bluuebunny/crossref_metadata_2025 huggingface dataset:
Activation of glutamate receptors in response to membrane depolarization of hair cells isolated from chick cochlea.
Authors: Kataoka Y , Ohmori H
Published: December 2014

1. Experiments were performed to identify the neurotransmitter released from hair cells of chick cochlea. An isolated hair cell was closely apposed to a cultured granule cell of the rat cerebellum, and both cells were whole‐cell voltage clamped by utilizing a nystatin perforated patch technique. 2. Depolarization of hair cells to potentials more positive than ‐20 mV induced currents in the granule cell in a 10 mM Ca2+ extracellular medium. Amplitudes of induced currents were dependent on the membrane potential of granule cells and showed an outward‐going rectification. The induced current in granule cells was reversibly suppressed by a local application of 2‐amino‐5‐phosphonovalerate (APV), which indicates that the current was generated through the activation of an NMDA subtype of the glutamate receptor expressed on the granule cell. 3. The current amplitude of the granule cell was dependent on the size of hair cell depolarization. The size of current induced in a granule cell held at +55 mV was progressively increased with hair cell depolarization from ‐20 to +10 mV. At more positive potentials, the current amplitude was decreased. This voltage dependence was similar to but did not exactly match that of Ca2+ currents in the hair cell. The granule cell current appeared at more positive membrane potentials than the Ca2+ current in hair cells. 4. When intracellular Ca2+ concentration was increased by UV irradiation of the hair cell loaded with a caged Ca2+ compound, nitr‐5, the closely apposed granule cell generated an outward current when voltage clamped at +55 mV. 5. These observations (paragraphs 2‐4) imply that the most likely neurotransmitter released from the hair cell at its synapse with the afferent nerve terminal is glutamate.
Signalling properties of identified deep cerebellar nuclear neurons related to eye and head movements in the alert cat.
Authors: Gruart A , Delgado-García J M
Published: December 2014

1. The spike activity of deep cerebellar nuclear neurons was recorded in the alert cat during spontaneous and during vestibularly and visually induced eye movements. 2. Neurons were classified according to their location in the nuclei, their antidromic activation from projection sites, their sensitivity to eye position and velocity during spontaneous eye movements, and their responses to vestibular and optokinetic stimuli. 3. Type I EPV (eye position and velocity) neurons were located mainly in the posterior part of the fastigial nucleus and activated antidromically almost exclusively from the medial longitudinal fasciculus close to the oculomotor complex. These neurons, reported here for the first time, increased their firing rate during saccades and eye fixations towards the contralateral hemifield. Their position sensitivity to eye fixations in the horizontal plane was 5.3 +/‐ 2.6 spikes s‐1 deg‐1 (mean +/‐ S.D.). Eye velocity sensitivity during horizontal saccades was 0.71 +/‐ 0.52 spikes s‐1 deg‐1 s‐1. Variability of their firing rate during a given eye fixation was higher than that shown by abducens motoneurons. 4. Type I EPV neurons increased their firing rate during ipsilateral head rotations at 0.5 Hz with a mean phase lead over eye position of 95.3 +/‐ 9.5 deg. They were also activated by contralateral optokinetic stimulation at 30 deg s‐1. Their sensitivity to eye position and velocity in the horizontal plane during vestibular and optokinetic stimuli yielded values similar to those obtained for spontaneous eye movements. 5. Type II neurons were located in both fastigial and dentate nuclei and were activated antidromically from the restiform body, the medial longitudinal fasciculus close to the oculomotor complex, the red nucleus and the pontine nuclei. Type II neurons were not related to spontaneous eye movements. These neurons increased their firing rate in response to contralateral head rotation and during ipsilateral optokinetic stimulation, and decreased it with the oppositely directed movements. 6. Saccade‐related neurons were located mostly in the fastigial and dentate nuclei. Fastigial neurons were activated antidromically from the medial longitudinal fasciculus, while dentate neurons were activated from the red nucleus. These neurons fired a burst of spikes whose duration was significantly related to saccade duration. Dentate neurons responded during the omni‐directional saccades, while some fastigial neurons fired more actively during contralateral saccades. 7. These three types of neuron represent the output channel for oculomotor signals of the posterior vermis and paravermis. It is proposed that type I EPV neurons correspond to a group of premotor neurons directly involved in oculomotor control.(ABSTRACT TRUNCATED AT 400 WORDS)
Effects of rapid changes of external Na+ concentration at different moments during the action potential in guinea‐pig myocytes.
Authors: Le Guennec J V , Noble D
Published: December 2014

1. A rapid solution‐changing system using a solenoid was set up. The half‐time for changing the external solution surrounding a ventricular cardiac cell was 7.2 +/‐ 1.4 ms, whereas the time needed to change 90% of this solution was 48.5 +/‐ 7.9 ms. This rapid switching system was used to reduce the external sodium concentration at different moments during the action potential (recorded using the whole‐cell method) to 50% of its original value. This was performed in order to investigate the effect on the shape and duration of the action potential of modifying the activity of the sodium‐calcium exchanger. 2. A diminution of the action potential duration was seen irrespective of the substitute used for reducing the NaCl concentration from 140 to 70 mM. The magnitude of this diminution depended on the presence or absence of EGTA (5 mM) in the pipette solution and also on the moment during the action potential at which the NaCl substitution occurred. 3. Some differences were observed depending on whether the NaCl substitute used was lithium chloride or choline chloride. When choline chloride or N‐methyl‐D‐glucamine was used as the NaCl substitute, the amplitude of the action potential was slightly reduced (by 2‐5 mV) when the solution was changed 40 ms before the action potential was triggered. This reduction was never observed when LiCl was used as the NaCl substitute. 4. The effects on the shape of the action potential of changing from a solution containing 140 mM NaCl to one containing 70 mM NaCl and 70 mM LiCl were much more rapid when these changes occurred at a later stage during the action potential. The rate of repolarization was more than doubled when the change occurred at a late stage of the action potential but was hardly changed at the beginning of the plateau. 5. These experiments confirm the role of the sodium‐calcium exchange current in determining the duration of the mammalian ventricular action potential. However, it is also possible that the sodium background current plays a significant role in determining the shape of the action potential.
Inhibitory synaptic potentials in guinea‐pig substantia nigra dopamine neurones in vitro.
Authors: Häusser M A , Yung W H
Published: December 2014

