bluuebunny/crossref_metadata_2025 huggingface dataset:
The effects of inorganic phosphate and creatine phosphate on force production in skinned muscles from rat ventricle.
Authors: Kentish J C
Published: December 2014

Thin ventricular trabeculae from rat hearts were skinned with the non‐ionic detergent Triton X‐100. The isometric force development of these muscles was investigated over a range of Ca2+ concentrations (0.2‐200 microM) in the presence of various concentrations of creatine phosphate (CP), creatine and inorganic phosphate (Pi). The addition of Pi (1‐30 microM) at constant ionic strength reduced the maximum Ca2+‐regulated force (obtained at 200 microM‐Ca2+). At 20 mM‐Pi force was 31 +/‐ 5% (mean +/‐ S.E. of mean, n = 7) of that in Pi‐free solution. This inhibitory action of Pi was not due to an inhibition of creatine kinase activity or to a reduction in the amount of free energy available for work from ATP hydrolysis. Increases in Pi concentration over the range 0‐20 mM shifted the sigmoid relationship between force and [Ca2+] to higher Ca2+ concentrations. The [Ca2+] required for 50% activation rose from 8.3 +/‐ 1.5 microM (mean +/‐ S.E. of mean, n = 7) in Pi‐free solutions to 19.8 +/‐ 4.2 microM in 20 mM‐Pi. An increase in CP concentration in the range 10‐30 mM had a small inhibitory effect on maximum Ca2+‐regulated force but did not alter the force‐‐[Ca2+] relationship. Creatine (0‐30 mM) was without effect on the skinned muscles. The inhibitory effects of Pi suggest that a net hydrolysis of CP to Pi and creatine in the myoplasm of intact cardiac cells could reduce force development at a given myoplasmic [Ca2+], especially if the latter was below the level needed to fully activate the myofibrils. This suggestion is discussed in relation to the CP hydrolysis and decrease in force development that are observed in cardiac muscle during hypoxia or ischaemia.
Electrophysiological properties of neonatal rat motoneurones studied in vitro.
Authors: Fulton B P , Walton K
Published: December 2014

The electroresponsive properties of neonatal lumbar spinal motoneurones were studied using isolated, hemisected spinal cords from neonatal rats aged 3‐12 days. The extracellular and intracellular responses to electrical stimulation of the ventral and dorsal root were studied as well as the intracellular response to current injection. Field potentials recorded in the lateral motor area following electrical stimulation of lumbar ventral roots had a triphasic positive‐negative‐positive wave form. The negative component did not return to the base line smoothly but exhibited a 'shoulder' where the negativity increased in duration. Following electrical stimulation of the dorsal root, presynaptic field potentials were recorded upon activation of the afferent axons as well as following synaptic activation of interneurones and motoneurones. The input resistances of neonatal motoneurones determined from the slope of current‐voltage plots were high compared with the adult. The resistance decreased with age with a mean of 18.1 M omega for animals 3‐5 days old, 8.8 M omega for animals 6‐8 days old and 5.4 M omega for animals 9‐11 days old. Values for the membrane time constant were similar to those in the adult with a mean of 4.5 ms. Action potentials elicited by ventral or dorsal root stimulation or by intracellular current injection were marked by a pronounced after‐depolarization (a.d.p.) and an after‐hyperpolarization (a.h.p.). The amplitude of the a.h.p. varied with that of the a.d.p. The amplitude of excitatory post‐synaptic potentials (e.p.s.p.s) elicited by electrical stimulation of the dorsal root was affected by intracellular current injection. Two types of e.p.s.p.s were distinguished: those with a biphasic reversal (early phase first) and those in which the early phase was unaffected by inward current injection while the later phase was reversed. Unlike in the adult, the reversals could be achieved with low current levels and the amplitude of both types of e.p.s.p. was increased by inward current injection. Inhibitory post‐synaptic potentials (i.p.s.p.s) were elicited by dorsal or ventral root stimulation. The amplitude of these i.p.s.p.s was diminished and reversed in sign with inward current injection and their amplitude was enhanced with outward current injection. Activation of neonatal motoneurones with long current pulses revealed that there is one steady‐state firing range.(ABSTRACT TRUNCATED AT 400 WORDS)
Modulation by applied electric fields of Purkinje and stellate cell activity in the isolated turtle cerebellum.
Authors: Chan C Y , Nicholson C
Published: December 2014

