bluuebunny/crossref_metadata_2025 huggingface dataset:
Release and recycling of calcium by the sarcoplasmic reticulum in guinea‐pig portal vein smooth muscle.
Authors: Bond M , Kitazawa T , Somlyo A P , Somlyo A V
Published: December 2014

The amplitude of interrupted contractions evoked by noradrenaline or caffeine in Ca2+‐free, high‐K+ solutions containing EGTA or La3+ was determined in small (40‐60 micron thick) bundles of guinea‐pig portal anterior mesenteric vein. Interrupted contractions were produced by removing the stimulating agent as soon as the amplitude of the tension record reached its peak. The distribution of intracellular Ca2+ was determined, with electron probe X‐ray microanalysis, in cryosections of preparations frozen in the relaxed state and at the peak of noradrenaline‐induced contractions. Interrupted contractions of maximal or near‐maximal amplitudes could be evoked every 2 min for up to 15 min in the virtual absence of extracellular Ca2+. If noradrenaline was allowed to remain in the solution throughout the period of spontaneous relaxation, a subsequent contraction could no longer be evoked in the absence of extracellular Ca2+. Interrupted contractions, similar to those evoked by noradrenaline, could also be stimulated by caffeine. The amplitude of reproducible interrupted contractions in Ca2+‐free, high‐K+ solution was graded with noradrenaline concentration. The ability of these smooth muscles to contract repeatedly and maximally in Ca2+‐free solutions indicates the recycling of Ca2+ released from an intracellular store. The occurrence of these contractions in high‐K+ (depolarizing) solutions supports the conclusion (Devine, Somlyo & Somlyo, 1972) that the release of intracellular Ca2+ is one of the mechanisms of pharmacomechanical coupling. The number of subplasmalemmal regions in which high Ca concentrations (greater than 10 mmol/kg dry wt.) were detected, with approximately 75 nm diameter electron probes, was reduced in muscles frozen at the peak of contraction, from 4.7/cell periphery in the relaxed to 1.4/cell periphery in the contracted preparations. In freeze‐substituted smooth muscles, in which the membranes of the junctional sarcoplasmic reticulum could be visualized, the regions containing high Ca were identified as part of the sarcoplasmic reticulum (s.r.), indicating that the s.r. is the store from which noradrenaline and caffeine release Ca2+.
Synaptic mechanisms involved in the inspiratory modulation of vagal cardio‐inhibitory neurones in the cat.
Authors: Gilbey M P , Jordan D , Richter D W , Spyer K M
Published: December 2014

The respiratory modulation of the activity of vagal cardio‐inhibitory neurones of the nucleus ambiguus of the cat has been investigated by electrophysiological and neuropharmacological techniques. All twenty‐four vagal efferent neurones studied had axons with conduction velocities indicative of B fibres and projected to the right cardiac branches of the vagus. Their spontaneous or DL‐homocysteic acid (DLH)‐evoked activity showed a marked reduction during the phase of inspiration and all showed signs of receiving a baroreceptor input. Ionophoretic application of DLH always excited cardiac vagal motoneurones (c.v.m.s). Application of acetylcholine to these same cells provoked a decrease in firing rate in twelve of the fifteen neurones tested. In ten of these twelve cells simultaneous application of atropine antagonized the effect of acetylcholine. Atropine applied alone enhanced neuronal firing, particularly in inspiration. Stable intracellular recordings have been made from two c.v.m.s. These were inhibited during inspiration. Input resistance fell markedly during inspiration and injection of chloride reversed this wave of hyperpolarization to a wave of depolarization indicating that this resulted from chloride‐mediated inhibitory post‐synaptic potentials (i.p.s.p.s). These c.v.m.s were activated during Stage I expiration, and showed a weak and variable wave of inhibition in Stage II expiration. Pulse‐rhythmic depolarizing potentials were reduced in their amplitudes during the periods of decreased neurone input resistance. It is concluded that c.v.m.s receive an excitatory input during post‐inspiration and a powerful inhibitory synaptic input during inspiration. The implications of these observations for the physiology of cardiorespiratory reflexes are discussed.
Voltage‐dependent dye coupling at a rectifying electrotonic synapse of the crayfish.
Authors: Giaume C , Korn H
Published: December 2014

At the crayfish giant motor synapse, the lateral giant axon (l.g.a.) and the giant motor fibre (g.m.f.) form an electrotonic junction which exhibits two states of ionic coupling (Furshpan & Potter, 1959a; Giaume & Korn, 1983). Junctional conductance is low at resting membrane potentials (i.e. with lateral axon more negative than the motor fibre) and high when the polarity of the voltage difference (delta V) across the synapse is reversed. For these two states of conductance, junctional permeability was investigated using the intercellular tracer Lucifer Yellow. The dye was ionophoretically injected into either the presynaptic (l.g.a.) or the post‐synaptic (g.m.f.) cell. In the high conductance state (delta V greater than 0), fluorescence was detected in both neurones whether Lucifer Yellow had been injected pre‐ or post‐synaptically. By contrast, at the resting junctional polarization (delta V less than 0) Lucifer Yellow spread from the giant axon to the g.m.f., but not from the g.m.f. to the giant axons. These data demonstrate that dye transfer at the giant motor synapse, like ionic coupling, is sensitive to junctional polarization and is more marked in the high conductance state. Possible explanations for the asymmetry observed in the low conductance state are discussed.
Changes in miniature end‐plate potentials after brief nervous stimulation at the frog neuromuscular junction.
Authors: Doherty P , Hawgood B J , Smith I C
Published: December 2014

