Neuro-muscular synapse ultrastructure in the Lambert-Eaton myasthenic syndrome]

A study was made of the ultrastructure of the neuro-muscular synapses in the patients with the myasthenic Lambert-Eaton syndrome. Most of the synapses displayed an increased content of synaptic vesicles in the axon terminals, and the anastomosing synaptic folds were increased in number and depth. Local destructive changes were found in the terminals of some synapses. The data obtained confirmed the fact that this syndrome was underlied by disorder of the transmitter release from the presynaptic structures.     

Ca-dependent slow action potentials in neuromuscular diseases

Slow Ca-dependent action potentials were studied in skeletal muscle fibers from different Neuromuscular Diseases (NMD). Biopsies were obtained from: 3 myopathies [Fascioscapulohumeral Dystrophy (FSH) and Polymyositis (PM)], 6 patients with other diseases (CD) [Amyotrophic Lateral Sclerosis (ALS), Central Core Disease, Mitochondrial Myopathy, Polyneuritis (PN), von Eulenberg's Paramyotonia], and 8 normal control muscles. Experiments were carried out in muscle fibers under current-clamp conditions. Membrane currents other than Ca ones were abolished or greatly diminished. Muscle fibers produced any of 3 types of responses, when stimulated by depolarizing pulses: fully developed Ca-action potentials (CaAP), abortive non-regenerative Ca responses (NrR), or only capacitive passive responses (WR). The 3 types of responses were not dependent on the basal conditions of the fibers. The frequency of observation of CaAPs was significantly higher in myopathic disease. In myopathies, 46% of the muscle fibers had CaAPs, while only 22% of fibers from CD and 15% of the fibers from normal muscles showed CaAPs. No differences were observed in the resting constants as well as in the CaAPs parameters between normal and diseased muscle fibers.     

Effects of 8-bromo-cyclic GMP on slow channel mediated action potentials of 3-days-old embryonic chick ventricle.

The role of cyclic GMP in modulation of cardiac slow channel activity was investigated by observing the effects of 8-bromo-cyclic GMP (8-Br-cGMP) on action potentials of isolated ventricle of 3-days-old chick embryo, which exhibit upstroke primarily due to slow channels. 8-Br-cGMP (0.5 & 1 mM) reduced the maximum diastolic potential, maximal upstroke velocity (+Vmax) and overshoot in 30-60 min. 8-Bromo-cyclic AMP (8 Br-cAMP, 0.5 & 1 mM), isoproterenol (Iso, 0.5-5 microM) and forskolin (0.5-2 microM) caused an increase in +Vmax and overshoot. 8-Br-cGMP antagonised this enhancement of +Vmax. Increase in +Vmax and overshoot by Bay-K-8644 (1 microM) was also blocked by 8-Br-cGMP. These findings show that 8-Br-cGMP inhibited the early embryonic cardiac slow channel activity, which contributes significantly to the upstroke of action potential, under basal conditions as well as after its accentuation by elevation of cyclic AMP levels (by 8-Br-cAMP, Iso & Forskolin) or by direct stimulation of the channel activity (by Bay-K-8644). It is suggested on the basis of these findings that cyclic GMP plays a key role in down modulation of the cardiac slow channel activity in early embryonic chick heart.     

Depressant action of Ca-antagonists on slow action potentials in guinea pig ventricular muscles

The depressant action of four Ca antagonists, including a novel drug, tiapamil, on Ca channels was investigated using a conventional microelectrode technique. "All or none" slow action potentials were recorded in K+-depolarized guinea-pig papillary muscles. Verapamil and diltiazem decreased the amplitude and maximum rate of rise (Vmax) of the slow action potentials at concentrations up to 2 microM. The depressant effect of a novel Ca-antagonist, tiapamil, on the slow action potentials was as marked as that of verapamil and diltiazem. However, prenylamine was less potent than the other 3 drugs. In addition, the action of all drugs on the slow action potentials was enhanced as the frequency of stimulation was increased between 0.0083 and 1 Hz. It was concluded that tiapamil, as verapamil and diltiazem, produced a frequency-dependent blockade of the slow Ca channel.     

