GUANOSINE 5'-THIOTRIPHOSPHATE MAY STIMULATE PHOSPHOINOSITIDE MESSENGER PRODUCTION IN SEA-URCHIN EGGS BY A DIFFERENT ROUTE THAN THE FERTILIZING SPERM

We show that microinjecting guanosine-5'-thiotriphosphate (GTP-gamma-S) into unfertilized sea urchin eggs generates an intracellular free calcium concentration ([Ca]i) transient apparently identical in magnitude and duration to the calcium transient that activates the egg at fertilization. The GTP-gamma-S-induced transient is blocked by prior microinjection of the inositol trisphosphate (InsP3) antagonist heparin. GTP-gamma-S injection also causes stimulation of the egg's Na+/H+ antiporter via protein kinase C, even in the absence of a [Ca]i increase. These data suggest that GTP-gamma-S acts by stimulating the calcium-independent production of the phosphoinositide messengers InsP3 and diacylglycerol (DAG). However, the fertilization [Ca]i transient is not affected by heparin, nor can the sperm cause calcium-independent stimulation of protein kinase C. It seems that the bulk of InsP3 and DAG production at fertilization is triggered by the [Ca]i transient, not by the sperm itself. GDP-beta-S, a G-protein antagonist, does not affect the fertilization [Ca]i transient. Our findings do not support the idea that signal transduction at fertilization operates via a G-protein linked directly to a plasma membrane sperm receptor.