Calcium mobilization by dual receptors during fertilization of sea urchin eggs

Fertilization is accompanied by a transient increase in the concentration of intracellular Ca²⁺, which serves as a signal for initiating development. Some of the Ca²⁺ appears to be released from intracellular stores by the binding of inositol trisphosphate (IP₃) to its receptor. However, in sea urchin eggs, other mechanisms appear to participate. Cyclic adenosine diphosphate-ribose (cADPR), a naturally occurring metabolite of nicotinamide adenine dinucleotide, is as potent as IP₃ in mobilizing Ca²⁺ in sea urchin eggs. Experiments with antagonists of the cADPR and IP₃ receptors revealed that both Ca²⁺ mobilizing systems were activated during fertilization. Blockage of either of the systems alone was not sufficient to prevent the sperm-induced Ca ²⁺ transient. This study provides direct evidence for a physiological role of cADPR in the Ca²⁺ signaling process.