Ca 2+ -Induced Ca 2+ Release in Sea Urchin Egg Homogenates: Modulation by Cyclic ADP-Ribose

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Abstract
Calcium-induced calcium release (CICR) may function widely in calcium-mediated cell signaling, but has been most thoroughly characterized in muscle cells. In a homogenate of sea urchin eggs, which display transients in the intracellular free calcium concentration ([Ca 2+ ] i ) during fertilization and anaphase, addition of Ca 2+ triggered CICR. Ca 2+ release was also induced by the CICR modulators ryanodine and caffeine. Responses to both Ca 2+ and CICR modulators (but not Ca 2+ release mediated by inositol 1,4,5-trisphosphate) were inhibited by procaine and ruthenium red, inhibitors of CICR. Intact eggs also displayed transients of [Ca 2+ ] i when microinjected with ryanodine. Cyclic ADP-ribose, a metabolite with potent Ca 2+ -releasing properties, appears to act by way of the CICR mechanism and may thus be an endogenous modulator of CICR. A CICR mechanism is present in these nonmuscle cells as is assumed in various models of intracellular Ca 2+ wave propagation.