1. The properties of stimulus‐evoked and spontaneous inhibitory synaptic potentials were examined in guinea‐pig substantia nigra dopamine neurones in sagittal and coronal midbrain slices in the presence of glutamate receptor antagonists. 2. Focal electrical stimulation within the substantia nigra, cerebral peduncle, internal capsule or the striatum evoked a biphasic IPSP consisting of a fast and a slow component, with peak latencies of about 30 and 250 ms, respectively. The fast component was sensitive to chloride injection, reversed polarity at ‐79.4 +/‐ 1.1 mV and was blocked by the GABAA receptor antagonists picrotoxin and bicuculline. The slow IPSP reversed at ‐99.3 +/‐ 5.4 mV and was blocked by the GABAB receptor antagonists 2‐hydroxysaclofen and CGP 35348. 3. Spontaneous IPSPs were observed in many neurones. These events reversed polarity at ‐77.5 +/‐ 2.6 mV and were completely blocked by bicuculline and/or picrotoxin. In the presence of TTX, small spontaneous events remained which probably represent miniature IPSPs. In coronal slices, application of 4‐aminopyridine raised the frequency of spontaneous IPSPs, presumably by activating nigral interneurones, but failed to reveal spontaneous biphasic IPSPs or spontaneous pure slow IPSPs. 4. The amplitude of the fast IPSPs fluctuated from trial to trial. Amplitude histograms of minimal fast IPSPs displayed evenly spaced peaks, suggesting that synaptic transmission is quantal at these synapses. The measured peak spacing depended on the driving force for Cl‐. 5. The fast IPSP showed little or no paired‐pulse depression, and in the presence of 2‐hydroxysaclofen (400‐600 microM) showed paired‐pulse facilitation. The GABAB agonist baclofen inhibited the fast IPSP via a presynaptic mechanism. The pharmacologically isolated slow IPSP showed marked paired‐pulse facilitation. 6. It is concluded that synaptic inhibition in the substantia nigra is mediated by GABA, is relatively resistant to frequency‐dependent depression and is regulated by presynaptic GABAB autoreceptors. Striatonigral and pallidonigral fibres activate both GABAA and GABAB receptors, while intranigral pathways appear to activate predominantly GABAA receptors.
Relaxation, [Ca2+]i and [Mg2+]i during prolonged tetanic stimulation of intact, single fibres from mouse skeletal muscle.
Authors: Westerblad H , Allen D G
Published: December 2014

1. In skeletal muscle there is generally a slowing of relaxation with increasing tetanus duration and it has been suggested that this is due to Ca2+ loading of parvalbumin (PA). To study this we have produced prolonged tetani in intact, single fibres from a mouse foot muscle which contain a high concentration of PA. We measured the rate of tension relaxation and also various aspects of Ca2+ handling. 2. During ‘interrupted’ tetani (15 repeated cycles of 100 ms with stimulation and 50 ms without) we observed a marked slowing of the relaxation both under control conditions and in acidosis (obtained by increasing the bath CO2 content). This slowing was not accompanied by any reduction of the initial rate of decline of the free myoplasmic Ca2+ concentration ([Ca2+]i), which was measured with indo‐1. 3. The functioning of the sarcoplasmic reticulum (SR) pump after tetani of various durations was analysed by plotting d[Ca2+]i/dt vs. [Ca2+]i during the final slow decline of [Ca2+]i after tetani. This analysis showed that the rate of SR Ca2+ pumping after a 1 s tetanus is less than half of that after a 100 ms tetanus. 4. The amplitude of the tail of [Ca2+]i 250 ms into relaxation was measured after tetani of various durations. This amplitude increased with tetanus duration and could be fitted to the sum of one exponential and one linear function. The exponential component increased with a time constant of 0.17 s and probably reflects Ca2+ loading of PA. 5. Ca2+ binding to PA will displace Mg2+ and hence the free myoplasmic concentration of Mg2+ ([Mg2+]i) will increase. To study this we used the fluorescent Mg2+ indicator furaptra. The results showed an increase of [Mg2+]i during prolonged tetani which, after removing the Ca2+ component of the fluorescent signal, amounted to about 0.5 mM. 6. A model of Ca2+ movements and tension production in skeletal muscle was used. The model showed that the increase of the amplitude of [Ca2+]i tails after tetani of various durations can be explained by the combined effect of Ca2+ loading of PA and slowed SR Ca2+ pumping. In contrast to the experimental data, the model predicted a slight reduction of the initial rate of [Ca2+]i decline with increased tetanus duration. 7. In conclusion, we observed a marked slowing of relaxation during prolonged tetanic stimulation. Altered Ca2+ handling, including Ca2+ loading of PA, seems not to be important for this slowing. Thus, the slowing appears to be due to altered cross‐bridge kinetics.
Non‐monosynaptic transmission of the cortical command for voluntary movement in man.
Authors: Burke D , Gracies J M , Mazevet D , Meunier S , Pierrot‐Deseilligny E
Published: December 2014

