Chloride conductance in normal and myotonic muscle fibres and the action of monocarboxylic aromatic acids

1. Cable parameters, component conductances, excitability and membrane potentials in isolated external intercostal fibre bundles at 38 degrees C from normal and myotonic goats were measured in normal and low-chloride Ringer, and in the presence of monocarboxylic aromatic acids that produce myotonic responses in mammalian muscle.2. The mean resting chloride conductance in mumho/cm(2) of myotonic fibres (range 0-147) was significantly less than that of normal fibres (range 376-951). The mean resting potassium conductance was higher in myotonic fibres (range 123-285) than in normal fibres (range 44-132). Potassium conductance increased about 10 mumho/cm(2) per mV increase in absolute resting potential.3. In normal fibres in normal Ringer 3-chloro-2,5,6-trimethylbenzoic acid; 5,6-dihydro-5,5-dimethyl-7-carboxybenz[c]acridine; phenanthrene-9-carboxylic acid; and anthracene-9-carboxylic acid at 10(-5)-10(-4)M decreased membrane conductance without consistently changing diameter or capacitance. In low-chloride Ringer 3-chloro-2,5,6-trimethylbenzoic acid (5 x 10(-5)M) increased potassium conductance in myotonic and normal fibres. It is concluded that these compounds block chloride conductance.4. The carboxylic acids produced myotonia in normal fibres similar to that in untreated myotonic fibres.5. Anthracene-9-carboxylic acid intravenously (8 mg/kg) in normal goats produced acutely a condition resembling myotonia congenita. The carboxylic acids produced no myotonic effects in frog muscle.     

Simulation of the Interaction Between Muscle Fiber Conduction Velocity and Instantaneous Firing Rate

In this study, the relationships between the early and late afterpotentials and velocity and amplitude recovery functions (VRF and ARF) in skeletal muscle were examined using model simulation. A mathematical model of the muscle fiber action potential, that incorporated a tubular slow potassium conductance, was developed and used to simulate muscle fiber action potentials at a range of interpulse intervals. The slow potassium conductance produced an afterhyperpolarization which resulted in supernormal action potential conduction velocity and amplitude for interpulse intervals >7 ms. Increasing the number of conditioning stimuli caused a further increase in conduction velocity and amplitude, and an additional phase of supernormality, with a peak at approximately 100 ms. Positive correlations between instantaneous firing rate and both conduction velocity and amplitude were also observed during simulation of repetitive stimulation of the muscle fiber. The relationships were eliminated when the slow potassium conductance channel was removed from the model. The results suggest that an afterhyperpolarization, possibly due to a slow tubular potassium conductance, could cause the VRF and ARF observed in muscle. They additionally suggest that the positive correlations between instantaneous firing rate, conduction velocity, and amplitude are directly related to the VRF and ARF.     

Electrical properties of white and red muscle fibres of the elasmobranch fish Scyliorhinus canicula

1. Standard electrophysiological techniques (including a method which controls the membrane potential at a point on a muscle fibre) were used to investigate the electrical properties of white and red muscle fibres of the elasmobranch fish Scyliorhinus canicula.2. The resting potential of the white fibres in the standard Ringer solution was - 85.2 +/- 0.4 mV. That of the red fibres was - 71.1 +/- 1.2 mV. The membrane resistance of white fibres was 1588 +/- 97 Omega cm(2) and that of red fibres was 5410 +/- 1070 Omega cm(2).3. White fibres always responded to direct stimulation with an action potential. It proved impossible, with two impaling micro-electrodes, to record action potentials from the red fibres, although on one occasion an abortive spike was seen.4. The resting membrane of the red fibres seemed less permeable to chloride than was the membrane of the white fibres. However, the resting potassium permeability showed the potential dependence called inward or anomalous rectification in both white and red fibres.5. White fibres responded to square depolarizing pulses with conductance changes to sodium and, subsequently, to potassium.6. All red fibres examined with the point voltage clamp showed a delayed increase in potassium conductance on depolarizing.7. Out of twenty-seven red fibres examined, six showed no sign of having any sodium conductance mechanism. Eight showed large sodium currents on depolarizing, and the remaining thirteen had small sodium currents. It seemed likely that the group of eight fibres might be able to propagate action potentials.     