The amplitude of miniature end‐plate potentials (m.e.p.p.s), recorded at the frog neuromuscular junction in a normal ionic environment and in absence of drugs, was examined following 10‐450 nerve impulses using conventional electrophysiological techniques and on‐line computational analysis. In both contracting preparations and non‐contracting preparations pre‐treated with glycerol, 100 or more nerve impulses resulted in a maximal fall in mean amplitude of about 20% with recovery apparent over the next 10‐20 min. In an altered ionic environment with a lowered Ca and raised Mg concentration, 450 nerve impulses did not produce a decrease in mean amplitude but a similar reduction was seen following a larger number of impulses. The reduction in amplitude was estimated to follow the release of the order of 5000‐10000 quanta at end‐plates in a normal ionic environment and on average 17000 quanta in the presence of a lowered Ca and raised Mg concentration. Changes in the mean size of the spontaneous quantal response is considered to be a presynaptic event and to reflect the loss and slow recovery of larger packets of transmitter from a vesicular store that is readily released by nerve impulses.
Two calcium currents in Neanthes arenaceodentatus egg cell membranes.
Authors: Fox A P , Krasne S
Published: December 2014

Two distinct types of Ca currents, Ca(I) and Ca(II), were found in the eggs of the marine polychaete Neanthes arenaceodentatus and studied under voltage‐clamp conditions. Ca(I) and Ca(II) channels differ in their selectivity sequences, show different sensitivities to blocking by Cd, and have different activation thresholds. These facts indicate that the channels responsible for Ca(I) and Ca(II) currents are unique and different. Both Ca(I) and Ca(II) currents decrease with time under a maintained depolarization. This relaxation exhibits different kinetics, with those for Ca(II) being an order of magnitude slower than for Ca(I). The Ca(I) current relaxation has been shown previously to be due to a voltage‐dependent inactivation. The magnitude of relaxation of the Ca(II) current elicited by a test‐voltage step immediately following a conditioning‐voltage step paralleled the magnitude of the peak of the Ca(II) current flowing during the conditioning pulse. The kinetics of the current relaxation depended upon bath Ca concentration, the kinetics slowing down as the bath Ca was increased. These observations are consistent with an external depletion being the cause of the current relaxation for the Ca(II) channel.
Membrane patches and whole‐cell membranes: a comparison of electrical properties in rat clonal pituitary (GH3) cells.
Authors: Fernandez J M , Fox A P , Krasne S
Published: December 2014

A comparison has been made between the electrical properties of excised 'outside‐out' patches and whole‐cell membranes of GH3 cells using the patch‐pipette technique. Despite a complicated surface morphology, which includes numerous microvilli, ruffles and blebs, high‐resistance seals (typically greater than 10(11) omega) were consistently formed between patch pipettes and GH3 cell membranes. When the internal solution contained 120 mM‐CsF, outward currents through K channels were blocked and large Na channel currents were consistently observed in the whole‐cell recording mode. Using the same solutions, single Na channel currents were readily observed in outside‐out patches. Averaging patch currents yielded macroscopic currents showing the same voltage‐dependent kinetics as those observed for the whole‐cell membrane. The current vs. voltage and inactivation time constant vs. voltage relationships for the Na channel shifted towards more negative potentials (25 mV or more) within approximately 30 min after going into the whole‐cell recording mode. These same relationships could be measured for outside‐out patches and their positions along the voltage axis coincided with the asymptotic values measured in the whole‐cell mode. When the internal solution contained 120 mM‐N‐methylglucamine fluoride and the external solution contained 120 mM‐Tris chloride, no ionic channel currents could be observed either for whole‐cell or outside‐out patch membranes. Under these conditions, displacement currents induced by tetraphenyl borate (TPB) were recorded in both types of membranes. The total charge moved showed a sigmoidal dependence upon the applied voltage for both whole‐cell and outside‐out patch membranes. The charge vs. voltage relationship showed a shift along the voltage axis similar to that observed for Na channels except that the magnitude of the shift was larger. A shift in this relationship was also observed for excised patches but the observable magnitude of the shift was smaller than that in the whole‐cell recording mode. The asymptotic values of the charge vs. voltage relationship were similar for whole‐cell and outside‐out patches, as were the asymptotic values for the translocation time constants. It is concluded that there are no fundamental differences in the properties of ionic channel and displacement currents between whole‐cell membranes and excised membrane patches.(ABSTRACT TRUNCATED AT 400 WORDS)
Fertilization‐induced ionic conductances in eggs of the frog, Rana pipiens.
Authors: Jaffe L A , Schlichter L C
Published: December 2014