Quasi steady‐state electric fields were applied across the isolated turtle cerebellum to study the relationship between applied field, neuronal morphology and the modulation of the neuronal spike firing pattern. Spiking elements were identified electrophysiologically using extracellular recording methods and by subsequent horseradish peroxidase injection, which revealed their dendritic morphology and orientation. The electric field was precisely defined by measuring the voltage gradients induced in the cerebellum by 40 s constant‐current pulses. The field was constant in the vertical (dorso‐ventral) axis and zero in the horizontal plane, in agreement with theory. Neurones were modulated by applying a sinusoidal field at frequencies between 0.05 and 1.0 Hz. Modulated cells exhibited an increase in firing frequency and fell into one of four classes, depending on the direction of the field that produced the modulation. Thus neurones were excited by: ventricle‐directed fields (V modulation), pia‐directed fields (P modulation), both of the above (V/P modulation) or showed no consistent modulation (non‐modulation). Most Purkinje somata and primary dendrites (nineteen out of twenty‐eight) and most Purkinje dendrites (eighteen out of thirty), were V modulated with maximum rate proportional to the peak field intensity. The dendrites of these cells were consistently oriented toward the pia. Among the stellate cells, the lower molecular layer stellates, with dendrites extending predominantly towards the pia, were mostly (nineteen out of thirty‐two) V modulated. The mid‐molecular layer stellates, which showed much variability in dendritic orientation, were distributed among all four of the modulation classes. The upper molecular layer stellates, with a mostly horizontal dendritic alignment, were mainly (nine out of sixteen) non‐modulated. All groups of spiking elements showed a correlation between patterns of modulation by applied fields and dendritic orientation, which suggests the degree of differential polarization of the extended cable elements of the neurone by the applied field as the basic mechanism for field‐induced excitation or inhibition. The threshold for modulation among all neurones was 15‐20 mV/mm, which is similar to the fields that modulate other nervous tissues. This suggests that many neurones can be modulated by fields of the order of 10‐20 mV/mm.
Inward rectification and low threshold calcium conductance in rat cerebellar Purkinje cells. An in vitro study.
Authors: Crepel F , Penit-Soria J
Published: December 2014

The bioelectrical properties of Purkinje cells were analysed in sagittal slices of adult rat cerebellum by the use of intracellular recordings performed at a somatic level in current or in voltage clamp. The passive electrical constants of Purkinje cells were determined by measuring the time course and the amplitude of the voltage responses induced by hyperpolarizing current pulses. The mean value of input resistance was 21 +/‐ 1 M omega. Mean values of the membrane time constant and of the total electrotonic length of Purkinje cells were 19.5 +/‐ 1.7 ms and 0.59 +/‐ 0.01 ms respectively. A time dependent inward rectification was present in all cells. In current‐clamp experiments it appeared as a sag in hyperpolarizing voltage responses which were followed by well developed anodal breaks. In voltage‐clamped cells, the inward relaxation induced by hyperpolarizing commands fitted to a single exponential. It was already present near resting potential and could reach an amplitude of up to 4 nA for jumps near to ‐120 mV. This relaxation was provisionally termed Ih. Tail current relaxations also fitted to a single exponential when they were recorded in the presence of tetrodotoxin (TTX) and of Co. The inward relaxation induced by hyperpolarizing commands was readily blocked by Cs, whereas it was unaffected when Ba replaced Ca in the bath, except near rest where it was strongly reduced. The Ca channel blockers Cd, Co and D600 also markedly depressed or even suppressed the inward rectification near resting potential, and up to about ‐85 mV, whereas this blocking effect was much less apparent or even absent at more negative potentials. Ih was clearly enhanced when the external K concentration was raised up to 20 mM. In the presence of TTX and Co in the bath, inward relaxations induced by hyperpolarizing jumps were unaffected in Na‐free solution, whereas the amplitude of tail currents was reduced. Furthermore, the reversal potential of Ih which ranged between ‐45 and ‐56 mV in the Co plus TTX containing solution, shifted toward more negative values in the Na‐free medium. In contrast, Ih remained unchanged in low Cl solution. From these experiments, it is likely that K and Na are the main charge carriers of Ih. Furthermore, this current seems to be contaminated near resting potential by a Ca‐dependent K current. Anodal breaks following hyperpolarizing commands were slightly attenuated when Cd or TTX were added to the bath.(ABSTRACT TRUNCATED AT 400 WORDS)
Factors influencing an increase in spontaneous transmitter release by hypoxia at the mouse neuromuscular junction.
Authors: Nishimura M
Published: December 2014

To test a possibility that the functional buffering of intracellular Ca2+ plays a primary role in the enhancement of spontaneous transmitter release during hypoxia, the frequency of miniature end‐plate potentials (m.e.p.p.s) was examined under several conditions. At 36 degrees C, hypoxia (bubbling with 95% N2 and 5% CO2) increased the average frequency of m.e.p.p.s from about 3 s‐1 to 100 s‐1 or more, in a standard Krebs‐Ringer solution. This effect declined with a decrease in the temperature and was much reduced at 24 degrees C. Removal of external Ca2+ (addition of 2 mM‐EGTA), increase of Mg2+ levels to 5 mM, and treatment with 20 microM‐ouabain, which gave a slight increase, did not reduce the rise in m.e.p.p. frequency during hypoxia. Pre‐incubation of the tissue in a solution containing 10 mM‐KCl at 24‐32 degrees C and its subsequent exposure to hypoxia caused a very marked increase in m.e.p.p. frequency, while incubation in 10 mM‐KCl alone caused a small rise in the frequency. These data indicate that this combination potentiates the individual effects of each treatment. These experiments suggest that the hypoxia‐induced increase in spontaneous transmitter release is primarily due to an increase in intracellular Ca2+ levels, probably because of inhibition of mechanisms which control buffering and extrusion of intracellular Ca2+. The release and influx mechanisms which elevate intraterminal Ca2+ may also be involved passively in the effect of hypoxia.
Reinnervation of the lateral gastrocnemius and soleus muscles in the rat by their common nerve.
Authors: Gillespie M J , Gordon T , Murphy P R
Published: December 2014

