always less than 8% of the mean Pa, CO2, the c.s.n.d. amplitude ranged from 40 to 186% of the mean discharge frequency. C.s.n.d. amplitude was divided by the corresponding Pa, CO2 amplitude to give an index of the sensitivity of the chemoreceptors to Pa, CO2. This sensitivity has been compared with that determined from the mean discharge frequency produced in response to changes in mean Pa, CO2 comparable in magnitude to the Pa, CO2 amplitude. The former sensitivity was usually at least 3 times greater than the latter. Examination of the fluctuations in chemoreceptor discharge frequency in relation to the corresponding pH oscillations revealed that whereas the minimum discharge frequency coincided with the alkaline peak of the pH oscillation (trough of the Pa, CO2 oscillation), the maximum discharge frequency did not invariably coincide with the acid trough of the pH oscillation (peak of the Pa, CO2 oscillation). On 30% of occasions, the maximum discharge frequency was associated with the region of maximum rate of fall of pH (maximum rate of rise of Pa, CO2). It was concluded that fluctuations in the discharge of carotid chemoreceptors cannot be accounted for on the basis of a simple proportional relationship to Pa, CO2. They contain a large rate of change component.
Contraction of human umbilical artery, but not vein, by oxygen.
Authors: McGrath J C , MacLennan S J , Mann A C , Stuart-Smith K , Whittle M J
Published: December 2014

The O2 tension of umbilical arterial blood in utero is 15 mmHg. The contractile effect of increasing the O2 tension above this value was studied quantitatively in vitro in preparations of human umbilical artery and vein. The umbilical arterial smooth muscle was contracted in a concentration‐related manner by stepped increments in O2 tension. The threshold for O2‐induced contraction was estimated to be 36 mmHg at pH 7.28 and the maximum contraction occurred at 297 mmHg. The sensitivity of the preparations to low O2 tensions (less than 100 mmHg) was reduced by increasing the pH of the bathing medium from 7.28 to 7.36, at which the threshold was 69 mmHg and the maximum contraction occurred at 282 mmHg. Reducing the pH to 7.18 did not significantly change the sensitivity from that found at pH 7.28. Indomethacin (0.1 microM) virtually abolished responses to O2. The results indicate that the increase in the physiological O2 tension at birth, from 15 to 100 mmHg, may be an adequate stimulus to effect the closure of the umbilical artery. In the physiological range O2 did not contract the umbilical venous smooth muscle. This may allow the transfusion of blood from the placenta to the fetus at birth when the O2 tension of umbilical cord blood increases after the onset of breathing.
Electrical properties of axons and neurohypophysial nerve terminals and their relationship to secretion in the rat.
Authors: Nordmann J J , Stuenkel E L
Published: December 2014

Isolated rat pituitary stalk‐neurohypophysial complexes were electrically stimulated and the evoked compound action potentials were recorded at the level of both axons and nerve terminals. The latency of the nerve terminal response increased during continuous stimulation of the stalk at frequencies as low as 1 Hz. At similar frequencies continuous stimulation of the stalk produced an increase in the latency of the response of the nerve fibres and a decrease in the amplitude of the compound action potential. The increase in the latency of the response of both axons and nerve terminals was related to the frequency and number of stimuli. The time necessary for full recovery of the response of the axons and the nerve endings, following stimulation at frequencies above 5 Hz, was not linearly related to the frequency of stimulation. Stimulation of the stalk with a pulse pattern (bursts) imitating the electrical activity of vasopressin‐containing magnocellular neurones showed that the latency of the compound action potential had increased by the end of the first burst. The latency of the response of axons and nerve endings was inversely proportional to the time interval between bursts. Prolonged stimulation of the isolated neural lobe with 'vasopressin'‐like bursts induced the release of vasopressin. Twelve bursts, separated by 3 min intervals, released more hormone than fifty bursts given during the same period of time, but separated by a 21 s interval. Leu‐enkephalin (10(‐5) M) did not modify the latency or the amplitude of the action potentials evoked with low frequency of stimulation (0.5 Hz) or with 'vasopressin'‐like bursts. In conclusion, it is suggested that the electrical properties of the nerve fibres and the nerve endings goes some way to explain the pattern of hormone release observed during sustained stimulation.
The regulation of cytosolic calcium in rat brain synaptosomes by sodium‐dependent calcium efflux.
Authors: Nachshen D A , Sanchez-Armass S , Weinstein A M
Published: December 2014

