Role of endocardial endothelium in positive inotropic action of vasopressin

Abstract

We have studied the effects of vasopressin on isolated cat papillary muscle both before and after damaging the endocardium. The experiments were performed in physiological conditions of temperature (35 degrees C) and calcium concentration (1.25 mM Ca2+). Isometric and isotonic twitches as well as maximal unloaded velocity of shortening (Vmax) were measured. In muscles with an intact endocardium (n = 13), vasopressin (10(-12) to 10(-6) M) induced early twitch relaxation with a significant reduction of time to half isometric tension decline and with concomitant significant decrease in peak twitch performance. In a second group of muscles (n = 13) the endocardial endothelial surface was damaged by briefly (1 s) exposing the muscles to a 0.5% Triton X-100 followed by abundant wash with Krebs-Ringer solution, thereby irreversibly decreasing time to half twitch relaxation and peak twitch tension without significantly affecting Vmax. After this intervention, vasopressin had a positive inotropic effect with a significant increase in peak twitch tension and Vmax with no significant changes in twitch duration. Accordingly, in the presence of a functional endocardium, the direct myocardial positive inotropic effect of vasopressin was reversed, with early twitch relaxation and diminished peak twitch performance. At the highest concentrations of vasopressin, vasopressin-induced functional and morphological damage of the endocardium was observed.