1. The possibility was investigated that, in man, some of the descending command for tonic voluntary wrist extension is transmitted to extensor motoneurones over a non‐monosynaptic pathway. 2. Stimulation of the cutaneous superficial radial nerve at 3 times perceptual threshold depressed the electromyogram (EMG) of extensor carpi radialis (ECR) and the discharge of single ECR motor units, both with a mean central delay of 4.2 ms. Such stimuli depressed the response to transcranial magnetic stimulation of the motor cortex, but had little effect on the H reflex. 3. The possibility that the relative sparing of the H reflex was due to an alteration in transmission of the afferent volley for the H reflex was excluded. 4. The central latency of the cutaneous‐induced depression of the discharge of single motor units in biceps brachii (C5‐C6) was shorter by about 1 ms than that of the more caudal wrist and finger extensor motor units. This suggests that the locus for the cutaneous‐induced effects was spinal but above the cervical enlargement. 5. The pattern of EMG depression (evoked by superficial radial but not palmar stimuli, in wrist extensors but not wrist flexors) is that previously described for the presumed propriospinal system of human subjects. 6. It is concluded that a significant component of the voluntary command for tonic wrist extension reaches the relevant motoneurone pool via a non‐monosynaptic pathway. It is suggested that the interposed neurones could be C3‐C4 propriospinal neurones.
Dynamic asymmetries of cardiac output transients in response to muscular exercise in man.
Authors: Yoshida T , Whipp B J
Published: December 2014

1. We determined the kinetics of cardiac output (Q) with respect to oxygen uptake (VO2) at the on‐ and off‐transients of constant‐load exercise. Six subjects performed constant‐load exercise which consisted of 5 min rest, 5 min one‐legged pedalling at 50 W and a 5 min recovery period. 2. The transient responses were characterized by first‐order kinetics. There was no significant difference between the time constants for VO2 (tau VO2) at the on‐ (33.9 +/‐ 3.5 s, mean +/‐ S.E.M.) and off‐transient (37.2 +/‐ 2.9 s). The time constant for Q (tau Q, 29.4 +/‐ 3.2 s) was consistently shorter than tau VO2 at the on‐transient. However, tau Q was appreciably longer at the off‐transient (44.3 +/‐ 3.6 s) than the on‐transient. 3. The results support the contention that the time constant for the on‐transient of Q is appreciably faster than that for VO2 and hence there seems little justification for the notion that the time constants for the kinetics of VO2 are determined by the limitations of blood flow in the transient. The asymmetry of Q kinetics, with the off‐transient tau Q being appreciably slower than the on‐transient tau Q, serves to maintain a sufficiently high oxygen flow to the muscle during recovery from exercise at a time when the muscle oxygen uptake remains high.
Stretch effects on whole‐cell currents of guinea‐pig urinary bladder myocytes.
Authors: Wellner M C , Isenberg G
Published: December 2014

1. By means of two patch‐pipettes, isolated urinary bladder myocytes were longitudinally stretched up to 20% beyond slack length (delta L = 20%). 2. Experiments were conducted using both voltage and current clamp configurations. In current clamped cells at 23 degrees C, delta L depolarized the membrane from ‐50 to ca ‐15 mV, the amplitude of depolarization increasing with the extent of delta L. At 36 degrees C, delta L induced action potentials or increased the frequency of spontaneous action potentials. 3. In voltage clamped cells at a holding potential of ‐50 mV, stretch induced an inward current (Iin) and increased the input conductance. Both effects increased with delta L. They were blocked by 40 microM gadolinium, suggesting stretch activation of non‐selective cation channels (SACs) as the underlying mechanism. 4. Stretch‐induced difference currents rectified outwardly and reversed at a reversal potential (Erev) of ‐28 +/‐ 10 mV. Twenty millimolar [TEA]o suppressed the rectification and shifted Erev to 0 +/‐ 1 mV. The result suggests that stretch can activate not only SACs but also TEA‐sensitive K+ channels. 5. Stretch changed the net current due to clamp steps to 0 mV as though it increased the potassium current (IK) and reduced the calcium current (ICa). While 20 mM intracellular BAPTA did not modify the stretch‐induced whole‐cell inward current (Iin) at ‐50 mV, it suppressed the stretch effects on IK and ICa as if these effects were mediated by an increase in the subsarcolemmal Ca2+ concentration. 6. The results support the hypothesis that longitudinal stretch can activate SACs and Ca2+ influx through them. In non‐clamped cells, stretch can also modulate Ca2+ influx through L‐type Ca2+ channels via changes in membrane potential.
The processing of human ballistic movements explored by stimulation over the cortex.
Authors: Palmer E , Cafarelli E , Ashby P
Published: December 2014