Optical evidence for a chloride conductance in the T-system of frog skeletal muscle

T-system action potentials were recorded optically from intact frog skeletal muscle fibers stained with the non-penetrating potentiometric dye NK-2367. The effect of chloride removal on the falling phase of the radially propagating tubular action potential was studied to determine whether a chloride conductance located in the T-system membranes contributes to tubular repolarization during activity. Our results show that, in chloride-free Ringer, repolarization of the tubular action potential is significantly slowed. Moreover, the late phase of tubular repolarization is characterized by a large afterpotential, which is highly temperature-dependent and appears as a secondary peak above 10° C. The optical data were compared with predicted T-system action potentials generated from a radial cable equivalent circuit model of the T-system, in which the effects of a distributed tubular leak conductance were tested. Results of this analysis are consistent with the proposal that some of the outward repolarization current during the T-system action potential is drawn across a chloride conductance located in the T-system membranes.     

Potassium conductance changes in skeletal muscle and the potassium concentration in the transverse tubules

1. When one hyperpolarizes a muscle fibre by passing electric current, the K conductance declines with time. Voltage-clamp experiments on frog sartorius muscle fibres showed that two components contribute to this decline.2. A rapid component operates when the fibre is hyperpolarized to potentials more negative than -120 mV. Decline by this mechanism is reversed completely within 200 msec. The large effect of temperature on the kinetics of this process indicates that it represents a time-dependent membrane permeability change.3. A slow component operates also at less negative potentials. Recovery at -65 mV takes place with half-times of about 0.4 sec. The Q(10) for the rate of recovery is 1.3, indicating that this process is diffusion limited.4. After prolonged hyperpolarization to potentials positive to -120 mV, membrane current at the resting potential is outward and persists for several seconds. At that time, the potential measured in the absence of membrane current is shifted in the negative direction by 3-5 mV.5. This shift and the time course of currents near the resting potential after hyperpolarization as well as the Q(10) of 1.3 indicate that the slow process is due to changes in tubular K concentration and not to a time-dependent membrane permeability change.6. At potentials less negative than -120 mV, tubular depletion can satisfactorily account for the decline of K conductance. At more negative potentials, the decline appears to be due to both depletion and a permeability change.     

Experimental myotonia in mammalian skeletal muscle: changes in membrane properties

Summary When myotonia is induced in rat diaphragms either by addition of 2,4-dichlorphenoxyacetate to the solution in which the excised muscle is bathed or by feeding animals on 20,25-diazocholesterol, the most significant change in membrane properties of the myotonic muscle fibres is an increase of the specific membrane resistance by a factor of 2 to 2.5. Membrane resting potential and capacitance are not altered. Like in healthy muscle, in 20,25-induced myotonic muscle the fibre membrane does not show rectifier properties ± 12 mV around the resting potential. The threshold for eliciting an action potential is unchanged but it is easier to start a burst of action potentials with a suprathreshold stimulus.This work was supported by the Deutsche Forschungsgemeinschaft.     

Radial spread of contraction in frog muscle fibres

1. The membrane potential of isolated muscle fibres in solutions containing tetrodotoxin (TTX) was controlled with a two-electrode voltage clamp. The striation pattern in the region of the electrodes was observed microscopically.2. With square steps of depolarization of increasing magnitude, contraction occurs first in the myofibrils just beneath the surface membrane, and then spreads inwards towards the axis of the fibre as the depolarization is increased.3. From the depolarizations which make the superficial and axial myofibrils contract it is possible to estimate a space constant (lambda(T)) for electrotonic spread in a transverse tubular network.4. lambda(T) was found to vary with fibre radius; for a 50 mu fibre it was about 60 mu. lambda(T) was not greatly affected by tetraethylammonium (TEA) chloride (111 mM), or by sucrose substitution of most of the sodium chloride in the Ringer solution.5. The ratio of the depolarization threshold for contraction of surface myofibrils and of central myofibrils was smaller for short (3 msec) than for long depolarization.6. Action potentials, recorded from a sartorius fibre, were used as the command signal for the voltage-clamped fibre in tetrodotoxin. The central myofibrils of this fibre did not appear to contract unless the imposed ;action potentials' were of normal size.7. The passive electrical characteristics of the transverse tubular system will just allow an action potential, at room temperature, to activate the myofibrils at the centre of a frog muscle fibre. An active potential change would be required to achieve a safety factor appreciably greater than one for this process.     

