SYNERGISTIC CALCIUM-RELEASE IN THE SEA-URCHIN EGG BY RYANODINE AND CYCLIC ADP RIBOSE

A transient rise in cytoplasmic Ca2+ activity in the sea urchin egg oc curs during fertilization due to release from an intracellular store. Two intracellular receptor Ca2+ channels for inositol 1,4,5-trisphosph ate (IP3) and ryanodine have been identified by physiological and immu nological techniques. While IP3 is the endogenous messenger for the IP 3 receptor, a corresponding physiological messenger for the ryanodine receptor is unknown. A variety of recent experimental evidences sugges t that cyclic ADP ribose (cADPR) may be a possible candidate. In this study using both egg homogenates and intact eggs, we show that subthre shold concentrations of cADPR and ryanodine can act synergistically to potentiate Ca2+ release. Addition of 10-20 nM cADPR, which causes lit tle net increase in Ca2+, generally enhances the action of subthreshol d concentrations of ryanodine. Similarly the addition of 60-80 mu M ry anodine causes a slight transient increase but potentiates maximal Ca2 + increase by a subsequent subthreshold addition of cADPR. While the t arget of Ca2+ release by ryanodine and cADPR may be the ryanodine rece ptor, their actions appear to be different and more complex than simpl y opening the release mechanism. There are significant differences in the kinetics of release by the two agonists. In addition we used a poo rly metabolized analog of IP3 and an inhibitor of endoplasmic reticulu m Ca2+ ATPase activity, to show that the unfertilized egg contains a r apidly filled Ca2+ store, which is commonly released by both IP3-media ted and ryanodine-mediated release mechanisms. (C) 1994 Academic Press , Inc.