1. When pinched‐off presynaptic nerve endings (synaptosomes) isolated from rat brain are incubated in a low‐Na (24‐36 mM) medium, they take up 45Ca in a time‐dependent manner. In a medium containing 1 mM‐Ca, this Na‐dependent 45Ca uptake amounts to approximately 10 nmol/mg protein at 1 min, and to approximately 40 nmol/mg protein at 20 min. The Na‐dependent Ca uptake is not reduced when the synaptosomes are loaded with concentrations of quin 2 as high as 2 mM. 2. The increase in 45Ca uptake is paralleled by an increase in the free cytosolic Ca concentration [Ca]i, as monitored with the fluorescent Ca indicators quin 2 or fura 2. [Ca]i increases from the value of approximately 200 to approximately 500 nM within 3‐5 min, and thereafter, remains at this elevated level. 3. When synaptosomes that have been loaded with 45Ca (for 1 min, in a low‐Na medium) are diluted into an Na‐containing medium, there is a rapid efflux of the Ca load. After correcting for Ca that is taken up during the efflux period, calculations show that the total Ca in the synaptosomes returns to the control level within 1 min. Measurements of total chemical Ca parallel the measurements made with radiotracer Ca, and confirm that the Ca loaded into the nerve terminals during a 5 min incubation in a low‐Na medium is extruded from the nerve terminals within 1 min in a normal‐Na medium. 4. The efflux of Ca from the synaptosomes is paralleled by a drop of [Ca]i to its basal level, also within 1 min. 5. The mitochondrial uncoupler, carbonyl cyanide p‐trifluoromethyloxy‐phenyl‐hydrazone (FCCP, 1 microM), has no effect on either Na‐dependent Ca uptake or efflux in synaptosomes. FCCP causes a slight (100‐200 nM) increase in [Ca]i in synaptosomes resuspended in either a Na or a low‐Na medium. This indicates that little of the Ca that is taken up by the synaptosomes in a low‐Na medium is sequestered by the mitochondria. 6. These results suggest that Na‐dependent Ca efflux (probably Na‐Ca exchange) plays an important role in allowing nerve terminals to recover rapidly from a Ca load.
Properties of potassium and sodium channels in frog internode.
Authors: Grissmer S
Published: December 2014

1. Voltage‐clamp experiments were performed on single frog internodes after acute demyelination with lysolecithin. The action of lysolecithin was stopped by washing out the lysolecithin with normal Ringer solution containing bovine albumin when the first delayed current was observed. After washing, the temperature was lowered from 25 to 15 degrees C. These procedures greatly prolonged the survival of the demyelinated internode up to 1 h. 2. External tetraethylammonium chloride (TEA+, 110 mM) reduced the K+ current in the internode only to 11% of the control value. 110 mM‐TEA+ increased the time constant tau n of K+ activation by a factor of two in the node and by a factor of four in the internode. 120 mM‐CsCl at the cut ends of the fibre also reduced the delayed outward current recorded at 60 mV in the internode to 11% of the control value, hardly changing the time constant tau n. 3. After a depolarization, the K+ tail current decayed in two phases, suggesting that the K+ conductance of the internodal membrane may be composed of at least two components, a slow one (gKs) and a fast one (gKf). As in the node, the fast K+ conductance of the internode can be further decomposed into two components (gKf1 and gKf2) with different activation potential ranges. The fast phase of the tail current was blocked by external application of 1 mM‐4‐aminopyridine (4‐AP). The slow phase was almost unaltered by 1 mM‐4‐AP. The extrapolated slow tail current was 33% of the total tail current in the internode and 15% at the node, i.e. the proportion of slow K+ channels is larger in the internode than in the node. 4. Tetrodotoxin (TTX)‐sensitive transient inward currents could be measured in the demyelinated internode, provided the large K+ currents were blocked by internal Cs+. The time course, TTX sensitivity, reversal potential and steady‐state inactivation of the transient early inward current indicate that this current is caused mainly by Na+ passing through Na+ channels. 5. The density of K+ and Na+ channels in the demyelinated internode is estimated from the size of the K+ and Na+ current, respectively, and the capacity of the demyelinated segment. The K+ channel density of the internode seems to be about 20 times smaller than in the node, whereas the Na+ channel density in the internode appears to be about 500 times smaller than in the node.