1. When seated human subjects abducted one arm rapidly in response to a tone there was successively a burst of electromyographic (EMG) activity in the deltoid and latissimus dorsi muscles followed by another burst in the deltoid muscle. This triphasic pattern is typical of a ballistic ‘focal movement’. There were also bursts of EMG activity in the contralateral latissimus dorsi, pectoralis major and abdominal muscles, which were assumed to be ‘associated postural adjustments’. The same bilateral pattern of muscle activity occurred in a deafferented subject. 2. When subjects abducted the left arm rapidly, magnetic stimulation over the left motor cortex delayed the onset of the EMG burst in the right latissimus dorsi relative to the initial burst in the left deltoid. When subjects abducted the right arm rapidly, magnetic stimulation over the left motor cortex delayed the onset of the initial EMG burst in the right deltoid relative to the burst in the left latissimus. In each case, the delay of an EMG burst was greatest (about 80 ms) when the stimulus was given just before the burst was expected to occur. The inhibition of voluntary movements by transcranial stimulation was not associated with a reduction in the excitability of spinal motoneurons. 3. We conclude that focal ballistic movements and their associated postural adjustments are generated in exactly the same way. We postulate that these movements are preprogrammed, held in a memory until the ‘go’ signal and then released through both motor cortices to spinal motoneurons.
Effect of insulin on area and Na+ channel density of apical membrane of cultured toad kidney cells.
Authors: Erlij D , De Smet P , Van Driessche W
Published: December 2014

1. The stimulation of transepithelial Na+ transport caused by insulin in A6 cultured toad kidney cells was investigated by determination of membrane capacitance (Cm), short circuit current (Isc) and current fluctuation analysis. Values of Cm are proportional to membrane area while blocker‐induced current fluctuation analysis provides an estimate of the number of active amiloride‐sensitive Na+ channels in the apical membrane. 2. Insulin simultaneously increased Cm, Isc and Gt (transepithelial conductance) in epithelia incubated with Na(+)‐containing solutions on both sides. 3. Analysis of 6‐chloro‐3,5‐diaminopyrazine‐2‐carboxamide (CDPC)‐induced noise showed that insulin increased the number of active Na+ channels in the apical membrane, without altering the single channel current. 4. When nystatin was used to permeabilize the apical membrane the impedance data revealed the presence of a second time constant. Analysis of these data indicated that the basolateral membrane capacitance (Cb) is much larger than the apical membrane capacitance (Ca). Insulin administered to nystatin‐treated epithelia increased the values for both capacitances. 5. We suggest that the stimulation of transepithelial Na+ transport caused by insulin may be associated with the exocytotic delivery of transporters to the apical membrane.
Veratridine‐induced oscillations of cytosolic calcium and membrane potential in bovine chromaffin cells.
Authors: López M G , Artalejo A R , García A G , Neher E , García-Sancho J
Published: December 2014

1. Veratridine (VTD) induced large oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential (Vm) in otherwise silent bovine chromaffin cells loaded with fura‐2. 2. Depletion of the intracellular Ca2+ stores by thapsigargin or ryanodine did not affect these oscillations. Caffeine had a complex effect, decreasing them in cells with high activity but increasing them in cells with low activity. 3. The [Ca2+]i oscillations required extracellular Ca2+ and Na+ and were blocked by Ni2+ or tetrodotoxin. They were antagonized by high external concentrations of Mg2+ and/or Ca2+. 4. The oscillations of Vm had three phases: (i) slow depolarization (20 mV in 10‐40 s); (ii) further fast depolarization (30 mV in 1 s); and (iii) rapid (5 s) repolarization. [Ca2+]i decreased during (i), increased quickly during (ii) with a 1 s delay with regard to the peak depolarization, and decreased during (iii). 5. Slight depolarizations increased the frequency of the oscillations whereas large depolarizations decreased it. 6. The Ca(2+)‐dependent K+ channel blocker apamin increased the duration and decreased the frequency of the oscillations. 7. We propose the following mechanism for the oscillations: (i) the membrane depolarizes slowly by a decrease of potassium conductance (gK), perhaps due to a gradual decrease of [Ca2+]i; (ii) the threshold for activation of Na+ channels (decreased by VTD) is reached, producing further depolarization and recruiting Ca2+ channels, and inactivation of both Ca2+ and VTD‐poisoned Na+ channels is slow; and (iii) gK increases, aided by activation of Ca(2+)‐dependent K+ channels by the increased [Ca2+]i, and the membrane repolarizes. The contribution of the Na+ channels seems essential for the generation of the oscillations. 8. Bovine chromaffin cells have the machinery required for [Ca2+]i oscillations even though the more physiological stimulus tested here (high K+, field electrical stimulation, nicotinic or muscarinic agonists) produced mainly non‐oscillatory responses.
NMDA receptor‐mediated sympathetic chemoreflex excitation of RVL‐spinal vasomotor neurones in rats.
Authors: Sun M K , Reis D J
Published: December 2014

1. We investigated the role of N‐methyl‐D‐aspartic acid (NMDA) receptors in mediating hypoxic excitation of the reticulospinal vasomotor neurones of the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata in paralysed ventilated rats. 2. Unilateral close arterial injection of sodium cyanide (100 nmol) into the carotid sinus region or ventilation with 100% N2 for 12 s rapidly, reversibly and reproducibly excited the RVL‐spinal vasomotor neurones, followed about 1‐2 s later by increases in sympathetic nerve activity and arterial pressure, effects abolished by denervation of the ipsilateral carotid sinus nerve. 3. Ionophoresis onto the RVL‐spinal vasomotor neurones of kynurenate (a wide‐spectrum antagonist of the excitatory amino acid receptors) or of 2‐amino‐5‐monophosphovaleric acid (APV; a selective NMDA receptor antagonist), but not of xanthurenate (an inactive analogue of kynurenate), blocked the excitation elicited by intracarotid cyanide or 12 s of hypoxia. Kynurenate completely and APV partially blocked the excitatory responses to ionophoretically applied L‐glutamate. APV, however, did not alter the excitatory responses of the vasomotor neurones to ionophoreses of kainate and quisqualate. 4. Bilateral microinjection of kynurenate (10 nmol, 50 nl per site) or APV (5 nmol, 50 nl per site) into the RVL blocked the increases in arterial pressure elicited by intracarotid cyanide or 12 s of 100% N2 ventilation. 5. Twenty seconds of intratracheal administration of 100% N2 resulted in complex and prolonged elevations of arterial pressure, the late component of which was affected neither by sinus denervation nor by microinjections of kynurenate or APV into the RVL. 6. We conclude that the sympathetic and cardiovascular responses to stimulation of arterial chemoreceptors result from excitation of RVL‐spinal vasomotor neurones via activation of the NMDA subtypes of the excitatory amino acid receptors of the neurones. In contrast, the failure of these antagonists to influence the delayed excitation of the RVL‐spinal vasomotor neurons by more prolonged exposure to N2 inhalation further supports the view that these neurones are directly stimulated by hypoxia.
Baclofen enhancement of acetylcholine release from amacrine cells in the rabbit retina by reduction of glycinergic inhibition.
Authors: Neal M J , Cunningham J R
Published: December 2014

