Effects of some calcium antagonists on the excitation-contraction coupling in the myocardium under steady-state and nonsteady-state conditions of operation.

The specific features of excitation-contraction coupling in guinea-pig myocardium were studied by applying two different calcium antagonists--Mn2+ and methoxyverapamil (D 600). The muscle preparations were placed under two different conditions of operation: upon stimulation with equal interstimulus intervals (0.5, 1, 5 and 10 s)--steady state and with presentation of the same test-stimuli with the same intervals after a priming high-frequency series--nonsteady-state. The well-known coupling takes place under steady-state conditions of operation under the effect of Mn2+ (5 mM) and D 600 (1-5 x 10(-6) M). However, under nonsteady-state conditions of operation, in spite of the presence of antagonists of the slow inward current, potentiated contractions are observed. It is assumed that under nonsteady-state regime conditions are created for activation of the intracellular calcium stores and that the two calcium antagonists act on different sites in the electro-mechanical coupling in the myocardium.