Fertilization of the frog egg (Rana pipiens) elicits a positive‐going shift in membrane potential (fertilization potential) that lasts 10‐20 min and functions as a fast block to polyspermy. We examined the ion conductances underlying the fertilization potential, using the voltage‐clamp technique. We measured the membrane capacitance during the fertilization potential by applying an alternating current. We also determined the intracellular K and Cl concentrations in the egg, using ion‐selective micro‐electrodes. The conductance is largest in the first 2 min after fertilization. Regardless of whether the stimulus is provided by one or by more than one sperm or by artificial activation, the size of the conductance increase is the same, reaching a maximum of about 40 microseconds. Two separate conductances are involved at fertilization: Cl and K. [K]i = 121 mM and [Cl]i = 44 mM. The natural external medium is pond water (approximated in our experiments by 10% Ringer solution); therefore, an increase in K and Cl conductances leads to an efflux of both ions. The equilibrium potential of the fertilization current is between the Cl and K equilibrium potentials (ECl and EK), closer to ECl. 10 mM‐external tetraethylammonium (TEA) brings the equilibrium potential close to ECl and reduces the maximum conductance by about half. The Cl conductance is not blocked by 4‐acetamido‐4'‐isothiocyanostilbene‐2,2'‐disulphonic acid (SITS). The time courses of the K and Cl conductances are similar. The TEA‐resistant conductance (primarily Cl conductance) activated at fertilization increases as the membrane potential becomes more positive. A voltage‐sensitive Na conductance present in the unfertilized egg disappears after fertilization. During fertilization this conductance is too small to contribute significantly to the fertilization potential. The membrane capacitance increases by an average of 1.9 times in the first 2 min following the rise of the fertilization potential, during the period of cortical vesicle exocytosis. Capacitance then gradually decreases; at 1 h after fertilization, capacitance is 82% of the value in the unfertilized egg. The conductance increase precedes the capacitance increase by several seconds. Therefore the initial appearance of Cl and K channels cannot be accounted for by addition of membrane by cortical vesicle exocytosis. The conductance subsequently decreases, suggesting that the disappearance of the Cl and K channels is not caused by membrane removal.
The sagittal vestibulocollic reflex and its interaction with neck proprioceptive afferents in the decerebrate cat.
Authors: Dutia M B , Hunter M J
Published: December 2014

The sagittal vestibulocollic reflex (v.c.r.) evoked by nose‐up, nose‐down movements of the head, has been studied in the neck extensor muscle biventer cervicis in the decerebrate cat. Nose‐down movements of the head increased, and nose‐up movements decreased, electromyographic (e.m.g.) activity in the biventer cervicis muscles of the left and right sides. At low frequencies of sinusoidal head movement (0.1‐0.5 Hz), the gain of the sagittal v.c.r. was approximately constant, and e.m.g. modulation showed a phase lead of about 40 deg with respect to head position. At higher frequencies (2‐5 Hz), v.c.r. gain increased at a rate close to 40 dB/decade, and phase lead increased to approach 150 deg. The relation between head movement and v.c.r. activity may be described by a transfer function containing two lead terms, with time constants of 0.07 and 0.23 s, and two lag terms, with time constants of 5.3 and 9.9 s. When movements of the head were accompanied by stretching of the biventer cervicis muscles, the gain of the sagittal v.c.r. was increased threefold, at all frequencies between 0.1 and 5 Hz, with no substantial change in phase. Sinusoidal stretching of the biventer cervicis muscles, with the head stationary, evoked a stretch reflex (cervicocollic reflex, c.c.r.) the behaviour of which was similar to that of a second‐order system described by a transfer function containing two lead terms with time constants of 0.07 and 0.16 s. This difference in dynamics between the v.c.r. and the c.c.r. indicates that the lag terms in the v.c.r. transfer function reflect the frequency‐response properties of the vestibular pathway to the biventer cervicis muscles, as they do not appear when the same muscles participate in the c.c.r. The vectorial differences between the frequency‐response of the sagittal v.c.r. with and without concomitant stretching of the biventer cervicis muscles is quantitatively similar to the frequency‐response of the c.c.r. evoked by sinusoidal stretching. The inputs from the vestibular and stretch receptors thus appear to sum linearly to produce the increase in v.c.r. gain, at least over the frequency range 0.1‐1 Hz. Since most head movements, and all voluntary head movements, involve a rotation of the head in relation to the neck, the potentiation of the gain of the v.c.r. by afferents from stretch receptors (presumably muscle spindles) in the neck muscles is an important factor in the normal reflex stabilization of head position.
Conduction velocity is related to morphological cell type in rat dorsal root ganglion neurones.
Authors: Harper A A , Lawson S N
Published: December 2014