Ca-dependent slow action potentials in human skeletal muscle

Slow Ca-action potentials (CaAP) were studied in normal human skeletal muscle fibers obtained during surgery (fibers with both ends cut). Control studies also were carried out with intact as well as cut rat skeletal muscle fibers. Experiments were performed in hypertonic Cl-free saline with 10 or 84 mM Ca and K-channel blockers; muscles were preincubated in a saline containing Cs and tetraethylammonium. A current-clamp technique with two intracellular microelectrodes was used. In human muscle, 14.5% of the fibers showed fully developed CaAPs, 21% displayed nonregenerative Ca responses, and 64.5% showed only passive responses; CaAPs were never observed in 10 mM Ca. In rat muscle, nearly 90% of the fibers showed CaAPs, which were not affected by the cut-end condition. Human and rat muscle fibers had similar membrane potential and conductance in the resting state. In human muscle (22-32 degrees C, 84 mM Ca), the threshold and peak potential during a CaAP were +26 +/- 6 mV and +70 +/- 3 mV, respectively, and the duration measured at threshold level was 1.7 +/- 0.5 sec. In rat muscle, the duration was four times longer. During a CaAP, membrane conductance was assumed to be a leak conductance in parallel with a Ca and a K conductance. In human muscle (22-32 degrees C, 84 mM Ca, 40 micron fiber diameter), values were 0.4 +/- 0.1 microS, 1.1 +/- 0.7 microS, and 0.9 +/- 0.4 microS, respectively. Rat muscle (22-24 degrees C, 84 mM Ca) showed leak and K conductances similar to those found in human fibers. Ca-conductance in rat muscle was double the values obtained in human muscle fibers.     

The effect of adrenaline on the temperature dependency of cardiac action potentials in pink salmon Oncorhynchus gorbuscha

Using sharp electrode impalement, action potentials recorded from atrial and ventricular tissue of pink salmon Oncorhynchus gorbuscha generally decreased in duration with increasing test temperature (6, 10, 16 and 20° C). Stimulation of the tissue using 500 nM adrenaline had no significant effect on the duration of the atrial action potential at any test temperature but lengthened the ventricular action potential by ~17%.     

Characteristics of cardiac action potentials in marsupials

Summary Standard microelectrode techniques were used to record action potentials from single atrial, ventricular and Purkinje fibers of hearts taken from three species of marsupial (Macropus rufus, Macropus robustus andMacropus eugenii) and from dogs, sheep and guinea-pigs. The major electrophysiological parameters of marsupial potentials were qualitatively similar to the values for placental mammals. The grouped data for ventricular action potentials from studies on 6 adult male red kangaroos (Macropus rufus) were (mean ±SD): Resting potential –69.5±5.0 mV; action potential amplitude 92.7±5.7 mV; action potential duration (to 90% repolarization): 182.5±17.5 ms; maximum rate of depolarization: 196.5±80.1 V/s. The major point of difference was the short duration of the red kangaroo ventricular action potential compared to those of the placental mammals, and compared to atrial cells from the kangaroos. It is suggested that this explains the short QT interval reported by others for kangaroo electrocardiograms, and that it may also be implicated in the high frequency of sudden death previously noted in these animals.     

Inhibition of cardiac slow action potentials by 8-bromo-cyclic GMP occurs independent of changes in cyclic AMP levels

Cyclic GMP inhibits the slow inward Ca current of cardiac cells. This effect could be due to a cyclic GMP-mediated phosphorylation of the Ca channel (or some protein modifying Ca channel activity), or alternatively, to enhanced degradation of cyclic AMP owing to stimulation of a phosphodiesterase by cyclic GMP. To test the latter possibility, we examined the effect of extracellular 8-bromo-cyclic GMP on cyclic AMP levels in guinea pig papillary muscles, in parallel with electrophysiological experiments. Isoproterenol (10(-6) M) significantly increased the cyclic AMP levels and induced Ca-dependent slow action potentials. Superfusion with 8-bromo-cyclic GMP (10(-3) M) inhibited the slow action potentials induced by isoproterenol. However, muscles superfused with 8-bromo-cyclic GMP had cyclic AMP levels identical to those of muscles superfused with isoproterenol alone. Similarly, 8-bromo-cyclic GMP had no effect on the increase in cyclic AMP levels of muscles treated with forskolin (10(-6) M) or histamine (10(-6) M). We conclude that the inhibitory effect of cyclic GMP on slow Ca channels in guinea pig ventricular cells is not due to a decrease in the cyclic AMP levels. We hypothesize that a cyclic GMP-mediated phosphorylation is the most likely explanation for the Ca channel inhibition observed in this preparation.     