To determine whether there is any specificity of regenerating nerves for their original muscles, the common lateral gastrocnemius soleus nerve (l.g.s.) innervating the fast‐twitch lateral gastrocnemius (l.g.) and slow‐twitch soleus muscles was sectioned in the hind limb of twenty adult rats. The proximal nerve stump was sutured to the dorsal surface of the l.g. muscle and 4‐14 months later, the contractile properties of the reinnervated l.g. and soleus muscles and their single motor units were studied by dissection and stimulation of the ventral root filaments. Contractile properties of normal contralateral muscles were examined for comparison and motor units were isolated in l.g. and soleus muscles for study in a group of untreated animals. Measurement of time and rate parameters of maximal twitch and tetanic contractions showed that the rate of development of force increased significantly in reinnervated soleus muscles and approached the speed of l.g. muscles but rate of relaxation did not change appreciably. In reinnervated l.g. muscles, contraction speed was similar to normal l.g. muscles but relaxation rate declined toward the rates of relaxation in control soleus muscles. After reinnervation by the common l.g.s. nerve, the proportion of slow motor units in l.g. increased from 10 to 31% and decreased in soleus from 80 to 31%. The relative proportions of fast and slow motor units in each muscle were the same as the proportions of fast and slow units in the normal l.g. and soleus muscles combined. It was concluded that fast and slow muscles do not show any preference for their former nerves and that the change in the force profile of the reinnervated muscles is indicative of the relative proportions of fast and slow motor units: fast units dominate the contraction phase and slow units the relaxation phase of twitch and tetanic contractions of the muscle.
Effect of potassium and acetylcholine on canine intestinal smooth muscle.
Authors: Hara Y , Szurszewski J H
Published: December 2014

Mechanical and intracellular electrical activity were recorded simultaneously from small intestinal smooth muscle of the dog. Tonic and phasic contractions due to exogenous acetylcholine and elevated external K+ concentration were spike‐dependent in longitudinal and inner circular muscle layers and spike‐independent in the outer circular muscle layer. Voltage‐tension curves were generated by graded depolarization of the membrane. In spike‐dependent longitudinal and inner circular muscle layers the threshold voltage for initiation of spikes and contraction was approximately ‐‐53 mV. In spike‐independent outer circular muscle layer the voltage threshold for contraction was approximately ‐42 mV. The resting membrane potential in longitudinal and inner circular muscle layers was close to the voltage threshold for initiation of spikes and contraction. In contrast, in the outer circular muscle it was approximately 20 mV more negative to the voltage threshold for contraction. In the outer circular muscle layer of whole‐thickness preparations an increase in the amplitude of phasic contractions caused by acetylcholine was associated with an increase in the amplitude of the slow waves. Tone was related to the resting membrane potential. In preparations of isolated outer circular muscle acetylcholine caused depolarization of the membrane potential, slow waves and phasic contractions; comparable depolarization by increases in external K+ concentration did not induce slow waves or phasic contractions. Comparison of the effect of acetylcholine on outer circular muscle with the voltage‐tension curve for this muscle layer showed that the top of the slow wave was associated with just the contractile force predicted by the voltage‐tension curve. This suggests that acetylcholine altered the force of phasic contraction of the outer circular muscle through a voltage‐dependent mechanism. In non‐neural cells located on the serosal side of the outer circular muscle layer of the dog, cat and rabbit, changes in external K+ concentration depolarized the membrane and abolished slow waves. There was no response to acetylcholine.
The effect of heart rate on in utero left ventricular output in the fetal sheep.
Authors: Anderson P A , Glick K L , Killam A P , Mainwaring R D
Published: December 2014

The effect of heart rate on left ventricular output was examined in seven fetal lambs at ages of 128 to 140 gestational days. The fetuses had been surgically instrumented at least 4 days previously with an ascending aortic flow probe, left ventricular dimension transducers, and left and right atrial pacing electrodes. Natural variations in heart rate of the lambs taken as a group correlated positively with left ventricular output, and negatively with ventricular end‐diastolic dimension and stroke volume (P less than 0.0001). Rate did not affect output with right atrial pacing. With left atrial pacing, it correlated negatively with output (P less than 0.0001). At both pacing sites, rate correlated negatively with end‐diastolic dimension and stroke volume (P less than 0.0001). The introduction of a longer interval during each pacing rate circumvented the rate‐related changes in dimension and allowed the ventricle to fill to the same end‐diastolic dimension. The systole at the end of the longer interval had a greater stroke volume than the preceding systoles. The faster the preceding paced rate, the greater was the stroke volume (P less than 0.0001). This study demonstrates that experimentally induced variations in heart rate produce changes in end‐diastolic volume and contractility which prominently affect stroke volume. Over a broad range of rates, however, the effect of rate on left ventricular output is either negative or absent. With naturally occurring rate changes, there are additional changes in contractility and venous return which affect stroke volume. These combine to produce a positive relation between heart rate and left ventricular output. These effects of heart rate on output are qualitatively similar to those described for the adult animal.
Effect of denervation on a steady electric current generated at the end‐plate region of rat skeletal muscle.
Authors: Betz W J , Caldwell J H , Harris G L
Published: December 2014

