Development of the second contraction component after restoration with isoprenaline of pre-inhibited contractile activity of isolated myocardium.

Transmembrane action potentials and contractile activity were registered under isometric conditions during in vitro experiments on the papillary muscle of guinea-pig myocardium. The blocking agents of the slow transmembrane inward current: D 600 (10(-6) to 5 X 10(-6) M) and Mn2+ (5 X 10(-3)M), induce strong inhibition of the contraction amplitude. Applied on this background, isoprenaline (5 X 10(-7) to 10(-6)M) completely restores the contraction amplitude. The action potential is characterized by a high-amplitude plateau, followed by fast repolarization. The muscle was stimulated with suprathreshold rectangular impulses, 1 ms duration and 1-60 s interstimulus intervals. Directly proportional dependence was observed between the duration of the action potentials and the interstimulus intervals. A second component appears in the relaxation phase of the contraction upon stimulation with 10 s interstimulus interval, being particularly pronounced for 40-60 s intervals. As a consequence, the contraction acquires a marked two-component configuration. The data obtained are interpreted in the light of the hypothesis about the existence of two pools providing calcium for the contraction, assuming the participation of intracellular stores for the first component and transmembrane calcium for the second one. A scheme of the possible pathways of the Ca-ions, through which the contractile process in mammalian muscles takes place, is proposed.