An electrophysiological investigation of the slow fibre system in the frog rectus abdominis muscle

1. Junctional potentials were recorded with a micro-electrode inserted into muscle fibres of the rectus abdominis muscle of the frog. Large and small nerve fibres were stimulated separately, using a selective stimulation technique. In a few muscle fibres rectangular current pulses were applied through a second micro-electrode to examine current-voltage relations.2. Two groups of muscle fibres could be distinguished: (a) Muscle fibres with resting potentials less negative than -75 mV, being localized at the ventral surface only and representing about 9% of the superficial fibres. These fibres were found to be innervated by small (high-threshold) nerve fibres and responded to single indirect stimuli with a typical s.j.p. (long latency, multiple components, after-hyperpolarization); upon repetitive stimulation summation of s.j.p.s and a slowly increasing tension could be recorded. The fibres showed a slow time course of electrotonic potentials, relatively high membrane resistance and ;delayed rectification'. (b) Muscle fibres with generally much higher resting potentials, being innervated by large (low-threshold) nerve fibres only. Upon indirect stimulation a short-latency end-plate or action potential (followed by a twitch) was recorded. The membrane time constant and input resistance of these fibres were similar to those of twitch fibres in other frog muscles.3. No evidence was found in the present work for the existence of an intermediate type of muscle fibre.4. It is concluded that the rectus muscle, similar to other frog muscles, contains two distinct types of muscle fibres, twitch and slow.     

Effects of histamine on hippocampal pyramidal cells of the rat in vitro

Summary The actions of bath applied histamine on CA1 pyramidal cells were investigated in hippocampal slices of the rat. Histamine caused a) a slight depolarization but no significant change in resting membrane conductance; b) an abbreviation of long afterhyperpolarizations after single action potentials, bursts of action potentials or TTX resistant spikes; c) a loss of accommodation of firing. In the presence of TEA or barium, histamine prolonged and increased the size and number of the slow TTX resistant spikes. A depolarizing plateau which follows such spikes was also increased by histamine. Evoked synaptic potentials were unaffected by histamine, but the population spike was increased. The frequency of spontaneous chloride dependent potentials, which reflect interneurone firing, was also increased. These effects considerably outlasted histamine application and were mimicked by the H₂-agonist impromidine but not the H₁-agonist thiazolethylamine, and blocked by the H₂-antagonists cimetidine and metiamide but not the H₁-antagonists mepyramine or the beta-antagonist propranolol. It is concluded that histamine, by activating H₂-receptors, antagonizes a calcium mediated potassium conductance in hippocampal pyramidal cells without affecting calcium current. By this mechanism histaminergic afferent fibres could effectively regulate cortical responsiveness by selectively potentiating large excitatory inputs of target neurones.     

Evidence for a transient potassium membrane current dependent on calcium influx in crab muscle fibre.

1. Voltage-clamp experiments were achieved on crab muscle fibre with the double sucrose-gap technique. 2. The accuracy of the imposed voltage has been controlled with an impaled micro-electrode connected to an external circuit. 3. Step depolarizations elicit two kinds of records. In type I fibres, the initial current exhibits only an inward calcium component. In type II fibres, the initial current exhibits a hump, transient outward current, mixed with the calcium current; these fibres exhibit always action potentials with fast repolarization. 4. A potassium origin is suggested for this outward current, due to its dependence on [K]o and its inhibition by TEA. 5. In fibres with a composite initial current, the voltage dependence of the availability of the measured inward current appears complex. It can be shown to be the sum of a simple calcium inactivation (which is observed alone in TEA solution) and a fast potassium inactivation. This potassium conductance is nearly half-available at the resting membrane potential. 6. The origin of the transient outward current is tentatively described. Consecutive to a transient internal increase of calcium ions (due to the calcium current) its activation curve is shifted in an hyperpolarizing direction resulting in an increased activation for an apparent identical depolarization. 7. This fast outward current which overlaps the calcium inward current can account for the low amplitude and the variability of the electrical activity of crab muscle fibres.     