Combining intracellular recording and dye‐injection techniques permitted direct correlation of neuronal soma size with peripheral nerve conduction velocity in individual neurones of the L4 dorsal root ganglion (d.r.g.) of the anaesthetized 5‐8‐week‐old rat. The conduction velocities fell into two main groups; those greater than 14 m/s (A alpha and beta fibres) and those less than 8 m/s (A delta and C fibres). Fibres with conduction velocities in the A delta range (2.2‐8 m/s) in the sciatic nerve between the sciatic notch and the neuronal soma in the d.r.g. often conducted more slowly, that is in the C‐fibre range (less than 1.4 m/s), in the periphery from the tibial nerve to the sciatic notch. For the fast‐conducting myelinated afferents, there was a loose positive correlation between cell size and the conduction velocity of the peripheral axon, whereas a clearer positive correlation existed between neuronal cell size and axonal conduction velocity both for A delta‐ and for C‐fibre afferents. The relationship of the cell cross‐sectional area (measured at the nucleolar level), to the cell volume for each neuronal soma was similar for the different conduction velocity groups. The somata of the fast‐conducting myelinated A alpha and A beta fibres had a similar mean and range of cross‐sectional areas to those of the large light cell population. The somata with A delta and C fibres were of a more uniform size and were restricted to the smaller cells within the ganglia. The mean and range of cross‐sectional areas of the C cells was similar to those of the small dark cell population. A delta somata had a larger mean and range of cell sizes than those of the small dark cell population. The relationships of peripheral axon type to the morphological cell types are discussed.
Sodium‐dependent control of intracellular pH in Purkinje fibres of sheep heart.
Authors: Ellis D , MacLeod K T
Published: December 2014

Intracellular pH (pHi) of Purkinje fibres from sheep heart was recorded with pH‐sensitive glass micro‐electrodes. The cells were acidified by one of three methods: (1) exposure to and subsequent removal of NH4Cl, (2) exposure to solutions containing 5% CO2 or (3) exposure to an acidic Tyrode solution. The pHi recovery from these acidifications was studied. The time constant of recovery from an acidification induced by NH4Cl was almost twice as long as that from one induced by CO2 or acid extracellular pH. Following an acidification induced by exposure to CO2 the time constant of pHi recovery was not changed when the cell was depolarized to ‐40 mV (by replacement of some Na+ by K+). An intracellular acidification was produced when extracellular Na+ was removed and replaced by quaternary ammonium ions or K+. Such Na+‐free solutions also inhibited pHi recovery from an acidification. A 50% inhibition of the rate of recovery was produced by lowering the [Na+]o to 8 mM. When used as a Na+ substitute, Li+ could permit recovery. Tris (22 mM) changed pHi in the alkaline direction. Amiloride (1 mM) or a decrease in temperature slowed the recovery from an acidification (Q10 = 2.65). There was no effect of SITS (4‐acetamido‐4'‐isothiocyanatostilbene‐2,2'‐disulphonic acid disodium salt; 100 microM) on the recovery. Na+‐sensitive glass micro‐electrodes were used to measure the intracellular Na+ activity when [Na+]o was lowered to levels used in our pHi recovery experiments. From these data we have calculated the apparent Na+ electrochemical gradient at different values of [Na+]o. If this gradient is responsible for H+ efflux from the cell then, by applying thermodynamic considerations, it can be shown that only low concentrations (1‐2mM) of extracellular Na+ are required. Solutions containing a very low [Ca2+]o (less than 10(‐8) M, buffered with EGTA) were used to prevent large rises of [Ca2+]i which may occur on removal of external Na+. Under these conditions pHi recovery is still dependent upon [Na+]o, and the apparent inhibition of pHi recovery by removal of Na+ is not simply due to rises in [Ca2+]i. The intracellular acidification which occurs on removal of Na+ does not occur when [Ca2+]o is very low (less than 10(‐8) M).(ABSTRACT TRUNCATED AT 400 WORDS)
Behaviour of motor units of human arm muscles: differences between slow isometric contraction and relaxation.
Authors: Denier van der Gon J J , ter Haar Romeny B M , van Zuylen E J
Published: December 2014

The behaviour of motor units in the m. biceps brachii (long head), in the m. brachialis and in the m. supinator during slow isometric contraction and relaxation was studied when subjects were performing different motor tasks. These tasks were: flexion of the elbow joint, supination of the forearm and exorotation of the humerus. Motor unit activity was recorded by means of bipolar fine wire electrodes. In the long head of the biceps, motor unit activity was recorded at medial, central and lateral sites. When the subject relaxed from flexion, the firing rate of motor units located in the biceps and the brachialis was always found to be lower than that at the corresponding level of flexion force during contraction. The firing rate during relaxation decreased slowly and almost linearly with force. However, during relaxation from supination or exorotation, the firing rate of motor units at medial and central locations in the biceps was more or less constant until decruitment. The firing rate of motor units of the supinator during relaxation from supination decreased slowly and was lower than during contraction. Motor units located medially and centrally in the biceps had decruitment thresholds for flexion that were lower than their recruitment thresholds. Motor units on the lateral side of the biceps did not show such a difference. In the brachialis decruitment thresholds for flexion were usually higher than the recruitment thresholds. Differences between decruitment and recruitment thresholds for motor units in the biceps were much more pronounced for supination and exorotation than for flexion. For motor units in the supinator the decruitment threshold during relaxation from supination was higher than the recruitment threshold. The time that had passed after the onset of firing of a motor unit did not influence its decruitment threshold. If, after complete relaxation, the exerted force was increased again, it appeared that the recruitment threshold was changed. It took about 4 s to reach the original recruitment threshold. It is concluded that the relation between the firing rate of a motor unit and total exerted force depends on the phase of contraction. This relation varies within a muscle and between muscles. Furthermore, the results indicate an interchange of activity within the motoneurone pools of the synergists involved in isometrical motor tasks.
Temperature‐dependent calcium sensitivity changes in skinned muscle fibres of rat and toad.
Authors: Stephenson D G , Williams D A
Published: December 2014