Simulated propagation of cardiac action potentials.

We have used numerical methods for solving cable equations, combined with previously published mathematical models for the membrane properties of ventricular and Purkinje cells, to simulate the propagation of cardiac action potentials along a unidimensional strand. Two types of inhomogeneities have been simulated and the results compared with experimentally observed disturbances in cardiac action potential propagation. Changes in the membrane model for regions of the strand were introduced to simulate regions of decreased excitability. Regional changes in the intercellular coupling were also studied. The results illustrate and help to explain the disturbances in propagation which have been reported to occur at regions of decreased excitability, regions with changing action potential duration, or regions with changing intercellular coupling. The propagational disturbances seen at all of these regions are discussed in terms of the changing electrical load imposed upon the propagating impulse.     

The course of ionic transmembrane currents during cardiac action potentials.

The course of the total transmembrane ionic current (Ii) during a natural action potential (AP) was reconstructed from a family of current traces recorded for single voltage clamp depolarization steps to various levels. The experiments were performed on 9 papillary cat muscles driven at 0.5 per second in oxygenated 31 degrees C Tyrode. Under varying experimental conditions very good agreement was found between the resulting Ii curve and another indicator of Ii, the first time derivative of the AP (dV/dt). Furthermore, the coefficient needed to adjust dV/dt to reconstructed Ii may serve as an indicator of the membrane capacity. The results suggest the validity of the employed approximation and, in general, the adequacy of the sucrose gap technique applied to cardiac muscle.     

Induction of action potentials in frog slow muscle fibres paralysed by alpha -bungarotoxin

Slow muscle fibres in the frog are normally incapable of generating action potentials. However, several days after an intramuscular injection of alpha - bungarotoxin, they acquire the ability to generate action potentials. It appears that alpha -bungarotoxin induces the action potential mechanism in slow fibres because it blocks acetylcholine receptors, and thus interferes with the action of non-quantal acetylcholine leaking from nerve terminals, or because the toxin has some other, as yet undefined, action on nerve or muscle.     

Arrhythmogenic effects of ultra-long and bistable cardiac action potentials

Contemporary accounts of the initiation of cardiac arrhythmias typically rely on after-depolarizations as the trigger for reentrant activity. The after-depolarizations are usually triggered by calcium entry or spontaneous release within the cells of the myocardium or the conduction system. Here we propose an alternative mechanism whereby arrhythmias are triggered autonomously by cardiac cells that fail to repolarize after a normal heartbeat. We investigated the proposal by representing the heart as an excitable medium of FitzHugh-Nagumo cells where a proportion of cells were capable of remaining depolarized indefinitely. As such, those cells exhibit bistable membrane dynamics. We found that heterogeneous media can tolerate a surprisingly large number of bistable cells and still support normal rhythmic activity. Yet there is a critical limit beyond which the medium is persistently arrhythmogenic. Numerical analysis revealed that the critical threshold for arrhythmogenesis depends on both the strength of the coupling between cells and the extent to which the abnormal cells resist repolarization. Moreover, arrhythmogenesis was found to emerge preferentially at tissue boundaries where cells naturally have fewer neighbors to influence their behavior. These findings may explain why atrial fibrillation typically originates from tissue boundaries such as the cuff of the pulmonary vein.     

The effects of myasthenic IgG on miniature end-plate currents in mouse diaphragm

In diaphragms from mice injected with purified IgG from a patient with myasthenia gravis, miniature end-plate currents (m.e.p.c.s) were reduced in size by about 50% and sensitivity to superperfused carbachol was also reduced. The ‘myasthenic’ m.e.p.c.s closely resembled m.e.p.c.s made small by α-bungarotoxin (α-BuTX) or (+)-tubocurarine with regard to alteration of height by poisoning of acetylcholinesterase and sensitivity to further receptor blockade. However, the time course of decay of the ‘myasthenic’ m.e.p.c.s was closer to normal than in m.e.p.c.s reduced in size by dTC or α-BuTX. It is postulated that myasthenic IgG acts to interfere with receptor function to open ionic channels more than it blocks binding of ACh to receptor.