Membrane properties underlying spontaneous activity of denervated muscle fibres

We have examined the events underlying the initiation of spontaneous action potentials (fibrillation) in fibres of previously denervated rat diaphragm maintained in organ culture for up to 10 days.1. Based on discharge pattern, two classes of spontaneously active fibres were found: rhythmically discharging fibres, and fibres in which action potentials occur at irregular intervals.2. Sites of action potentials initiation were located by exploration along the fibre length with two independent extracellular recording electrodes. The majority of sites of origin in both regular and irregular fibres were at the former end-plate zone; however, there was no region along the length that could not, at least in some fibres, be a site of origin.3. Intracellular recording at or near sites of origin of action potential discharge showed two types of initiating events. Irregularly discharging fibres were brought to threshold by discrete depolarizations of up to 15 mV in amplitude, while regularly occurring action potentials were associated with oscillations of the membrane potential.4. Discrete depolarizations (called fibrillatory origin potentials or f.o.p.s) at sites of origin in irregularly discharging fibres have the following properties: (a) random occurrence and nearly constant amplitude outside a refractory period during which both amplitude and probability of a second f.o.p. are reduced; (b) associated inward current flow which is localized to about 100 mum or less along the fibre length, and (c) dependence of amplitude and frequency on membrane potential.5. Oscillation of membrane potential found at sites of origin of action potential discharge in regular fibres also occurred locally along the fibre length and was sensitive to changes in membrane potential.6. Both f.o.p.s and oscillations of membrane potential were reversibly abolished by low Na(+)-Ringer fluid or tetrodotoxin.7. Neither type of initiating event was appreciably affected by concentrations of D-tubocurarine which blocked extrajunctional sensitivity to acetylcholine.8. We conclude that spontaneous action potentials under these conditions arise from a localized Na(+)-conductance change in the membrane of the active fibre; this conductance change is distinct from the increased Na(+)-conductance which follows the interaction of acetylcholine with its receptor. Spontaneous activity in single, denervated muscle fibres is cyclical and self-inhibiting (Purves & Sakmann, 1974); thus the Na(+)-conductance change underlying the initiation of spontaneous action potentials is affected by muscle fibre activity.     

How Gymnodinium breve red tide toxin(s) produces repetitive firing in squid axons

Partially purified toxin(s), GbTX, extracted from Gymnodinium breve red tide organisms elicits a spontaneous train of action potentials in the squid giant axon. The spikes have a shape similar to that in the normal seawater control except for an increase in the rate of recovery from the afterhyperpolarization. With this more rapid recovery, the membrane potential overshoots the resting potential and threshold, triggers another spike, and thus produces repetitive firing. Voltage-clamp studies revealed that the toxin has no effect on the normal sodium or potassium conductance changes produced by step depolarization. However, consistent with the faster recovery after an action potential, GbTX speeds recovery of the "shut-off" currents to their steady-state values after a depolarization. The most likely mechanism by which the toxin accelerates recovery after an action potential (leading to repetitive firing) is the induction of a small additional inward current which was found to be reduced by prehyperpolarization. This toxin-induced current which speeds recovery is blocked by tetrodotoxin and hence presumably flows through the sodium channel.     

Chloride conductance in the transverse tubular system of rat skeletal muscle fibres: importance in excitation-contraction coupling and fatigue

Contraction in skeletal muscle fibres is governed by excitation of the transverse-tubular (t-) system, but the properties of the t-system and their importance in normal excitability are not well defined. Here we investigate the properties of the t-system chloride conductance using rat skinned muscle fibres in which the sarcolemma has been mechanically removed but the normal excitation-contraction coupling mechanism kept functional. When the t-system chloride conductance was eliminated, either by removal of all Cl(-) or by block of the chloride channels with 9-anthracene carboxylic acid (9-AC) or by treating muscles with phorbol 12,13-dibutyrate, there was a marked reduction in the threshold electric field intensity required to elicit a t-system action potential (AP) and twitch response. Calculations of the t-system chloride conductance indicated that it constitutes a large proportion of the total chloride conductance observed in intact fibres. Blocking the chloride conductance increased the size of the twitch response and was indicative that Cl(-) normally carries part of the repolarizing current across the t-system membrane on each AP. Block of the t-system chloride conductance also reduced tetanic force responses at higher frequency stimulation (100 Hz) and greatly reduced twitch responses in the period shortly after a brief tetanus, owing to rapid loss of t-system excitability during the AP train. Blocking activity of the Na(+)-K(+) pump in the t-system membrane caused loss of excitability owing to K(+) build-up in the sealed t-system, and this occurred approximately 3-4 times faster when the chloride conductance was blocked. These findings show that the t-system chloride conductance plays a vital role during normal activity by countering the effects of K(+) accumulation in the t-system and maintaining muscle excitability.     