An electric current flows continuously out of the synaptic region of rat lumbrical muscle fibres. It is generated apparently as a result of a non‐uniform Cl‐ conductance (GCl), with GCl being lowest at the end‐plate. We investigated the effects of denervation on this current. The current persisted with little change after denervation. This was somewhat unexpected, since GCl falls dramatically after denervation, and in acute experiments on normal muscles, the steady current is greatly reduced by agents which block GCl. The steady current was blocked in denervated muscle, as in normal muscle, by low‐Cl‐ solutions, Na+‐free and K+‐free solutions, and treatment with furosemide and 9‐anthracene‐carboxylic acid. The current in denervated muscle appears to be generated by the same general mechanism as in normal muscle. The results suggest that the [Cl‐]i is significantly higher in denervated than in normal muscle fibres. Preliminary experiments with Cl‐ ‐selective micro‐electrodes have confirmed this: [Cl‐]i rises from about 12 mM to about 23 mM after denervation. This has the effect of moving the Cl‐ equilibrium potential (ECl) in a positive direction, so that the driving force for passive Cl‐ efflux is increased. The increased driving force compensates for the reduced GCl, allowing the steady current to persist in denervated fibres.
Incoming synapses and size of small granule‐containing cells in a rat sympathetic ganglion after post‐ganglionic axotomy.
Authors: Case C P , Matthews M R
Published: December 2014

A quantitative ultrastructural study has been made of the reaction of the incoming synapses of small granule‐containing cells after axotomy of the major post‐ganglionic branches of the superior cervical ganglion of the young adult rat. These cells are intrinsic and interneurone‐like in this ganglion, receiving a preganglionic input and giving outgoing synapses to principal post‐ganglionic neurones. Unlike their outgoing synapses, which are lost after post‐ganglionic axotomy (Case & Matthews, 1986), the incoming synapses of the small granule‐containing cells in axotomized ganglia increased in incidence post‐operatively. The increase first became clearly evident 5‐7 days post‐operatively and was greater, being both more sustained and progressive, after bilateral than after unilateral axotomy. After bilateral axotomy the incidence of incoming synapses rose to more than four times that of normal ganglia and was still elevated at 128 days post‐operatively, but was within normal limits at 390 days. After a unilateral lesion, increases of similar extent and time course to those in the axotomized ganglia were seen in the incoming synapses of small granule‐containing cells in the uninjured contralateral ganglia. The incoming synapses of the small granule‐containing cells are multifocal, i.e. show several points or active foci of synaptic specialization. The increase in synapses expressed itself both through an increased incidence of these synaptic active foci per nerve terminal and through an increase in the number of presynaptic nerve terminal profiles associated with the cells. Control observations indicated that the increase in synapses was not due to surgical stress, nor was it attributable solely to post‐operative ageing. The nerve terminals which were presynaptic to the small granule‐containing cells post‐operatively were all of preganglionic origin: no incoming synapses or presynaptic nerve terminals remained at 2 days after a preganglionic denervation of axotomized or contralateral ganglia, at whatever stage this was performed throughout the range of survival intervals. There was some evidence that the synapses had increased by sprouting, including terminal sprouting, of the preganglionic nerve fibres. In the shorter term there was an increase in the proportion of small nerve terminal profiles. In the longer term the mean size of the terminal profiles increased, and very large terminals of unusual form were seen. After post‐ganglionic axotomy, and in particular after a bilateral lesion, the small granule‐containing cells became hypertrophied.(ABSTRACT TRUNCATED AT 400 WORDS)
Induction of action potentials by denervation of tonic fibres in rat extraocular muscles.
Authors: Bondi A Y , Chiarandini D J , Jacoby J
Published: December 2014

The effects of denervation by nerve section on the electrical properties of tonic and twitch fibres of rat extraocular muscles were examined. Normally innervated tonic fibres lack action potentials. Upon direct stimulation they generate graded, voltage‐dependent responses or slow peak potentials (s.p.p.s). However, one week after denervation the s.p.p.s are transformed into action potentials which are slower and broader than those of twitch fibres. The action potentials are Na dependent and partially resistant to blockade with 10(‐5) M‐tetrodotoxin and 10(‐6) M‐saxitoxin. Changing the holding potential of the fibres from ‐80 mV to more negative levels increases the maximal rate of rise of the action potential. This effect is not observed on the s.p.p.s of normally innervated fibres. Following denervation the resting potential of tonic and twitch fibres becomes about 10‐15 mV less negative. In denervated muscles stimulation with pulses of hyperpolarizing current evokes graded responses in tonic fibres and action potentials in twitch fibres. In normally innervated muscles, these anodal break responses are never observed in tonic fibres and are very rare in twitch fibres. By two weeks after nerve section, reinnervation is present. The action potentials of tonic fibres are still present but stronger stimulation is needed to evoke anodal break responses. By three weeks, direct stimulation of tonic fibres evokes normal s.p.p.s in about 25% of the studied fibres and action potentials in the rest. By four weeks, most tonic fibres have lost the action potential but small anodal break responses can be evoked in most. It is suggested that following denervation a new population of Na channels appears in tonic fibres. The properties of these channels are different from those of the channels normally present in innervated tonic fibres but they are in some ways similar to those of the channels which appear in twitch fibres following denervation.
Jejunal myoelectrical activity in the conscious neonatal pig.
Authors: Burrows C F , Merritt A M , Tash J
Published: December 2014