Single mechanically skinned muscle fibres of different types (fast‐ and slow‐twitch mammalian; slow and twitch amphibian) were successively activated in solutions of various Ca2+ concentrations at different temperatures. An increase in temperature from 5 to 22 degrees C reversibly shifted the isometric steady‐state force‐pCa curves towards higher Ca2+ concentration for individual fibres of each of the muscle types. A further increase in temperature to 35 degrees C in mammalian fibres resulted in an additional decrease in Ca2+ sensitivity. The temperature dependence of Ca2+ sensitivity was greater in the 'faster' fibre types: fast‐twitch greater than slow‐twitch; twitch greater than slow. The maximum isometric force response, P0, of both rat and toad skinned fibres was found to be strongly dependent on temperature below 22 degrees C. No detectable force could be induced by Ca2+ in mammalian muscle fibres at 0‐1 degree C while in toad fibres P0 decreased by about 90% when temperature dropped from 20 to 0 degree C. Since in mechanically skinned fibres of other amphibians (Bufo bufo, Rana species) P0 is only marginally affected it is likely that the P0‐temperature relations are indicative of the range of temperature over which the muscles are normally functional. The P0‐temperature relations of skinned muscle fibres closely resembled the P0‐temperature relations of tetanically stimulated intact muscle preparations from the same species of animals suggesting that the contractile apparatus is mainly responsible for the variation of force response with temperature in intact muscle.
Slow depolarizing and hyperpolarizing currents which mediate bursting in Aplysia neurone R15.
Authors: Adams W B
Published: December 2014

Interruption of normal bursting activity by application of a voltage clamp reveals that action potentials in Aplysia neurone R15 are followed by two slow currents that long outlast the currents produced during the action potentials. Similar currents are seen following simulation of an action potential with a brief depolarizing pulse delivered under continuous voltage clamp. One of these currents, herein called ID, is an inward, or depolarizing current 0.5‐5 nA in amplitude that reaches a peak 300‐500 ms after the action potential. It produces the depolarizing after‐potential that follows action potentials in this cell and is responsible also for the grouping together of action potentials into bursts. The second current, herein called IH, is an outward, or hyperpolarizing current 0.1‐2 nA in amplitude that reaches a peak in 2‐10 s and is still present for many tens of seconds following the action potential. IH mediates the interburst hyperpolarization. Both currents summate temporally during the burst. Despite changes in the amplitude and duration of action potentials during the burst, each action potential adds nearly constant increments to the summated amplitudes of ID and IH. The summated amplitude of ID grows during the first few action potentials and gives rise to the increased rate of depolarization and the increased firing rate seen during the first half of the burst. Due to its slower kinetics, IH summates throughout the burst until its summated amplitude is large enough to cause the cell to hyperpolarize, thereby bringing the burst to an end. When the normal burst is interrupted by application of the voltage clamp, the ID and IH current peaks are followed by a current which approaches a more negative steady‐state level with a time course that consists of at least two phases. The first phase is exponential with a time constant of 15‐30 s. Under continuous voltage clamp, the current following a train of depolarizing pulses returns to the holding current with a similar time course. These observations, together with time constants for IH that are longer than the interburst interval, suggest that IH is always partially activated during normal bursting. A computer simulation demonstrates that opposing inward and outward currents with different kinetics, i.e. ID and IH, are sufficient to give rise to bursting activity, in the absence of non‐linear voltage‐dependent conductances. Such voltage‐dependent conductances, which are present in the normal cell, contribute to but are not necessary for bursting activity.
Transport of sodium and chloride across rat gastric mucosa in vitro.
Authors: Jackson M J , Norris S H
Published: December 2014

The effects of ion substitution, inhibitors and variations in transmural p.d. on the movements of sodium and chloride across an in vitro preparation of rat gastric mucosa have been studied. The tissue maintained net steady‐state transport of sodium in the mucosal‐to‐serosal direction in the absence of transmural gradients of electrochemical potential. Sodium transport was independent of the presence of chloride, and was abolished by 1 X 10(‐5) M‐amiloride. The inhibitor produced a decrease in short‐circuit current equivalent to the depression of sodium transport, indicating that the sodium transport process was electrogenic. Variations in transmural p.d. showed that the sodium transport process included two components: one that varied with p.d. and one that was independent of it. These findings have been interpreted in terms of a system for sodium transport composed of three components: two rate‐limiting entry mechanisms at the apical membrane, one of which can be represented as a conductive channel for sodium diffusion and the other as a neutral process possibly a sodium‐hydrogen exchanger, and a voltage‐independent pump at the basolateral membrane analogous to the constant‐current pump models described in some other epithelia. The tissue maintained a net secretory movement of chloride in the short‐circuited condition. The process responsible for net transport of chloride could be resolved into two components: one that was sodium dependent, electrogenic, and abolished by 8 X 10(‐3) M‐acetazolamide, and one that was independent of the presence of sodium, electrically silent and abolished by 5 X 10(‐4) M‐SITS (4‐acetamido‐4'‐isothiocyano‐2,2'‐disulphonic acid stilbene). Both components of the chloride transport process varied with p.d. These findings were interpreted in terms of a system of three components: two entry mechanisms at the basolateral membrane including a coupled sodium‐chloride influx process and a chloride‐bicarbonate exchanger in parallel, and a rate‐limiting conductive channel at the apical membrane. In addition, the studies on the effects of variations in transmural p.d. on chloride fluxes revealed a symmetrical voltage‐independent component, dependent on the presence of chloride in the trans compartment, and it was suggested that this component may reflect the presence of a chloride‐chloride exchange mechanism.
Sodium and potassium distribution in the lactating mouse mammary gland in vivo.
Authors: Berga S E , Neville M C
Published: December 2014

