Cyclic ADP-ribose (cADPR) is a naturally occurring cyclic nucleotide and a potent mediator of calcium mobilization in many mammalian tissues.'Recent studies suggest that cADPR may play an important role as a second messenger, analogous to inositol triphosphate (IP# in Ca2+ signaling, but targeting a different intracellular Ca2+ channel, the ryanodine re~ eptor.~ cADPR is a metabolite of/?-NAD+, and its synthesizing enzyme,/?-NAD+ cyclase, ...