The purpose of this study was to define and quantify patterns of normal jejunal myoelectrical activity in the conscious neonatal pig. Twelve 3‐day‐old piglets were obtained from a local herd. At 7 days of age four bipolar Ag‐AgCl electrodes were surgically implanted at 5 cm intervals on the jejunum. Piglets were divided into fed (n = 7) and fasted (n = 5) groups and a minimum of four daily recordings were made from each pig between 7 and 14 days of age. Slow wave and spike activity were seen in all animals. Slow waves occurred at a frequency of 17.0 +/‐ 0.3 cycles/min (c.p.m.) in fed and 16.8 +/‐ 0.2 c.p.m. in fasted piglets. Spike activity predominated, occurring in characteristic migrating myoelectrical complexes (m.m.c.s.) and occupying 79% of the recording time in fed and 73% in fasted piglets (P less than 0.05). The activity front (phase 3) of the m.m.c. recurred every 47.7 +/‐ 2.4 min in fed, and 50.9 +/‐ 2.3 min in fasted piglets. The m.m.c. periodicity in both groups was irregular, with a range of 4‐145 min in fed, and 8‐104 min in fasted piglets. Activity fronts lasted 4.8 +/‐ 0.1 min in fed and 4.4 +/‐ 0.1 min in fasted piglets. All piglets demonstrated intense, short (2.5‐5.0 s), distinct bursts of intense spike activity (migrating action potential complexes, m.a.p.c.s.). These were rapidly conducted in an aboral direction at a velocity of 1.8 cm/s in fed, and 0.8 cm/s in fasted piglets (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)
Calcium and delayed potassium currents in mouse pancreatic beta‐cells under voltage‐clamp conditions.
Authors: Rorsman P , Trube G
Published: December 2014

Pancreatic islets of NMRI mice were dissociated into single cells which were kept in tissue culture for 1‐3 days. The whole‐cell configuration of the patch‐clamp technique was used to study inward and delayed outward currents of beta‐cells under voltage‐clamp conditions at 20‐22 degrees C. Outward currents were suppressed by substituting the impermeant cation N‐methyl‐D‐glucamine for intracellular K+. The remaining inward current had a V‐shaped current‐voltage relation reaching a peak value of 39 +/‐ 4 pA (mean +/‐ S.E. of mean) around ‐15 mV. It was identified as a Ca2+ current, because the peak amplitude was increased 1.6 times by increasing external [Ca2+] ([Ca2+]o) from 2.6 mM to 10 mM and it was blocked by Co2+ (5 mM) or nifedipine (5 microM) but not by TTX (20 microM). The activation time constant of the inward current at ‐10 mV was 1.28 +/‐ 0.08 ms. The relation between the degree of activation (estimated from the size of the tail currents) and membrane potential V followed the sigmoidal function f = 1/(1 + exp [(Vh‐V)/k]) with half‐maximal activation potential, Vh = 4 +/‐ 1 mV and slope factor, k = 14 +/‐ 1 mV (for [Ca2+]o 10 mM). The inward current inactivated only weakly during depolarizing pulses of 0.1‐1 s duration. The delayed outward current (in experiments with 155 mM‐internal [K+] ([K+]i)) had a linear voltage dependence at potentials above ‐20 mV; its amplitude at ‐10 mV was 210 +/‐ 30 pA. Tail currents related to the activation of the outward current had K+‐dependent reversal potentials. The current was blocked by extracellularly applied tetraethylammonium (20 mM) and 4‐aminopyridine (2 mM). It was not affected by glibenclamide (3 microM), tolbutamide (0.2 mM) and alterations of intracellular [Ca2+] (1 nM‐1 microM). The activation time constant of the outward current at ‐10 mV was 21 +/‐ 3 ms. The voltage dependence of activation could be described by the sigmoidal function (see above) with Vh = 19 +/‐ 1 mV and k = 5.6 +/‐ 0.4 mV. The outward current inactivated during long (15 s) depolarizing pre‐pulses (time constant at ‐10 mV: 2.6 +/‐ 0.6 s). 50% inactivation occurred at Vh = ‐36 +/‐ 2 mV, k was ‐4.1 +/‐ 0.2 mV. Inward and outward currents during depolarizing voltage pulses in beta‐cells are similar to Ca2+ and delayed K+ currents in other cell types. These currents seem sufficient to generate the action potentials of the beta‐cell.
Cyclic adenosine monophosphate, calcium, acetylcholine and the current induced by adenosine in the Xenopus oocyte.
Authors: Stinnakre J , Van Renterghem C
Published: December 2014