Slow changes in potassium permeability in skeletal muscle

1. Voltage clamp experiments on sartorius muscle fibres at 3 degrees C showed that the potassium current is divisible into three components, namely:(a) Current in the delayed rectifier channel, which reached a maximum in about 0.1 sec at -30 mV, and declined with a time constant of about 4 msec when the fibre was repolarized to -100 mV; this component had an approximately linear instantaneous current-voltage relation and an equilibrium potential E(1) at 10-15 mV positive to the resting potential.(b) A slow component which reached a maximum in about 3 sec at -30 mV, and declined with a time constant of about 0.5 sec when the fibre was repolarized to -100 mV; this component had an approximately linear instantaneous current-voltage relation and a mean equilibrium potential E(2) at -83 mV in fibres where E(1) averaged -75 mV.(c) Current in the inward rectifier channel which decreased with a time constant of about 0.25 sec when the fibre was hyperpolarized to -150 mV. This component had an equilibrium potential close to the resting potential and an instantaneous current-voltage relation which was that of an inward rectifier.2. The general characteristics of the late after-potential in muscles in hypertonic solutions at 3 degrees C are consistent with those of the slow conductance change. The sign of the late after-potentials was reversed by depolarizing below -80 mV.3. The decline of current during a maintained hyperpolarization cannot be attributed solely to a decrease in tubular potassium concentration, since there may be a large decrease in current without much alteration of equilibrium potential. The negative slope conductance often seen at -150 mV is also difficult to reconcile with the tubular depletion hypothesis.4. Replacement of 10 mM-K by 10 mM-Rb abolished inward rectification but had less effect on the fast and slow components of the potassium conductance.     

The membrane capacity of mammalian skeletal muscle fibres

Summary Membrane capacity was measured as a function of fibre diameter in mammalian skeletal muscle fibres under normal conditions and under conditions designed to reduce the membrane chloride conductance, i.e. in solutions in which choride ions were replaced by sulphate or methylsulphate ions, or in normal Krebs solutions containing 2,4-dichlorophenoxyacetic acid (2.5mm). The experiments were done on rat sternomastoid, extensor digitorum longus and soleus muscle fibres. The average membrane capacity of fibres in each muscle was greater than normal when chloride conductance was reduced and the slope of the relationship between membrane capacity and fibre diameter increased. The results were consistent with the hypothesis that the space constant of the transverse tubule system in mammalian fibres is normally short because the transverse tubule membrane has a high chloride conductance. The experimental results imply that the space constant of the transverse tubule system was less than 40 µm for fibres in normal Krebs solution and greater than 100 µm for fibres with low membrane chloride conductance. The space constant was calculated using measured geometrical parameters of the transverse tubule, and measured membrane conductance, and the values were close to 20 µm for fibres in normal Krebs solution and between 50 and 120 µm for fibres with low chloride conductance.     

On the regulation of repetitive firing in lumbar motoneurones during fictive locomotion in the cat

