At fertilization in sea urchin, the free radical nitric oxide (NO) has recently been suggested to cause the intracellular Ca²⁺ rise responsible for egg activation. The authors suggested that NO could be a universal activator of eggs and the present study was set up to test this hypothesis. Intracellular NO and Ca²⁺ levels were monitored simultaneously in eggs of the mouse or the urochordate ascidian Ascidiella aspersa. Eggs were either fertilized or sperm extracts microinjected. Sperm-induced Ca²⁺ rises were not associated with any global, or local, change in intracellular NO, although we were able to detect NO produced by the addition of a NO donor. Furthermore, the NO synthase inhibitor NG-nitro-image-arginine methyl ester had no effect on sperm-induced Ca²⁺ release but did block completely ionomycin-induced NO synthase activation. Therefore, we suggest that the current data provide evidence that NO has no role in the fertilization of these two chordate eggs.
Developmental Biology Vol. 234, Issue 1, p. 216-230