The ratio of intracellular potassium to sodium in the mammary cells of the lactating mouse was compared with the ratio of potassium to sodium in the mouse milk to determine whether the sodium and potassium concentrations in milk are governed by a Donnan equilibrium as postulated earlier (Peaker 1977a, 1978). An efflux technique was used to determine the average intracellular sodium of 23.0 +/‐ 1.2 mumol/g tissue (+/‐ S.E. of mean). The intracellular potassium, determined by calculating the amount of potassium contained in both the interstitial and milk spaces and subtracting these values from the total tissue potassium, was 62 +/‐ 1 mumol/g tissue. The mean intracellular potassium to sodium ratio, calculated from individual efflux experiments, was 2.7 +/‐ 0.2. The total, interstitial, and milk water spaces were measured by tissue drying, sodium efflux, and lactose efflux, respectively. The average values (+/‐ S.E. of mean) obtained were 0.700 +/‐ 0.004 ml/g tissue, 0.150 +/‐ 0.016 ml/g tissue and 0.064 +/‐ 0.004 ml/g tissue. Based on these values the intracellular water space was 0.49 +/‐ 0.02 ml/g tissue. Intracellular concentrations of sodium and potassium calculated from the intracellular amounts of sodium and potassium and intracellular water space were 47 +/‐ 3 mM and 129 +/‐ 5 mM, respectively. The concentrations of potassium and sodium in mouse milk were 47 +/‐ 1 mM and 26 +/‐ 1 mM. The mean potassium to sodium ratio (+/‐ S.E. of mean) calculated from individual milk samples was 1.8 +/‐ 0.1. The milk ratio of potassium to sodium is significantly different (P less than 0.001) from the intracellular water ratio of potassium to sodium. This finding, in a tight epithelium such as the lactating mouse mammary gland, suggests that both sodium and potassium cannot be distributed passively across the apical membrane and an active transport process must exist for one or both of these ions in this membrane.
Calcium‐activated potassium channels in isolated presynaptic nerve terminals from rat brain.
Authors: Bartschat D K , Blaustein M P
Published: December 2014

86Rb efflux was examined in isolated presynaptic nerve terminals (synaptosomes) from rat brain in a study designed to assess K permeability (PK) changes sensitive to alterations in internal Ca activity. Rb efflux from 86Rb‐loaded synaptosomes into nominally Ca‐free physiological saline (PSS) containing 5 mM‐K was about 0.3‐0.4%/s. Raising extracellular K concentration [( K]o), to depolarize the synaptosomes, stimulated the 86Rb efflux. Addition of Ca to the 5 mM‐K PSS had no effect, but Ca did further stimulate 86Rb efflux into K‐rich solutions. The effect of Ca was graded, with apparent half‐maximal activation, KA approximately equal to 0.5 mM‐Ca. These data fit the view that, during depolarization, Ca enters the terminals through voltage‐regulated Ca channels, and that the rise in intracellular Ca concentration opens certain (Ca‐activated) K channels. The Ca‐dependent stimulation of 86Rb efflux was greatest during the initial seconds of incubation (component CT), and then declined to a much lower rate (component CS). Much of this change in rate could be attributed to inactivation of voltage‐regulated Ca channels and reduced entry of Ca. The Ca‐dependent increase in 86Rb efflux was completely inhibited by 100 microM‐La. In the presence of Ca, but not in its absence, the Ca ionophore A23187 stimulated 86Rb efflux both in 5 and 100 mM‐K PSS. The effect in 100 mM‐K was quantitatively greater, perhaps because of the increased outward driving force on Rb in depolarized synaptosomes. When synaptosomes were suspended in media containing the voltage‐sensitive fluorescent dye, DiS‐C3‐(5) (1,1'‐dipentyl‐2,2'‐thiocarbocyanine), the addition of Ca+ A23187 decreased the fluorescence intensity (= synaptosome hyperpolarization) when the media contained 5 mM‐K but not 100 mM‐K. This implies that in the presence of Ca + A23187, PK was increased, and the membrane potential moved closer to the K equilibrium potential, EK. Quinine sulphate, a blocker of Ca‐activated K channels, reduced the Ca‐stimulated 86Rb efflux with high affinity (apparent half‐maximal inhibition, KI approximately equal to 1 microM). Tetraethylammonium chloride, another agent known to block Ca‐activated K channels, was also a relatively potent inhibitor of Ca‐stimulated 86Rb efflux (KI approximately equal to 0.2 mM). The K‐channel blocker, 4‐aminopyridine, partially inhibited Ca‐stimulated 86Rb efflux at concentrations below 0.5 mM, but stimulated this efflux at concentrations greater than or equal to 1 mM.(ABSTRACT TRUNCATED AT 400 WORDS)
Effect of excitatory amino acids on gamma‐aminobutyric acid release from frog horizontal cells.
Authors: Cunningham J R , Neal M J
Published: December 2014

