Because previous findings showed that in human embryonic stem cell-derived cardiomyocytes (hESC-CM) the machinery for Ca2+-induced release of calcium is immature, we tested the hypothesis that hESC-CM contain functional 1,4,5-inositol triphosphate (IP3)-operated intracellular Ca2+ ([Ca2+]i) stores. We investigated the effects of angiotensin II (AT-II) and endothelin 1 (ET-1), which activate the 1,4,5-IP3 pathway, on [Ca2+]i transients and contractions in hESC-CM. Our major findings were that in hESC-CM, both AT-II (10(-9)-10(-7) M) and ET-1 (10(-9)-10(-7) M) exert inotropic and lusitropic effects. The involvement of 1,4,5-IP3-dependent intracellular Ca2+ release in AT-I-induced effects was supported by these findings: the effects of AT-II were blocked by 2-aminoethoxyphenyl borate (2-APB, a 1,4,5-IP3 receptor blocker) and U73122 (a phosopholipase C blocker); and hESC-CM express AT-II type 1 and IP3 type I and II receptors as determined by fluorescence immunostaining. In conclusion, hESC-CM exhibit functional AT-II and ET-1 signaling pathways, as well as 1,4,5-IP3-operated releasable Ca2+ stores.