MODULATION OF CA2-RAT CARDIOMYOCYTES BY ANGIOTENSIN-II AND ENDOTHELIN-1( TRANSIENTS IN NEONATAL AND ADULT)

Vasoactive peptides may exert inotropic and chronotropic effects in ca rdiac muscle by modulating intracellular calcium. This study assesses effects of angiotensin II (ANG II) and endothelin-1 (ET-1) on intracel lular free calcium concentration ([Ca2+](i)) in cultured cardiomyocyte s from neonatal and adult rats. [Ca2+](i) was measured microphotometri cally and by digital imaging using fura 2 methodology. Receptor subtyp es through which these agonists induce responses were determined pharm acologically and by radioligand binding studies. ANG II and ET-1 incre ased neonatal atrial and ventricular cell [Ca2+](i) transients in a do se-dependent manner. ANG II(10(-11) to 10(-7) M, failed to elicit [Ca2 +](i) responses in adult cardiomyocytes, whereas ET-1 increased [Ca2+] (i) in a dose-dependent manner. The ET(A) receptor antagonist BQ-123 s ignificantly reduced (P < 0.05) ET-1-induced responses, and the ET(B) receptor agonist IRL-1620 (10(-7) to 10(-5) M) significantly increased (P < 0.05) [Ca2+](i) in neonatal and adult cardiomyocytes. ET-1 bindi ng studies demonstrated 85% displacement by BQ-123 and similar to 15% by the ET(B) receptor agonist sarafotoxin S6c, suggesting a predominan ce of ET(A) receptors. Competition binding studies for ANG II failed t o demonstrate significant binding on adult ventricular myocytes, indic ating the absence or presence of very few ANG II receptors. These data demonstrate that ANG II and ET-1 have stimulatory [Ca2+](i) effects o n neonatal cardiomyocytes, whereas in adult cardiomyocytes, ANG II-ind uced effects are insignificant, and only ET-1-induced responses, which are mediated predominantly via ETA receptors, are preserved. Cardiomy ocyte responses to vasoactive peptides may thus vary with cardiac deve lopment.