Mechanism of positive inotropic actions of hydrazinium ions on bullfrog heart muscles.

Abstract

An investigation was carried out on isolated bullfrog's ventricular muscles so as to study the mechanism of an increasing positive inotropic effect of hydrazine, using a conventional microelectrode technique and a force-displacement transducer; membrane potentials and isometric twitch tensions were observed. A low concentration, 1.5 to 15mM, of hydrazinium ions produced an increase of isometric twitch tension, having a shorter contractile duration with an increasing velocity of contraction and relaxation. The developments of twitch contractions illustrated a typical sigmoid curve. However, the shortened contractile duration with the deeper negative deflection of dT/dt remained after the increased contraction was returned to the initial value. These increasing twitches were markedly enhanced in the Ca++-rich solution as compared with those observed in normal Ringer's solution. The action potentials generated in the bullfrog's ventricular muscles revealed the shorter duration and the decreased maximum rate of rise of the action potential. Phase II, the late portion of the upstroke of the action potential, was augmented by hydrazine in the Na+-deficient and the Ca++-rich solutions. The resting potentials were slightly reduced.In conclusion, it is postulated that the positive inotropic actions of hydrazinium ions on this tissue are mainly due to the depolarization of cell membrane and the increasing permeability of cell membrane to calcium ions. In one respect, the mode of action of hydrazinium ions on these cells would be essentially similar to that of adrenaline.