CALMODULIN IS A SELECTIVE MEDIATOR OF CA2+-INDUCED CA2+ RELEASE VIA THE RYANODINE RECEPTOR-LIKE CA2+ CHANNEL TRIGGERED BY CYCLIC ADP-RIBOSE

The ryanodine receptor-like Ca2+ channel (RyRLC) is responsible for Ca2+ wave propagation and Ca2+ oscillations in certain nonmuscle cells by a Ca2+-induced Ca2+ release (CICR) mechanism. Cyclic ADP-ribose (cADPR), an enzymatic product derived from NAD(+), is the only known endogenous metabolite that acts as an agonist on the RyRLC. However, the mode of action of cADPR is not clear. We have identified calmodulin as a functional mediator of cADPR-triggered CICR through the RyRLC in sea urchin eggs. cADPR-induced Ca2+ release consisted of two phases, an initial rapid release phase and a subsequent slower release. The second phase was selectively potentiated by calmodulin which, in turn, was activated by Ca2+ released during the initial phase. Caffeine enhanced the action of calmodulin. Calmodulin did not play a role in inositol 1,4,5-trisphosphate-induced Ca2+ release. These findings offer insights into the multiple pathways that regulate intracellular Ca2+ signaling.