The K+ current response to bath‐applied adenosine has been studied on follicle‐enclosed full grown oocytes from Xenopus laevis, using the two electrodes voltage‐clamp technique. The response to adenosine was mimicked by forskolin, an activator of adenylate cyclase. Forskolin applied at low concentration potentiated the response to adenosine. At low concentration, isoprenaline, a beta‐adrenergic agonist known to induce a potassium current via a rise of adenosine 3',5'‐phosphate (cyclic AMP) into the oocyte, potentiated the response to adenosine. Progesterone (10(‐5) M) reversibly induced a slight decrease (‐24%) of the response to adenosine. The calcium ionophore A23187 applied in normal external medium reduced the response to adenosine (about ‐70%). Intracellular injection of EGTA induced an increase (+64%) of the peak response to adenosine. Acetylcholine (0.5‐10 microM) inhibited the response to 3‐10 microM adenosine by 44‐91%. This inhibition was suppressed by atropine and was seen even on cells which did not show any current in response to acetylcholine application. The inhibition by ACh of the sensitivity to adenosine was long lasting (more than 1 h after the wash‐out of ACh). A long term inhibition (‐28 to ‐90%) also occurred when ACh was applied alone and washed before adenosine application. It is concluded that in Xenopus oocyte: increased cyclic AMP synthesis mediates the potassium response to adenosine; intracellular calcium ion concentration modulates this response; muscarinic stimulation induces a long‐lasting inhibition of the sensitivity to adenosine.
Differentiation of voltage‐gated potassium current and modulation of excitability in cultured amphibian spinal neurones.
Authors: Barish M E
Published: December 2014

Gigaohm‐seal whole‐cell voltage‐clamp techniques were used to study the development of ionic currents in the membrane of embryonic amphibian (Ambystoma) spinal neurones during in vitro differentiation. Dissociated neural plate cells, some of which are neuronal precursor cells, were placed into culture. Cells became excitable at the time of neurite outgrowth, 2‐3 days later, and over the next 2‐10 days the duration of the action potential shortened from about 100 ms to about 1 ms. Voltage‐clamp recordings demonstrated that at the time of appearance of neurites, activatable Na, Ca and voltage‐gated K channels were present in the membrane (Ca‐dependent K channels were not studied). Over succeeding days in culture, records of total membrane current indicated that the amplitudes of peak inward and steady‐state outward currents both increased. As a result of these increases, the pattern of total membrane current came to be increasingly dominated by outward currents. With inward Na and Ca currents blocked, a voltage‐gated K current (IK(V] could be studied in isolation. The reversal potential of this current varied in good agreement with the equilibrium potential for K ions predicted by the Nernst relation. The wave form of IK(V) activation was sigmoidal. Activation was more rapid at more positive voltages (relative to the usual holding potential of ‐70 mV), and deactivation was more rapid at more negative voltages. The amplitude of IK(V) increased during neural development, while cell size remained approximately constant. Increases in rates of activation and deactivation were observed in parallel with the increase in current density. When measured at 0 mV, cells studied on day 4 of culture or earlier showed steady‐state chord conductances (gK(V] of less than 20 nS, and one‐half activation times (t1/2) of 2 X 5‐10 ms. Older cells showed gK(V)s of 10‐80 nS, and t1/2s of 0 X 8‐2 X 5 ms. As Na, and to a lesser extent Ca, current amplitudes were also increasing during differentiation, these observations concerning IK(V) suggested that its amplitude and kinetic changes might in part be responsible for the observed decrease in action potential duration during development. This hypothesis was tested by modelling Na, Ca and voltage‐gated K currents and testing the effects of changes in amplitude and kinetics of IK(V) on the duration and ionic dependence of reconstructed action potentials. The results obtained using this model suggested that the increase in IK(V) amplitude and activation rate was sufficient to change action potential duration and apparent ionic sensitivity.
The variability of the maintained discharge of cat dorsal lateral geniculate cells.
Authors: Levine M W , Troy J B
Published: December 2014

Maintained discharge in the presence of a steady background luminance was analysed from forty‐eight cells in the A laminae of the dorsal lateral geniculate nucleus of cat. Cells were categorized as XG or YG and, in most cases, as on‐or off‐centre. The temporal contrast sensitivity function of twenty‐seven of the cells was measured using drifting gratings of the optimal spatial frequency. The maintained discharge was characterized by several simple descriptors, including the interval distribution, mean firing rate, and coefficient of variability. Its temporal organization was revealed by two indicators of correlational properties, the normalized autocovariance and the serial correlogram, and more effectively, by the less familiar plot of the standard deviation of firing rate versus sample duration. The statistics revealing temporal organization of the maintained discharge indicated that the variability of firing was nearly, but not quite, derived from a renewal process. The maintained discharges of seven cells were studied for more than one luminance level. Mean luminance did not appear to have any consistent effect upon the statistics of the maintained discharge. The temporal filtering properties of lateral geniculate cells were deduced from a comparison of the temporal contrast sensitivities of geniculate neurones (Troy, 1983b) and retinal ganglion cells (Lennie, 1980). Analyses showed that the maintained discharge of retinal neurones passed through this filter could not account for the observed statistics of the maintained discharge of geniculate neurones. It is proposed that additional noise is added to the retinal signal at the level of the lateral geniculate. Models are presented to explain how the signals might be filtered in a way that does not also affect the added noise.
Contractile activation in myotomes from developing larvae of Xenopus laevis.
Authors: Huang C L
Published: December 2014

