cADP ribose and [Ca2+]i regulation in rat cardiac myocytes

cADP ribose (cADPR)-induced intracellular Ca2+ concentration ([Ca2+](i)) responses were assessed in acutely dissociated adult rat ventricular myocytes using real-time confocal microscopy. In quiescent single myocytes, injection of cADPR (0.1-10 mu M) induced sustained, concentration-dependent [Ca2+](i) responses ranging from 50 to 500 nM, which were completely inhibited by 20 mu M 8-amino- cADPR, a specific blocker of the cADPR receptor. In myocytes displaying spontaneous [Ca2+](i) waves, increasing concentrations of cADPR increased wave frequency up to similar to 250% of control. In electrically paced myocytes (0.5 Hz, 5-ms duration), cADPR increased the amplitude of [Ca2+](i) transients in a concentration-dependent fashion, up to 150% of control. Administration of 8-amino- cADPR inhibited both spontaneous waves as well as [Ca2+](i) responses to electrical stimulation, even in the absence of exogenous cADPR. However, subsequent [Ca2+](i) responses to 5 mM caffeine were only partially inhibited by 8-amino- cADPR. In contrast, even under conditions where ryanodine receptor (RyR) channels were blocked with ryanodine, high cADPR concentrations still induced an [Ca2+](i) response. These results indicate that in cardiac myocytes, cADPR induces Ca2+ release from the sarcoplasmic reticulum through both RyR channels and via mechanisms independent of RyR channels.