Possibility of involvement of nitric oxide in induction of Ca²⁺-transients at fertilization of sea urchin eggs

Abstract

A transient increase of the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) (Ca²⁺-transient) in the early stage of fertilization is requisite for the activation of sea urchin eggs. We have found that immediately after insemination, cGMP begins to increase, followed by cADP-ribose and IP₃, all preceding the explosive increase of [Ca²⁺]_i (Kuroda et al., Development 128, 2001). Kuo et al. (Nature 406, 2000) have claimed that nitric oxide synthase (NOS) or nitric oxide (NO) is the primary egg activator. NO could activate the formation of cGMP/cADP-ribose, and S-nitrosylation of src kinase could accelerate the IP₃ production. On the contrary, Leckie et al. (J. Biol. Chem. 278, 2003) have argued that NO production was not the cause but the effect of [Ca²⁺]_i increase. We examined two inconsistent reports by simultaneous measurements of the NO level and [Ca²⁺]_i and those of cGMP and IP₃ contents of eggs during fertilization in the presence or absence of inhibitors against NOS and NO-dependent soluble guanylyl cyclase (sGC), and NO scavenger. We obtained the following results: (1) NOS inhibitors could not block the Ca²⁺-transients and NO production without lethal effects on sperm; (2) freshly prepared oxyhemoglobin could not block Ca²⁺-transients; (3) on the other hand, an inhibitor of NO-sensitive sGC elongated the latent period of Ca²⁺-transients, abolished the cGMP production immediately after insemination, and significantly delayed the IP₃ production. Even now the situation is confused. [Jpn J Physiol 54 Suppl:S71 (2004)]