1. The mechanism by which the GABAB‐receptor agonist, baclofen, enhances the light‐evoked release of [3H]acetylcholine (ACh) from cholinergic amacrine cells was studied using an eye‐cup preparation in anaesthetized rabbits and isolated retinas. 2. When applied locally to the rabbit retina, baclofen increased the release of ACh evoked by a flickering light (3 Hz) by over 40%. 3. In isolated retinas, baclofen strikingly inhibited the K(+)‐evoked release of glycine but had no effect on GABA release. 4. In the rabbit eye cup, strychnine enhanced the light‐evoked release of ACh to a similar degree to that produced by baclofen. The effects of baclofen and strychnine on the light‐evoked release of ACh were not additive. In contrast, bicuculline did not affect the enhancing action of baclofen on the light‐evoked release of ACh. 5. In order to see whether the glycinergic amacrine cells might be stimulated by ACh, isolated rat and rabbit retinas were exposed to muscarine. This cholinergic agonist potentiated the K(+)‐evoked release of glycine by 54%. 6. We suggest that baclofen enhances the light‐evoked release of ACh from amacrine cells by inhibiting glycine release from glycinergic amacrine cells which are stimulated by ACh and form an inhibitory feedback loop to the cholinergic neurones.
Absence of mechanical evidence for attached weakly binding cross‐bridges in frog relaxed muscle fibres.
Authors: Bagni M A , Cecchi G , Colomo F , Garzella P
Published: December 2014

1. Passive force responses to ramp stretches at various velocities were measured in intact and skinned single muscle fibres isolated from the lumbricalis muscle of the frog. Force was measured using a fast capacitance transducer and sarcomere length was measured using a laser light diffraction technique at a point very close to the fixed end so as to avoid effects of fibre inertia. Experiments were performed at 15 degrees C with sarcomere length between 2.13 and 3.27 microns under high (170 mM) and low (20 mM) ionic strength. 2. The analysis shows that the force response is the sum of at least three components: (i) elastic (force proportional to the amount of stretch), (ii) viscous (force proportional to rate of stretch), and (iii) viscoelastic (resembling the response of a pure viscous element in series with an elastic element). 3. The amplitude of all these components increased progressively with sarcomere length in the whole range measured. 4. A further component, attributable to the short‐range elasticity (SREC), was present in the force response of the intact fibres. 5. The amplitude of the force response decreased substantially upon skinning at high ionic strength but increased again at low ionic strength. The SREC was completely abolished by skinning. 6. None of the components of the force response was found to have the properties expected from the previously postulated ‘weakly binding bridges’.
Muscarine inhibits high‐threshold calcium currents with two distinct modes in rat embryonic hippocampal neurons.
Authors: Toselli M , Taglietti V
Published: December 2014

1. Ca2+ channel modulation by muscarine was investigated in primary cultured embryonic rat hippocampal neurons using the whole‐cell variant of the patch‐clamp technique. 2. Muscarine produced a reversible and concentration‐dependent decrease in the Ba2+ current amplitude. In 65% of neurons sensitive to the agonist, current inhibition was time and voltage dependent, being maximal between ‐20 and 0 mV and decreasing at depolarizing potentials. In the remaining 35% of neurons, the effects of muscarine were voltage independent, inhibition being constant in a wide potential range between ‐20 and +80 mV. 3. Different receptors might be involved in the two modes of modulation. Muscarine‐induced voltage‐dependent inhibition of Ba2+ current was best suppressed by the muscarinic receptor antagonist 4‐diphenylacetoxy‐N‐methyl‐piperidine methiodide (81% suppression), while voltage‐independent inhibition was best suppressed by AFDX116 (75% suppression). 4. In cells treated with omega‐conotoxin (omega‐CgTX), the voltage‐independent mode of inhibition was strongly prevented, suggesting that the two modulatory mechanisms (voltage dependent and voltage independent) operate on separate classes of high‐voltage‐activated (HVA) Ca2+ channels. 5. A pertussis toxin‐sensitive G‐protein is involved in both modes of action of muscarine, since both modes were prevented by pretreatment of the cells with 50 ng ml‐1 pertussis toxin. 6. Both modes of modulation were mimicked in different cells by intracellular application of GTP‐gamma‐S. However, the onset of voltage‐independent inhibition was about 5 times slower than that of voltage‐dependent inhibition, suggesting involvement of a more complex metabolic pathway for the former mode of channel modulation. 7. Relief of the voltage‐dependent inhibition was obtained by depolarizing voltage prepulses and occurred with kinetics that depended on agonist concentration. 8. The voltage‐dependent inhibition could be simulated by a kinetic model in which the time course of Ca2+ entry was assumed to be regulated by both the concentration of muscarine and membrane potential.
Conductance and kinetics of single cGMP‐activated channels in salamander rod outer segments.
Authors: Taylor W R , Baylor D A
Published: December 2014