The effects of excitatory amino acids, analogues and K on [3H]gamma‐aminobutyric acid [3H]GABA) release from horizontal cells of the isolated superfused frog retina were studied. Exposure of the retina to medium containing high concentrations (25‐100 mM) of KCl increased the release of [3H]GABA to a maximum which was 40 times the spontaneous resting release. The K‐evoked release of [3H]GABA was almost abolished in high‐Mg/low‐Ca medium. Glutamate, aspartate, kainate and quisqualate also stimulated the release of [3H]GABA from horizontal cells, the maximum evoked release being similar to that produced by KCl. The release of [3H]GABA evoked by glutamate, aspartate, kainate and quisqualate was abolished in high‐Mg/low‐Ca medium and by Na‐free medium. The evoked releases of [3H]GABA were not reduced by tetrodotoxin. N‐Methyl‐D‐aspartate (NMDA) at concentrations up to 10 mM had virtually no effect on [3H]GABA release from horizontal cells. In Mg‐free medium, NMDA stimulated [3H]GABA release, but the maximum release was only 10% of that produced by other agonists. Mg‐free medium did not significantly affect the evoked release of [3H]GABA by other agonists. NMDA apparently possessed affinity for the kainate receptor, because in normal medium it antagonized the effects of kainate but not glutamate, aspartate or quisqualate. The non‐selective antagonist of excitatory amino acids, (+/‐)‐cis‐2,3‐piperidine dicarboxylic acid (PDA) antagonized the action of glutamate, aspartate, kainate and quisqualate on horizontal cell [3H]GABA release. D(‐)‐2‐Amino‐4‐phosphonobutyrate (APB) and D‐gamma‐glutamylglycine (D‐gamma‐GG) antagonized the actions of kainate on horizontal cell [3H]GABA release at concentrations which had little affect on quisqualate‐evoked responses. Approximate estimates of pA2 values (Schild, 1947) showed that the specificity and potency of the antagonists was low. Nevertheless, the retinal 'non‐NMDA' receptors can probably be subdivided into kainate and quisqualate types. Glutamate diethylester (GDEE) did not affect the action of any agonist. We conclude that glutamate (and aspartate) probably stimulate the release of [3H]GABA from frog horizontal cells by activating receptors of the non‐NMDA type. This activation may trigger the opening of tetrodotoxin‐insensitive Na channels, resulting in the depolarization of the cell membrane and an increase in the conductance of voltage‐sensitive Ca‐channels. An influx of Ca ions would then trigger the release of [3H]GABA. Our results are not consistent with previous suggestions that GABA release from horizontal cells involves an outwardly directed transport process.
Effects of certain metabolites on pancreatic endocrine responses to stimulation of the vagus nerves in conscious calves.
Authors: Bloom S R , Edwards A V
Published: December 2014

The effects of exogenous glucose (0.05 mmol/kg . min) and vamin (0.02 mmol/kg . min) on the pancreatic endocrine responses to stimulation of the peripheral ends of the vagus nerves have been investigated in conscious 3‐6‐week‐old calves with cut splanchnic nerves. Exogenous glucose potentiated both the basal release of insulin and that which occurred in response to vagal stimulation, while inhibiting both the basal release of glucagon and that during vagal stimulation. Vamin significantly inhibited basal release of insulin but not that which occurred during vagal stimulation although it significantly inhibited vagal release of glucagon. The inhibitory effect of exogenous glucose on the basal and vagally stimulated release of pancreatic glucagon were both significantly reduced in the presence of vamin. Neither glucose nor mixed amino acids were found to affect the release of pancreatic polypeptide either at rest or during nerve stimulation. It is concluded that the effects of vagal activity on the alpha‐ and beta‐cells of the islets of Langerhans are normally modified by the existing concentration of both glucose and amino acids in these animals.
Signals from the oropharynx may contribute to the diuresis which occurs in man to drinking isotonic fluids.
Authors: Gebruers E M , Hall W J , O'Brien M H , O'Leary D , Plant W D
Published: December 2014

The drinking of a variety of isotonic salt solutions provoked a short load‐dependent diuretic response in man, similar in latency to that of a water diuresis and reaching its maximum 40‐60 min from the start of drinking. Subjects differed in the threshold load at which the response became evident, and also in the magnitude of their responses. Changes in free water clearance matched the increases in urinary minute volume, with no significant change in either urinary sodium or potassium excretion. Changes in urinary pH and in ammonium excretion were similar to those described for a water diuresis. Drinking an isotonic solution of mannitol provoked a diuresis similar to that of the salt solutions. All subjects had diarrhoea after drinking the mannitol. Signals arising from the oropharynx might be partly responsible for eliciting the diuresis. The significantly smaller urinary responses to infusions of isotonic salt solutions directly into the stomach support this view. The absence of a significant response to 'sham drinking' appears inconsistent with this, but oropharyngeal signals may only have a priming role in man and interact with other signals to give the full response.
The reduction of calcium current associated with early differentiation of the murine embryo.
Authors: Mitani S
Published: December 2014

