Apelin decreases the SR Ca2+ content but enhances the amplitude of [Ca2+]i transient and contractions during twitches in isolated rat cardiac myocytes

Apelin has been reported to have a positive inotropic action in the isolated rat heart. However, the effect of apelin on sarcoplasmic reticulum (SR) Ca2+ content and its influence on intracellular Ca2+ transient during excitation-contraction coupling remains poorly understood. In the present study, we determined the effect of apelin on Ca2+ transient and contractions in isolated rat cardiomyocytes. When compared with control, treatment with apelin caused a 55.7 +/- 13.9% increase in sarcomere fraction shortening and a 43.6 +/- 4.56% increase in amplitude of electrical-stimulated intracellular Ca2+ concentration (ECa2+]i) transients (n = 14, P < 0.05). But SR Ca2+ content measured by caffeine-induced Ca2+]i (CCa2+]i) transient was decreased 8.41 +/- 0.92% in response to apelin (n = 14, P < 0.05). Na+/Ca2+ exchanger (NCX) function was increased since half-decay time of CCa2+]i was decreased 16.22 +/- 1.36% in response to apelin. Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) activity was also increased by apelin. These responses can be partially or completely blocked by chelerythrine chloride, a PKC inhibitor. In addition, to confirm our data, we used indo-1 as another Ca2+ indicator and rapid cooling as another way to measure SR Ca2+ content, and we observed similar results. So we conclude that apelin has a positive inotropic effect on isolated myocytes, and increased amplitude of ECa2+]i is at least partially involved in the mechanism. NCX function and SERCA activity are increased by apelin, and the SR Ca2+ content is decreased by apelin during twitches. PKC played an important role in these signaling mechanisms.