Paradoxical after-potentiation of the myocardial contractility by lanthanum.

Abstract

La 0.1-0.2 mM caused a twitch inhibition and further showed a marked, long-lasting twitch potentiation during washout in both ventricular strips of bullfrog and atrial or papillary muscles of guinea-pig. The effect was more conspicuous in the frog heart. A similar potentiation was observed by appying Gd instead of La. The participation of endogenous catecholamine release is unlikely since the positive inotropic effect was never affected by the presence of .beta.-adrenergic blocker. The potentiation was not accompanied by any increse in the action potential plateau, suggesting independence of the phenomenon on the Ca current. A marked elevation of the resting tension was produced during a long-term La perfusion, reflecting the increase in the intracellular Ca level. Transient increase in the resting tension was frequently observed on returning to normal Ringer solution. The latter change seemed to result from a transient increase in the resting permeability to Ca2+. The negative or positive inotropic effect by increasing or lowering the external Na+ concentration observed under the control condition was strongly depressed during the twitch potentiation. Na+-free contracture induced by solutions isotonically replaced with sucrose or KCl was augmented both during La perfusion and during its washout. The elevation of the intracellular Ca level due to inhibition of Na-Ca exchange mechanism and/or increase in the Ca permeability may be the principal cause for the twitch potentiation caused by La washout.