Membrane currents of intact oocytes and early embryos of the mouse and the hamster were analysed with voltage‐clamp techniques. In both mouse and hamster the amplitude of Ca inward currents decreased with time during early development, and they were undetectable by the 8‐cell stage, while the threshold potential, alkaline earth cation selectivity, and activation‐inactivation kinetics remained unchanged. The reduction of Ca currents was further confirmed in the 2‐cell embryo whose cleavage was arrested with use of cytochalasin D, but the process was slightly delayed by comparison with that of the intact embryo. Early differentiation of cytochalasin‐D‐treated embryos was comparable to that of the intact embryo in terms of intercellular couplings and intercellular fluid accumulation. But these processes were also delayed as in the case of Ca current reduction. The outward current in the hamster embryo which was reflected in the resting membrane conductance began to increase abruptly after the 2‐cell stage and seemed to reach the maximum at the end of the 4‐cell or 8‐cell stage. The increase apparently occurred reciprocally with the decrease in Ca inward current. A similar but much smaller increase in resting membrane conductance also occurred in the cleavage‐arrested mouse 2‐cell embryo almost at the same development stage at which the abolition of Ca current was found. The possibility is discussed that Ca channels have a role in cell differentiation in early murine embryos.
The luminance origin of the pattern electroretinogram in man.
Authors: Riemslag F C , Ringo J L , Spekreijse H , Verduyn Lunel H F
Published: December 2014

Electroretinograms (e.r.g.s) and visually evoked potentials (v.e.p.s) to pattern stimuli were recorded simultaneously from healthy subjects. The stimuli were produced by a configuration in which the luminance of two sets of spatial elements (checks) could be modulated independently. Experiments were designed to distinguish between contrast responses and non‐linear luminance responses. In the first of two basic experiments, the luminance of only one set of checks was modulated, at a constant level in every trial. The other set was not modulated, but its luminance was set at various levels. Under these conditions the local luminance stimulation was kept equal for every trial whereas the contrast stimulation varied. Therefore, local luminance responses in these experiments were expected to be constant and contrast responses were expected to vary. The e.r.g.s were identical for all luminance settings of the unmodulated checks, suggesting that luminance rather than contrast determines the response. The v.e.p.s showed, on the contrary, the behaviour expected for contrast responses. In the second basic experiment the local luminance stimulation was also kept constant, but the phase difference between the modulations of the two sets of checks was varied between 0 deg (pure luminance stimulation) and 180 deg (pattern reversal). In this type of experiment the second harmonic responses to local luminance modulation are expected to decrease to a minimum as phase difference goes from 0 to 90 deg and increase again as phase difference goes to 180 deg. Contrast responses are expected to increase monotonically from zero to maximal at phase difference shifts from 0 deg (no contrast stimulation) to 180 deg (contrast reversal). The e.r.g.s decreased to a minimum at 90 deg phase difference and increase again with phase difference going to 180 deg. At 0 and 180 deg the same value was recorded. Consequently, the e.r.g. behaviour suggests a luminance origin of the responses. The v.e.p.s monotonically increased as phase difference went from 0 to 180 deg, thus suggesting a contrast origin of the responses. Two additional control experiments were performed. The first experiment compared the responses to homogeneous field red/green exchange and pattern red/green exchange, with the luminances of the red and green sources matched by heterochromatic flicker photometry. The exchange of luminance‐matched red and green checks (pattern reversal) did not produce different e.r.g. responses from those recorded in response to homogeneous field red/green exchange. The v.e.p. showed a significant increase for the pattern stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)
Potassium movements associated with amino acid and sugar transport in enterocytes isolated from rabbit jejunum.
Authors: Brown P D , Sepúlveda F V
Published: December 2014

Active transport of amino acids and the sugar, alpha‐methyl‐D‐glucoside (alpha‐MG) caused an increase in the rate of K efflux from isolated rabbit enterocytes. These effects were inhibited by apamin (5 X 10(‐7) M), quinidine (10(‐3) M), Ba (5 X 10(‐3) M) and trifluoperazine (5 X 10(‐5) M) but not by the loop diuretic furosemide (10(‐4) M). None of these drugs affected the basal rate of K efflux. The stimulatory effects of amino acids or alpha‐MG on K efflux are too great to be explained in terms of an increase in the electrical driving force across the plasma membrane of these cells and a change in membrane permeability is envisaged. An apparent Ca‐dependent K permeability in isolated enterocytes can be demonstrated using the Ca ionophore A23187. The effect of the ionophore on K efflux is abolished by apamin or Ba. It is proposed that Ca‐dependent K channels mediate the sugar and amino acid induced increases of K efflux. Under control conditions there is a decrease in intracellular K concentration during accumulation of alanine or alpha‐MG. Ba by itself does not alter K concentration but it did produce a marked increase when used in conjunction with alanine or alpha‐MG. The accumulation of alpha‐MG was inhibited in the presence of Ba. This is consistent with an interference with the driving force for sugar accumulation. It is suggested that the increase in K permeability described has a role in both maintaining ion homoeostasis during Na‐coupled transport and contributing to the driving force for sugar and amino acid absorption.