The Cyclic GMP-Mediated Calcium Release Pathway in Sea Urchin Eggs Is Not Required for the Rise in Calcium during Fertilization

Abstract

The mechanisms required for cGMP-induced Ca2+release in the sea urchin egg were investigated using both egg homogenates and intact eggs. The postulated pathway of cGMP-dependent protein kinase (PKG) activation of ADP-ribosyl cyclase for production of cADPR to activate the ryanodine receptor Ca2+channel was tested with a variety of activators (cGMP analogs and cIMP) and inhibitors (Rp-8-pCPT-cGMPS, 3-aminopyridine NAD, nicotinamide, and spermine). Our observations are consistent with Ca2+release by cGMP in the egg being dependent on an isoform of PKG that is distinct from the mammalian enzyme. PKG activity in the sea urchin egg was activated by cIMP, but was insensitive to cGMP analogs, which are potent activators of mammalian isoenzymes. Surprisingly, it appears the activation of the cGMP-dependent Ca2+release pathway was unnecessary during fertilization. Inhibitors of either PKG or ADP-ribosyl cyclase activities did not prevent the transient rise in intracellular Ca2+activity in heparin-loaded eggs during fertilization. These results suggest the synthesis of cADPR during fertilization is not necessary for regulating the Ca2+event.