Ursachen und Ausl�sungsbedingungen von Myokard-Kontraktionen ohne regul�res Aktionspotential

Abstract

1. In isolated guinea pig's atria or papillary muscles, cooled to a temperature below 20°C, rhythmic variations of tension are recorded immediately after the end of previous electrical stimulation, sufficiently great in duration and frequency. These tonus changes consist in a number of gradually decreasing mechanical oscillations which are quite distinct from the preceeding all-or-none responses since they are slow, nonconducted and graded. 2. The appearance as well as the amplitude and number of such mechanical oscillations can be considerably enhanced by an excess of Ca++ ions, by lowering the external sodium concentration or by treatment with heart glycosides. Under these conditions a tension maximum of the tonus waves of about 50 percent of the force of a regular beat may be reached. In addition, high Ca++ or heart glycosides increase the wave frequency. 3. Intracellular potential measurements with microelectrodes reveal that the rhythmic tonus oscillations correspond to small oscillatory after-potentials which decrease in a regular manner and disappear after 1 to 5 waves exactly like the mechanical changes. Obviously the electrical and mechanical oscillations run closely parallel, a decrease in resting potential always being followed, with a slight delay, by a rise in tension and vice versa. 4. The experiments indicate that low temperature, high Ca++ or heart glycosides will labilize the resting potential of mammalian myocardial fibres thus producing a pre-fibrillatory state in which local automaticity can be evoked even by electrical stimulation of moderate strength. Furthermore high Ca++ and heart glycosides, especially in a medium with a reduced sodium content, largely amplify the mechanical response of the myocardium even if the underlaying oscillatory depolarisation is very small. This effect can be fully explained by the special role of Ca++ ions in the excitation-contraction coupling.