1. The conductance and kinetics of single 3',5'‐cyclic guanosine monophosphate (cGMP)‐activated channels of retinal rod outer segments were studied in inside‐out membrane patches. The size of the single channel currents was increased by using low concentrations of divalent cations. 2. At saturating cGMP concentration, the current flickered at high frequency. Occasionally, the current was interrupted by closures lasting tens or hundreds of milliseconds. At +50 mV the maximum current during an opening was slightly more than 1 pA, but the open channel level was poorly resolved due to the speed of the gating transitions. 3. Amplitude histograms confirmed the presence of a sublevel of current, roughly a quarter the size of the peak current, at low cGMP concentrations. The fraction of time in the sublevel decreased with increasing cGMP concentration, suggesting that the sublevel may be due to opening by the partially liganded channel. 4. Consistent with previous macroscopic current recordings, single channel activation by cGMP had an apparent dissociation constant of 8.6 microM, and a Hill coefficient of 2.8. 5. At saturating cGMP concentrations, the channel was modelled as a two‐state system with the following parameters. The open channel conductance was 25 pS. The opening rate constant, beta, was 1.5 x 10(4) s‐1 at 0 mV, and had a voltage sensitivity equivalent to the movement of 0.23 electronic charges outward through the membrane electric field. The closing rate constant, alpha, was 2.1 x 10(4) s‐1 and was voltage insensitive. Assuming that the open‐state chord conductance was voltage independent, the inferred voltage dependence of beta largely accounted for the outward rectification in the steady‐state macroscopic current‐voltage relation of multichannel patches, at saturating cGMP concentration.
Prevention of the excitatory actions of bradykinin by inhibition of PGI2 formation in nodose neurones of the guinea‐pig.
Authors: Weinreich D , Koschorke G M , Undem B J , Taylor G E
Published: December 2014

1. Intracellular recordings were made from neurones in intact guinea‐pig nodose ganglia in vitro and from acutely isolated adult guinea‐pig and rabbit nodose neurons to study a bradykinin (BK)‐mediated block of a slow spike after‐hyperpolarization (AHPslow) that is prominent in 30‐40% of these neurones. 2. BK (100 nM) reversibly blocked the AHPslow, resulting in an ablation of the spike accommodative properties of these neurones. The B1 BK receptor agonist [des‐Arg9]‐BK did not mimic or prevent the actions of BK. In contrast, the B2 BK receptor antagonist D‐Arg‐[Hyp3,Thi5,D‐Tic7,Oic8]‐BK (HOE 140) prevented BK‐induced block of the AHPslow and the effect of BK on spike frequency adaptation. 3. The BK block of the AHPslow in acutely dissociated neurones was prevented by indomethacin, indicating that this BK effect was dependent upon a cyclo‐oxygenase metabolite intrinsic to these neurones. 4. One to twenty femtomoles of the prostanoids PGE2, PGD2, 9 alpha, 11 beta‐PGF2 (a metabolite of PGD2), PGF2 alpha, TxB2 and PGI2 were released spontaneously from a nodose ganglion in 15 min. BK (100 nM) selectively increased PGI2 release 2.8‐fold without affecting the release of the other prostanoids. Treatment with 10 microM tranylcypromine (TCP), a putative PGI2 synthase inhibitor, completely prevented the BK‐induced release of PGI2. 5. In the presence of 10 microM TCP, BK no longer produced significant effects on the AHPslow. In contrast, 10 microM TCP did not prevent PGI2 from blocking the AHPslow. 6. These results suggest that vagal afferents that exhibit AHPslow also possess the B2 type of BK receptor. Activation of these BK receptors results in the production of PGI2, which in turn controls spike frequency adaptation by affecting the amplitude of the AHPslow.
Functional classification of afferent phrenic nerve fibres and diaphragmatic receptors in cats.
Authors: BaBkowiec A , KukuBa K , Szulczyk P
Published: December 2014

1. Single afferent fibres with receptive fields in the diaphragm (272 units) dissected from the right phrenic nerve were classified according to the following properties: reaction to contraction of the diaphragm, resting activity, conduction velocity, location and properties of receptive fields, and reaction to injection of bradykinin and lactic acid into the internal thoracic artery. Nine additional fibres dissected from the phrenic nerve had receptive fields outside the diaphragm. The experiments were performed on chloralose‐anaesthetized cats. 2. Ninety‐six fibres (36%) had high resting activity when unloaded by contraction of the diaphragm, had low‐threshold receptive fields in the muscle and were mostly group II and III fibres. They probably innervated muscle spindles. 3. Eighty‐eight fibres (32%) were vigorously activated by contraction of the diaphragm. They had low‐threshold receptive fields located in the musculotendinous border and central tendon. Their conduction velocity was in the range for group II and III fibres. We infer that they may innervate tendon organs. 4. Eighty‐eight fibres (32%) were slightly affected or not affected by diaphragmatic contraction. They had low‐ and high‐threshold receptive fields located mostly in the muscular part of the diaphragm, and negligible resting activity. Most of them were group III and IV afferent fibres and were activated when bradykinin and lactic acid were applied to their receptive fields. Possibly these low‐ and high‐threshold receptors innervated diaphragmatic ergo‐ and nociceptors, respectively. 5. Sensory outflow from the diaphragm was found to be somatotopically organized, so that fibres with receptive fields in the sternocostal portion were predominantly located in the upper phrenic nerve root, and those with lumbar receptive fields were in the lower root. 6. It is concluded that the phrenic nerve contains fibres from several distinct classes of sensory receptors: muscle spindles, tendon organs, ergoceptors and nociceptors. The sensory diaphragmatic outflow to the spinal cord is somatotopically organized.
Resting load regulates cytosolic calcium‐force relationship of the contraction of bovine cerebrovascular smooth muscle.
Authors: Miyagi Y , Kobayashi S , Nishimura J , Fukui M , Kanaide H
Published: December 2014