Summary Repetitive firing of motoneurones was examined in decerebrate, unanaesthetised, paralysed cats in which fictive locomotion was induced by stimulation of the mesencephalic locomotor region. Repetitive firing produced by sustained intracellular current injection was compared with repetitive firing observed during fictive locomotion in 17 motoneurones. During similar interspike intervals, the afterhyperpolarisations (AHPs) during fictive locomotion were decreased in amplitude compared to the AHPs following action potentials produced by sustained depolarising current injections. Action potentials were evoked in 10 motoneurones by the injection of short duration pulses of depolarising current throughout the step cycles. When compared to the AHPs evoked at rest, the AHPs during fictive locomotion were reduced in amplitude at similar membrane potentials. The post-spike trajectories were also compared in different phases of the step cycle. The AHPs following these spikes were reduced in amplitude particularly in the depolarised phases of the step cycles. The frequency-current (f-I) relations of 7 motoneurones were examined in the presence and absence of fictive locomotion. Primary ranges of firing were observed in all cells in the absence of fictive locomotion. In most cells (6/7), however, there was no relation between the amount of current injected and the frequency of repetitive firing during fictive locomotion. In one cell, there was a large increase in the slope of the f-I relation. It is suggested that this increase in slope resulted from a reduction in the AHP conductance; furthermore, the usual elimination of the relation is consistent with the suggestions that the repetitive firing in motoneurones during fictive locomotion is not produced by somatic depolarisation alone, and that motoneurones do not behave as simple input-output devices during this behaviour. The correlation of firing level with increasing firing frequency which has previously been demonstrated during repetitive firing produced by afferent stimulation or by somatic current injection is not present during fictive locomotion. This lends further support to the suggestion that motoneurone repetitive firing during fictive locomotion is not produced or regulated by somatic depolarisation. It is suggested that although motoneurones possess the intrinsic ability to fire repetitively in response to somatic depolarisation, the nervous system need not rely on this ability in order to produce repetitive firing during motor acts. This capability to modify or bypass specific motoneuronal properties may lend the nervous system a high degree of control over its motor output.     

An evaluation of the membrane constants and the potassium conductance in metabolically exhausted muscle fibres.

1. The membrane characteristics of metabolically poisoned and mechanically exhausted frog skeletal muscle fibres were investigated with intracellular micro-electrodes. 2. When cyanide plus iodoacetate were applied as metabolic poisons twitch tension declined towards zero after 150-300 stimuli (0-3 Hz; temperature = 0 degrees C). At the beginning of stimulation the mean resting potenial fell from -75 to -69 mV; it rose subsequently to -83 mV. The membrane resistance decreased during this stimulation period along a sigmoid time course to 4-6% of the original value. 3. In completely exhausted fibres the following membrane constants were estimated (23 degrees C): length constant, 0-31 mm; input resistance, 31 komega; membrane resistance, 58 omega.cm2. These values were calculated under the assumption of a constant internal resistivity of 170 omega. cm. The Q10 values of these constants were similar to those in normal fibres. Afew experiments revealed that the membrane capacity remained roughly constant under these conditions. 4. The current-voltage relation of exhausted fibres was approximately linear in the range between -60 and -100 mV. At less negative potentials the conductance increased slightly while at more negative potentials it decreased. The latter, in particular, became more evident when the imput current was converted into membrane current density by applying Cole's theorem. 5TEA+ and Rb+ in the external solution increased the membrane resistance of exhausted fibres by more than one order of magnitude. The major part of the membrane conductance induced by exhaustion, however, could not be blocked by these ions or Zn2+. 6. Chloride-free test solutions were used to measure the relative contributions of potassium and chloride ions to the membrane conductance. The relation GK:GC1 changed from 2:3 in normal fibres to 5:1 in exhausted ones. In absolute terms GK rose from ca. 130 to 14,300 mumho/cm2 and GC1 from ca. 200 to 2900 mumho/cm2. The discrimination between K+ and Na+ by the resting membrane in exhausted fibres was probably equal to or even higher than that under normal conditions. 7. In normal fibres the input resistance decreased by up to 20% after the external application of 1-2 mM caffeine, which is known to release calcium ions from internal stores. The elevation in internal Ca2+ by direct injection caused a small and, as a rule, irreversible decrease in input resistance which was probably partly due to local damage to the surface membrane. 8. It is concluded that in metabolically exhausted muscle fibres the surface and tubular membranes are still intact and that the observed decrease in membrane resistance is mainly due to an increase in potassium conductance. In addition, the results indicate that the gating mechanism of the potassium channels (presumably those with the characteristics of the slow component) is affected when energy reserves diminish.     

