Die Trennung von Erregungsvorgang und Kontraktion am Skelett und Herzmuskel durch 2,4-Dinitrophenol

Anoxia and ventricular fibrillation; with a summary of evidence on the cause of fibrillation

Experiments are described showing that ventricular fibrillation is more readily produced in isolated rabbit hearts by electrical stimulation when the oxygen supply to the heart is reduced. This evidence completes investigations which have been made into factors affecting the production of fibrillation. These investigations have shown that factors which shorten the duration of the action potential, particularly those which cause the "plateau" to disappear, facilitate fibrillation, and those which lengthen the duration of the action potential prevent fibrillation. The reason for the length of the cardiac action potential may therefore be to prevent fibrillation. When the action potential is of normal length the fact that two adjacent fibres are out of phase does not matter; the one which is first to contract is not re-excited by the one which is second to contract, because at that moment the first is inexcitable. If the action potential is short, then the first may be already repolarized and may be excited by spread of excitation from the second. Factors which inhibit metabolism shorten the action potential.

INHIBITION OF CONTRACTION IN A MOLLUSCAN SMOOTH MUSCLE BY THIOUREA, AN INHIBITOR OF THE ACTOMYOSIN CONTRACTILE MECHANISM

The action of thiourea on a molluscan 'catch’ muscle was investigated, to determine whether the actomyosin contractile system can be inactivated directly in the surviving muscle by a poison known to inactivate actomyosin in vitro . Thiourea in a concentration of 0.5 to 0.6 M abolishes reversibly all contractile responses after stimulation with acetylcholine, cathodal direct current, isotonic potassium chloride, caffeine, and chloroform. The inhibiting effect is not caused by blocking the excitation mechanism at the membrane, since although thiourea inhibits the acetylcholine—and KCl contraction, it does not inhibit depolarization by acetylcholine and KCl. It also inhibits caffeine contracture although the latter is independent of the polarization of the membrane. The action of thiourea is not due to an osmotic effect. Thiourea diffuses into the muscle cell: the inside concentration reaches half the outside concentration in about 6 min. The rate of outflux is similar. Since the degree of inhibition depends on the thiourea concentration inside rather than outside it may be concluded that the site of action lies inside. Unlike DNP in certain muscles, thiourea does not reduce the ATP -store in concentrations in which it produces full inhibition of contractile responses. Again, quite unlike DNP , it inhibits the ATP contraction of the isolated contractile apparatus and both the ATP -ase of fibrils and the tension development of actomyosin threads. The dependence of the inhibition on the inhibitor concentration is similar in isolated systems and in the surviving ABRM after equilibration with thiourea. Urea, which produced little inhibition of the ATP contraction in isolated systems, had very little effect on the contrac­tility of living muscles, in concentrations of 0.6 M. The simpliest and probably the most plausible hypothesis to account for all these findings is that thiourea inhibits contractility in the living muscle by inhibiting the contractile and enzymic activity of the actomyosin contractile system. Such a direct action may or may not be implemented by an additional effect on the coupling between excitation and contraction.

Lack of direct antiarrhythmic electrophysiological effects of salicylate on isolated guinea-pig myocardium

Conventional microelectrode techniques were used to study the influence of Na-salicylate, Na-benzoate, Na-2,6-dihydroxybenzoate and 2,4-dinitrophenol (2,4-DNP) on the action potential (AP) of guinea-pig papillary muscles and atria. In papillary muscles, Na-salicylate (0.19-1.87 mmol/l) concentration-dependently shortened the AP duration and the functional refractory period. The AP amplitude decreased slightly with the largest concentration, while the resting potential and the maximum depolarisation velocity (Vmax) were not affected. A concentration-dependent negative inotropic effect was also seen. All drug effects were reversible after washout. In atria, 6.24 mmol/l Na-salicylate induced a slight shortening of the AP duration, a decrease of the AP amplitude and Vmax, but no decrease of the contractile force. The effects of the uncoupling agent, 2,4-DNP (10 mumol/l), were similar to those of the largest concentration of Na-salicylate in papillary muscles and in atria. Na-benzoate and Na-2,6-dihydroxybenzoate had no significant influence on AP duration, AP amplitude, resting potential, Vmax, refractory period or force of contraction of either papillary muscles or atria. These results suggest that Na-salicylate exerts its effects on isolated guinea-pig myocardium by uncoupling the oxidative phosphorylation, whereas two other possible mechanisms of action, namely an increase of membrane surface charge and an inhibition of prostaglandin synthesis, seem to be of minor importance.

Studies on electrical and mechanical activity in hypoxic papillary muscles of the guinea-pig

The hypoxia-induced changes in transmembrane potentials and in force of contraction of isolated papillary muscles of the guinea-pig were studied. With different glucose concentrations in the hypoxic medium both the extent and the time course of the reduction in force of contraction and action potential duration could be modified. A time lag of 10 minutes was observed in the onset of action potential shortening. The membrane potential decreased only at advanced stages of hypoxia. Lack of glucose was tolerated for a considerable longer period of time when the preparations were mainly quiescent during hypoxia, although membrane depolarization could not be prevented. Memembrane depolarization was basent only if the mucles were completely unloaded when exposed to hypoxia. The dissociation of events during hypoxia is interpredted as evidence for compartmentalization of energy supply within the cells.