1. We determined the effects of a change in preload, or resting load, on the development of force and on cytosolic calcium concentration ([Ca2+]i) during the contraction induced by high K+ depolarization and by the stable analogue of thromboxane A2, U‐46619, using front‐surface fluorometry and medial strips of bovine middle cerebral artery loaded with fura‐2. 2. Increase in resting load of strips from 0.495 mN (low load; 0.85L(max)) to 1.98 mN (optimal load; L(max)) elevated resting levels of [Ca2+]i slightly, and, significantly, after a delay of a few seconds. The force developed by high K+ depolarization at resting load of 1.98 mN was much greater than that at a resting load of 0.495 mN (191.8% of that at 0.495 mN); however, there was no difference in [Ca2+]i elevation induced by high K+ depolarization between resting loads of 0.495 and 1.98 mN. 3. Both in the presence and absence of extracellular Ca2+, the force developed by U‐46619 (0.1 microM) at resting load of 1.98 mN was also much greater than that at a resting load of 0.495 mN. Both in the presence and absence of extracellular Ca2+, there was no difference in the [Ca2+]i transients induced by U‐46619 between two different resting loads. 4. The [Ca2+]i‐force relationship during the contraction induced by high K+ depolarization was shifted to the left when the resting load was increased from 0.495 to 1.98 mN. At 0.495 mN resting load, this Ca(2+)‐force relationship was shifted to the left by U‐46619. This left‐side shift of the curve by U‐46619 was further enhanced by the increase in resting load from 0.495 to 1.98 mN. 5. The augmentation of force development induced by the change in resting load (from 0.495 to 1.98 mN) was not affected by a relatively specific inhibitor of protein kinase C, 1‐(5‐isoquinolinesulphonyl)‐2‐methylpiperazine dihydro‐chloride (H‐7; 10 microM). 6. We conclude that (1) at a given degree of [Ca2+]i elevation, the developed force induced by U‐46619 was greater than that induced by high K+ depolarization, and (2) preload, or resting load (below optimal load), regulates contractile responsiveness of cerebrovascular smooth muscle to various stimulations, mainly by modulating the [Ca2+]i‐force relationship, without affecting the extent of [Ca2+]i elevation. The results suggest the possibility that Ca(2+)‐insensitive pathways may be involved in the stretch‐dependent regulation of Ca2+ sensitivity of the contractile apparatus in smooth muscle.
Mechano‐sensitive linkage in excitation‐contraction coupling in frog skeletal muscle.
Authors: Bruton J D , Lännergren J , Westerblad H
Published: December 2014

1. Single skeletal muscle fibres of Xenopus laevis were used to investigate the involvement of a mechano‐sensitive link in excitation‐contraction coupling (EC coupling). 2. Fibres were stimulated by intermittent tetani until tension fell to about 40% of its initial level. Fibres were then stressed either by briefly stretching the fibres to 120% of their resting length or by exposing them to hypotonic Ringer solution ([NaCl] reduced to 80%) for 5 min. 3. In six of thirty‐five stretched fibres and in all fourteen fibres exposed to hypotonic solution, a long‐lasting depression of tension ensued. Tetanic tension then recovered slowly, often taking more than 10 h to return to its initial level. 4. During the period of minimal tension production, 12 mM caffeine induced a maximum contracture; 190 mM K+ induced a contracture larger than previous or subsequent tetani, and perchlorate (1 mM) slightly augmented tetanic tension. 5. Neither protease inhibitors nor a protein synthesis inhibitor altered the long‐lasting period of tension depression and slow recovery. A free‐radical scavenger was also without effect. 6. It is concluded that there is a mechano‐sensitive link involved in EC coupling which can be damaged easily in fatigued muscle fibres.
Latency of effects evoked by electrical and magnetic brain stimulation in lower limb motoneurones in man.
Authors: Nielsen J , Petersen N , Ballegaard M
Published: December 2014

