Abstract
Summary
The electrophysiological effects of a myocardial depressant factor (MDF) were studied on isolated cat papillary muscles. MDF decreased developed tension 58% within 6-11 min. There were no significant changes in the amplitude of either the resting membrane potential or the action potential of individual myocardial cells over this 11-min period. There was a significant prolongation of the action potential within 3 min of addition of MDF to the papillary muscles. However, this prolongation of the action potential cannot account for the negative inotropic effect of MDF. Addition of free calcium to the bathing medium from 2.5 mM to 7.5 mM completely and rapidly reversed the negative inotropic effect of MDF. We conclude that MDF acts at a site other than the excitation of the cell membrane, possibly by depressing excitation-contraction coupling or by impairing the contractile machinery directly.
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