NAADP and InsP₃ play distinct roles at fertilization in starfish oocytes

NAADP participates in the response of starfish oocytes to sperm by triggering the fertilization potential (FP) through the activation of a Ca²⁺ current which depolarizes the membrane to the threshold of activation of the voltage-gated Ca²⁺ channels. The aim of this study was to investigate whether this Ca²⁺ influx is linked to the onset of the concomitant InsP₃-mediated Ca²⁺ wave by simultaneously employing Ca²⁺ imaging and single-electrode intracellular recording techniques. In control oocytes, the sperm-induced membrane depolarization always preceded by a few seconds the onset of the Ca²⁺ wave. Strikingly, the self-desensitization of NAADP receptors either abolished the Ca²⁺ response or resulted in abnormal oocyte activation, i.e., the membrane depolarization followed the Ca²⁺ wave and the oocyte was polyspermic. The inhibition of InsP₃ signaling only impaired the propagation of the Ca²⁺ wave and shortened the FP. The duration of FP was also reduced in low-Na⁺ sea water. Finally, uncaged InsP₃ produced a Ca²⁺ increase, which depolarized the membrane upon the activation of a Ca²⁺-sensitive cation current. These results support the hypothesis that Ca²⁺ entry during the NAADP-triggered FP is required for the onset of the Ca²⁺ wave at fertilization. The InsP₃-mediated Ca²⁺ wave, in turn, may interact with the NAADP-evoked depolarization by activating a Ca²⁺-dependent Na⁺ entry.