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

Wang C, Du JF, Wu F, Wang HC. Apelin decreases the SR Ca content but enhances the amplitude of [Ca ]i transient and contractions during twitches in isolated rat cardiac myocytes. Am J Physiol Heart Circ Physiol 294: H2540–H2546, 2008. First published April 18, 2008; doi:10.1152/ajpheart.00046.2008.—Apelin has been reported to have a positive inotropic action in the isolated rat heart. However, the effect of apelin on sarcoplasmic reticulum (SR) Ca content and its influence on intracellular Ca transient during excitation-contraction coupling remains poorly understood. In the present study, we determined the effect of apelin on Ca 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 Ca concentration (E[Ca ]i) transients (n 14, P 0.05). But SR Ca content measured by caffeine-induced [Ca ]i (C[Ca ]i) transient was decreased 8.41 0.92% in response to apelin (n 14, P 0.05). Na /Ca exchanger (NCX) function was increased since half-decay time of C[Ca ]i was decreased 16.22 1.36% in response to apelin. Sarco(endo)plasmic reticulum Ca -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 Ca indicator and rapid cooling as another way to measure SR Ca content, and we observed similar results. So we conclude that apelin has a positive inotropic effect on isolated myocytes, and increased amplitude of E[Ca ]i is at least partially involved in the mechanism. NCX function and SERCA activity are increased by apelin, and the SR Ca content is decreased by apelin during twitches. PKC played an important role in these signaling mechanisms.