Contractile activation in response to application of different perfusing solutions was observed in developing myotomes from larvae of Xenopus laevis. Contractures resulted from treatment with isotonic solutions containing high K+ concentrations in myotomes from embryos from stage 24 onwards. These developed over 5‐10 s and inactivated over 20‐30 s. Contractures persisted in curarized preparations. The applied K+ concentrations at the estimated mechanical threshold in embryos at stages 26 and 34 were about 30 and 20 mM respectively. Maximum activity was achieved at K+ concentrations of 78 and 48 mM respectively. The lyotropic ion thiocyanate (10 mM) potentiated K+ contractures, and shifted the threshold K+ concentration to lower values. Contractures persisted in the presence of 3 mM‐Mn2+ and in Ca2+‐free solutions, with unaltered mechanical thresholds. K+ contractures were reversibly abolished by 2 mM‐tetracaine. Comparisons of resting potentials in 3 and 20 mM‐K+ confirmed that the resting potential remained sensitive to external K+ concentration, whether in control, tetracaine‐containing, or low‐Ca2+ bathing solutions. Caffeine (10‐20 mM) caused sustained contractures from stage 24 onwards. Caffeine sensitivity was greatest at stages 26‐28, then appeared to decline at stages 33‐36. These observations suggest that mechanisms for excitation‐contraction coupling develop in Xenopus embryos at the same time as do completed sarcomeres, close to stage 24. Activation of contraction then assumes the adult pattern, involving voltage‐dependent release of the intracellular store of activator, independent of entry of extracellular Ca2+.
The dependence on external cation of sodium and potassium fluxes across the human red cell membrane at low temperatures.
Authors: Blackstock E J , Stewart G W
Published: December 2014

The fluxes of Na and K across the human red cell membrane have been studied as functions of temperature and external cation composition. In media containing any of a variety of organic compounds as the principal cation (choline, N‐methyl‐D‐glucamine (NMDG), arginine, L‐lysine and trimethyl‐phenylammonium), the ouabain plus bumetanide‐insensitive influxes and effluxes of Na and K displayed marked paradoxical temperature dependence such that the flux minimum, which normally occurs at about 8‐10 degrees C in a NaCl medium, was shifted up to about 20 degrees C. Inhibitor and anion replacement studies excluded contributions by the major carrier‐mediated systems evident at 37 degrees C. At 0 degrees C in NMDG, about 1 mM‐external Na and 10 mM‐external K were required half‐maximally to inhibit the K and Na influxes respectively. When the K(86Rb) efflux in NMDG media at 0 degrees C was measured in the presence of low concentrations of a series of external inorganic ions and guanidine, the order of potency for reduction of the efflux was Li greater than Mg = Ca greater than Ba greater than Sr greater than Na greater than Rb = Cs = guanidine greater than K. The influxes of the neutral amino acid L‐leucine and the cationic species L‐lysine both showed simple monotonic temperature dependence in Na and NMDG media. These effects show that the permeability of the human red cell membrane to inorganic univalent cations at low temperatures is markedly dependent on the external ionic conditions. Low permeability is favoured by the presence of cations with a high charge density.
Tubocurarine suppresses slow calcium‐dependent after‐hyperpolarization in guinea‐pig inferior mesenteric ganglion cells.
Authors: Dun N J , Jiang Z G , Mo N
Published: December 2014

Intracellular recordings were made from neurones of the isolated guinea‐pig inferior mesenteric ganglia. Single‐spike potentials evoked by either depolarizing current pulses applied through the recording micro‐electrode or stimulation of the hypogastric nerves were followed by an after‐hyperpolarization (a.h.). The spike a.h. in 40% of the neurones, referred to herein as type I, had a relatively short duration (less than 50 ms) and exhibited a monophasic decay with a mean time constant (tau) of 11.4 ms. In the remaining cells (type II), the spike was followed by a long a.h. (greater than 100 ms) having a double‐exponential decay; the fast and slow components of the a.h. are termed a.h.f and a.h.s, respectively, and they had mean tau values of 11.4 and 74 ms, respectively. A.h.f and a.h.s of type II neurones were reduced by membrane hyperpolarization and reversed their polarities between ‐80 and ‐90 mV. The reversal potentials shifted in a manner closely predicted by the Nernst equation as external K+ concentration was increased. Superfusion of low‐Ca2+ high‐Mg2+ solution to type II neurones reduced the a.h.f and a.h.s by 32 and 82%, respectively, indicating that a.h.s is largely Ca2+‐dependent. Application of (+)‐tubocurarine (10‐100 microM) reversibly suppressed the a.h.s without affecting a.h.f in a concentration‐dependent manner. Following a short train of action potentials evoked from type II neurones, the post‐tetanic hyperpolarization (p.t.h.) was similarly depressed by (+)‐tubocurarine in a dose‐dependent manner. (+)‐tubocurarine did not significantly change the amplitude of Ca2+‐dependent spike potentials evoked in neurones bathed in Na+‐free high‐Ca2+ plus tetraethylammonium (5‐10 mM) solution. The results indicate that (+)‐tubocurarine selectively suppresses a.h.s, a slow Ca2+‐dependent a.h., the consequence of which is a facilitation of repetitive discharges of the cells.
Barium ions induce prolonged plateau depolarizations in neurosecretory neurones of the adult rat supraoptic nucleus.
Authors: Bourque C W , Brown D A , Renaud L P
Published: December 2014

