[Positive inotropic effect of amiloride and inhibition of Na+/Ca2+ exchange: dependence on temperature].

Na+/Ca2+ exchange plays a fundamental role in the regulation of intracellular Ca2+ levels and thus myocardial contractility. The influence of temperature variations on Na+/Ca2+ exchange activity in bovine heart sarcolemmal vesicles has been studied. Furthermore, the temperature dependence of positive inotropic response induced by amiloride, an inhibitor of Na+/Ca2+ exchange activity, has been investigated in isolated guinea-pig left atria driven at 1 Hz. Our results indicate that cooling from 37 degrees to 20 degrees C inhibits Na+/Ca2+ exchange activity in sarcolemmal vesicles, whereas does not change the extent of Na+/Ca2+ exchange inhibition by amiloride. In addition, the positive inotropic effect induced by amiloride in guinea-pig left atria decreases and eventually disappears when temperature is progressively reduced from 35 degrees to 23 degrees C. A possible relationship between the decrease in Na+/Ca2+ exchange activity induced by cooling and the temperature dependence of positive inotropic effect of amiloride is discussed.