Sodium dependence of the inward spread of activation in isolated twitch muscle fibres of the frog

1. The excitatory process travelling along the T-system may be either electrotonic or regenerative. If Na(+) dependent action potential is present in the tubular membranes, high frequency of stimulation might cause a Na(+) depletion in the tubules sufficient to abolish this process.2. We tested this hypothesis by recording tension in isolated muscle fibres stimulated tetanically (up to 60 shocks/sec). In low [Na(+)] solutions, output tension was initially similar to that in normal Ringer, but then fell smoothly to a substantially lower value.3. The activity of individual myofibrils was recorded directly with ciné-micrographs during isotonic contractions while the fibres were stimulated at high frequencies. In low [Na(+)](o) wavy myofibrils appeared in the centre of the fibre and spread towards the periphery, indicating failure of activation. Wavy myofibrils never appeared in normal Ringer.4. Intracellular action potentials recorded during the tetanic stimulation indicated that the inactivated myofibrils present in low [Na(+)] solutions cannot be explained by the changes in size and duration of the action potential.5. Our results strongly suggest the existence of a regenerative Na(+) conductance in the tubular membrane during the inward spread of an excitatory process.     

Zinc and copper influence excitability of rat olfactory bulb neurons by multiple mechanisms

Zinc and copper are highly concentrated in several mammalian brain regions, including the olfactory bulb and hippocampus. Whole cell electrophysiological recordings were made from rat olfactory bulb neurons in primary culture to compare the effects of zinc and copper on synaptic transmission and voltage-gated ion channels. Application of either zinc or copper eliminated GABA-mediated spontaneous inhibitory postsynaptic potentials. However, in contrast to the similarity of their effects on inhibitory transmission, spontaneous glutamate-mediated excitatory synaptic activity was completely blocked by copper but only inhibited by zinc. Among voltage-gated ion channels, zinc or copper inhibited TTX-sensitive sodium channels and delayed rectifier-type potassium channels but did not prevent the firing of evoked single action potentials or dramatically alter their kinetics. Zinc and copper had distinct effects on transient A-type potassium currents. Whereas copper only inhibited the A-type current, zinc modulation of A-type currents resulted in either potentiation or inhibition of the current depending on the membrane potential. The effects of zinc and copper on potassium channels likely underlie their effects on repetitive firing in response to long-duration step depolarizations. Copper reduced repetitive firing independent of the initial membrane voltage. In contrast, whereas zinc reduced repetitive firing at membrane potentials associated with zinc-mediated enhancement of the A-type current (-50 mV), in a significant proportion of neurons, zinc increased repetitive firing at membrane potentials associated with zinc-mediated inhibition of the A-type current (-90 mV). Application of zinc or copper also inhibited voltage-gated Ca(2+) channels, suggesting a possible role for presynaptic modulation of neurotransmitter release. Despite similarities between the effects of zinc and copper on some ligand- and voltage-gated ion channels, these data suggest that their net effects likely contribute to differential modulation of neuronal excitability.     

Development of potassium conductances in perinatal rat phrenic motoneurons

Prior to the inception of inspiratory synaptic drive transmission from medullary respiratory centers, rat phrenic motoneurons (PMNs) have action potential and repetitive firing characteristics typical of immature embryonic motoneurons. During the period spanning from when respiratory bulbospinal and segmental afferent synaptic connections are formed at embryonic day 17 (E17) through to birth (gestational period is approximately 21 days), a pronounced transformation of PMN electrophysiological properties occurs. In this study, we test the hypothesis that the elaboration of action potential afterpotentials and the resulting changes in repetitive firing properties are due in large part to developmental changes in PMN potassium conductances. Ionic conductances were measured via whole cell patch recordings using a cervical slice-phrenic nerve preparation isolated from perinatal rats. Voltage- and current-clamp recordings revealed that PMNs expressed outward rectifier (I(KV)) and A-type potassium currents that regulated PMN action potential and repetitive firing properties throughout the perinatal period. There was an age-dependent leftward shift in the activation voltage and a decrease in the time-to-peak of I(KV) during the period from E16 through to birth. The most dramatic change during the perinatal period was the increase in calcium-activated potassium currents after the inception of inspiratory drive transmission at E17. Block of the maxi-type calcium-dependent potassium conductance caused a significant increase in action potential duration and a suppression of the fast afterhyperpolarizing potential. Block of the small conductance calcium-dependent potassium channels resulted in a marked suppression of the medium afterhyperpolarizing potential and an increase in the repetitive firing frequency. In conclusion, the increase in calcium-mediated potassium conductances are in large part responsible for the marked transformation in action potential shape and firing properties of PMNs from the time between the inception of fetal respiratory drive transmission and birth.