The occurrence and ionic basis of prolonged plateau depolarizations were studied during intracellular recordings obtained from thirty‐nine supraoptic nucleus (s.o.n.) neurosecretory neurones in perfused explants of rat hypothalamus. Replacement of Ca2+ by Ba2+ in the perfusion media enhanced the shoulder on the repolarization phase of the action potentials in all of eleven cells tested. In Ba2+, spike durations increased as the holding membrane potential was made more positive, resulting in plateaux lasting up to 100 s. These plateaux were characterized by a sustained but slowly decaying absolute potential near 0 mV from which there appeared frequent spontaneous hyperpolarizing transients. A membrane resistance decrease of more than 50% was observed at the onset of a plateau, with gradual restoration during the plateau. Injection of Cs+ into twenty‐one cells abolished the spike frequency adaptation and after‐hyperpolarization associated with current‐evoked bursts of action potentials. Mean spike duration after Cs+ injection increased from 2.5 +/‐ 0.3 ms to 68 +/‐ 9 ms (S.E. of mean). Addition of 4‐5 mM‐tetraethylammonium (TEA) to the perfusion media further increased the spike duration of nine Cs+‐injected cells to 320 +/‐ 70 ms. No further increase could be obtained by doubling the concentration of TEA and/or by the addition of 0.2‐0.5 mM‐4‐aminopyridine to the media. Although spike duration was greatly prolonged during such extensive blockade of K+ channels, plateau potentials lasting for longer than 1 s were observed only when Ba2+ was eventually added to the perfusion media. The special property of Ba2+ that leads to the formation of plateau potentials in s.o.n. neurones is therefore not restricted to its ability to reduce K+ conductances but may reside in its reduced effectiveness as a mediator of Ca2+‐dependent inactivation of Ca2+ channels. Injection of Cs+ into s.o.n. neurones increased the slope of their current‐voltage relationship below ‐60 mV from 148 +/‐ 15 to 257 +/‐ 41 M omega (S.E. of mean) and eliminated the outward rectification present at potentials above ‐60 mV. K+ currents are presumably active near the resting potential of these cells. Addition of Ba2+ to the perfusion media revealed a Cd2+‐sensitive inward rectification above but not below ca. ‐55 mV. A slowly inactivating Ba2+ current is therefore carried through Ca2+ channels at potentials above ‐55 mV.(ABSTRACT TRUNCATED AT 400 WORDS)
Patterns of breathing in response to alternating patterns of alveolar carbon dioxide pressures in man.
Authors: Cunningham D J , Howson M G , Metias E F , Petersen E S
Published: December 2014

The time profile of alveolar PCO2 within the respiratory cycle has been forced to follow contrasting patterns in alternate breaths, in two different ways. Within‐breath changes (w.b.c., with a CO2‐rich inspirate supplied early or late in alternate inspirations) involved minimal alternation of end‐tidal PCO2. Between‐breath changes (b.b.c., with whole inspirates of CO2‐free or CO2‐rich gas) involved large swings of end‐tidal PCO2. As previously reported (Metias, Cunningham, Howson, Petersen & Wolff, 1981), both patterns of forcing were associated with alternation of ventilation, but only when hypoxia was present. The patterns of the alternating reflex responses in 118 runs on four human subjects in steady hypoxia are described in terms of alternation of inspiratory and expiratory tidal volume, time and mean flow. These patterns often disappeared, or changed unpredictably in mid‐run. The inspiratory pattern of reflex alternation depended in part on the type of forcing, but alternation of inspiratory tidal volume was usually observed with both types. No single pattern of expiratory alternation emerged as predominant. The pattern of reflex expiratory alternation was surprisingly independent of the pattern of inspiratory alternation: indeed, in w.b.c., but not in b.b.c., alternation of mean expiratory flow and of mean inspiratory flow were mutually exclusive. It is concluded that in man, as in cats and dogs, the arterial chemoreceptor pathway has access to various parts of the respiratory pattern generator, the exact response depending to some extent on the timing within the respiratory cycle. In particular, expiratory variables may be influenced directly through the arterial chemoreceptor pathway, i.e. without any supposedly mediating inspiratory alternation being demonstrable. The results are discussed briefly in relation to some current views on the organization of respiratory control.
Binding of triiodothyronine by fully differentiated rat enterocytes.
Authors: Hewitt J E , Smith M W
Published: December 2014

The cellular binding characteristics of triiodothyronine (T3) have been measured in a preparation of isolated fully differentiated enterocytes prepared from thyroidectomized rats. Specific binding of [125I]T3, defined as that portion displaced by the presence of 20 microM‐non‐radioactive hormone, was shown to become maximal after 100 min incubation with cells at a temperature of 15 degrees C. The cellular release of lactic dehydrogenase measured under these conditions was found to be 18%. Thin layer chromatography of radioactive compounds recovered from enterocytes and from incubation medium showed less than 2% radioactivity to be as free iodide. The amount of [125I]T3 specifically bound to enterocytes showed a curvilinear dependence on the concentration of T3 present in the incubation medium. Scatchard plot analysis of this data revealed the presence of both high (KD 1.2 +/‐ 1.1 X 10(‐9) M) and low (KD 1.9 +/‐ 0.8 X 10(‐7) M) affinity binding sites in these cells. Previous injection of non‐radioactive T3 into thyroidectomized rats, at a time chosen to ensure negligible levels of circulating T3 at the time of the experiment, caused a 3‐fold reduction in the maximal binding capacity of the low‐affinity site (83 compared with 28 pmol/mg protein). Neither the affinity nor the number of high‐affinity binding sites for T3 were significantly affected by this treatment. These results are compared with those obtained using other types of cell responsive to T3. The possible physiological importance of these findings is discussed.