1. The latency of effects in the tibialis anterior (TA) and soleus (Sol) muscles evoked by electrical and magnetic stimulation of the motor cortex was evaluated in human subjects by H reflex testing. Post‐stimulus time histograms (PSTHs) were established for the discharge of single voluntarily activated motor units and motor‐evoked potentials (MEPs) in the surface electromyogram. 2. At rest both electrical and magnetic stimulation evoked an inhibition of the Sol H reflex at the lowest intensities of stimulation. In some subjects a facilitation with an earlier onset was seen when increasing the stimulation strength. When the anode for the electrical stimulation was placed at the vertex directly above the leg motor area, the inhibition or facilitation often had the same latency as when evoked by magnetic stimulation. However, when the anode was placed 2‐3 cm lateral to the vertex, effects evoked by the electrical stimulus often occurred 1‐2 ms earlier. 3. Short‐latency peaks in the PSTH of the discharges of single TA motor units also tended to occur earlier when evoked by electrical stimulation with the anode lateral to the vertex than when evoked by magnetic stimulation or electrical stimulation with the anode at the vertex. 4. In one subject, near‐maximal electrical stimulation evoked MEPs with a latency corresponding to that seen following stimulation of the brainstem by electrodes placed bilaterally over the mastoid processes approximately 16 cm more distal. Maximal magnetic stimulation, in contrast, never resulted in responses with a latency shorter than that seen with the weakest electrical stimuli at the vertex. 5. The initial facilitation of the Sol H reflex evoked by magnetic stimulation and by electrical anodal stimulation at the vertex increased when the subject performed a voluntary plantarflexion. In contrast, the earlier facilitation evoked by electrical anodal stimulation 2‐3 cm lateral to the vertex had the same size both at rest and during contraction. 6. We suggest that magnetic stimulation and electrical anodal stimulation at the vertex may preferentially activate descending cortical cells at, or close to, the cell soma. The initial responses evoked by these two stimuli may therefore be influenced by the excitability of the cortical cells. On the other hand, electrical stimulation with the anode 2‐3 cm lateral to the vertex seems to often activate the axons at a deeper level. The initial responses evoked by this type of stimulation may therefore not be influenced by the excitability of the cortical cells.
A role for gamma‐glutamyl transpeptidase and the amino acid transport system xc‐ in cystine transport by a human pancreatic duct cell line.
Authors: Sweiry J H , Sastre J , Viña J , Elsässer H P , Mann G E
Published: December 2014

1. The roles of the gamma‐glutamyl cycle and the anionic amino acid transport system xc‐ in mediating L‐cystine uptake were investigated in cultured human pancreatic duct PaTu 8902 cells. This cell line exhibits morphological features of normal pancreatic duct cells and expresses gamma‐glutamyl transpeptidase (gamma‐GT, EC 2.3.2.2), an enzyme involved in the metabolism and regulation of intracellular glutathione (GSH). 2. Uptake of L‐cystine (10 microM) was linear for up to 10 min, temperature dependent, Na+ independent, saturable (Michaelis‐Menten constant (Km), 86 +/‐ 25 microM; maximal velocity (Vmax), 109 +/‐ 33 nmol (mg protein)‐1 h‐1) and reduced by 80‐90% by a 50‐fold excess concentration of L‐glutamate and L‐homocysteic acid, but not L‐aspartate. These transport properties resemble those described for system xc‐, which exchanges cystine for intracellular glutamate. 3. Acivicin, a known inhibitor of gamma‐GT, decreased gamma‐GT activity from 2.58 +/‐ 0.96 to 0.97 +/‐ 0.11 mU (mg protein)‐1 and decreased the initial rates of L‐cystine and L‐glutamine uptake by 41‐55%. Anthglutin (1‐gamma‐L‐glutamyl‐2‐(2‐carboxyphenylhyl)hydrazine), a structurally different inhibitor of gamma‐GT, also caused a concentration‐dependent (0.01‐1 mM) decrease in gamma‐GT activity and L‐cystine uptake. 4. Neither acivicin nor anthglutin inhibited the uptake of L‐glutamate, a poor substrate for gamma‐GT. 5. In the presence of a 500‐fold excess concentration of glutamate, which should abolish entry of cystine via system xc‐, the remaining fraction of cystine transport was inhibited by 50% by acivicin, suggesting that transport is, in part, dependent on the activity of gamma‐GT. 6. Cystine transport was also 60‐80% inhibited by a series of gamma‐glutamyl amino acids (5 mM) including gamma‐glutamyl‐glutamate, gamma‐glutamyl‐glutamine and gamma‐glutamyl‐glycine. alpha‐Dipeptides inhibited cystine transport by only 6‐22%. 7. These findings demonstrate that in human pancreatic duct PaTu 8902 cells, cystine uptake is mediated by system xc‐ (50‐60%) and the gamma‐glutamyl cycle. Our results provide the first evidence linking gamma‐GT with cystine transport in human epithelial cells and are of relevance in view of the importance of cystine as a sulphur amino acid source for GSH synthesis in cells exposed to oxidative stress.
Coding of pulsatile motor output by human muscle afferents during slow finger movements.
Authors: Wessberg J , Vallbo A B
Published: December 2014

1. Impulse activities of thirty‐eight muscle spindle and tendon organ afferents from the finger extensor muscles were recorded in the radial nerve of human subjects while the subjects performed voluntary flexion and extension finger movements at a single metacarpophalangeal joint. 2. The afferent firing was analysed in relation to the 8‐10 Hz discontinuities which previously have been shown to characterize these movements. Spike‐triggered averaging and frequency domain analyses demonstrated that all Ia muscle spindle afferents and a large proportion of group II spindle afferents responded in close association with local peaks in the joint acceleration. During muscle lengthening the impulses appeared during phases of rapid muscle stretch, whereas they appeared during the phase of minimal speed during muscle shortening. 3. The Golgi tendon organ (Ib) afferents displayed a reverse pattern of activity in relation to the discontinuities, i.e. the impulses tended to appear in the phase of minimal speed during lengthening movements and close to maximal shortening speed during shortening movements. Hence, their firing often coincided with the phasic increases of the parent muscle activity which account for the 8‐10 Hz discontinuities. 4. A close analysis of the time relations between spindle firing and the kinematics of the 8‐10 Hz discontinuities revealed that the population spindle response was too delayed and too dispersed to support the hypothesis that the discontinuities are accounted for by the stretch reflex. 5. If, as suggested in a previous paper, the 8‐10 Hz discontinuities are produced by a pulsatile descending motor command, the coding of the periodic but tenuous kinematic events by the population of proprioceptors may have a role in relation to an alleged pulsatile command generator.