SPERM INCREASE INOSITOL 1,4,5-TRISPHOSPHATE MASS IN XENOPUS-LAEVIS EGGS PREINJECTED WITH CALCIUM BUFFERS OR HEPARIN

Although it is known that all fertilization events are due to elevated intracellular calcium ([Ca2+](i)), it is not known how sperm induce an increase in zygote [Ca2+](i). We report that sperm can increase inositol 1,4,5-trisphosphate in Xenopus laevis eggs that have been preinjected with calcium buffers to prevent the increase in [Ca2+](i) (both the initial increase and the subsequent wave) after fertilization. After buffering [Ca2+](i) to levels well below basal, IP3 production was not blocked, whereas IP3 metabolism may be inhibited. Also, heparin (an inhibitor of inositol 1,4,5-trisphosphate action and the fertilization response) does not prevent a normal increase in IP3 after fertilization. Conditions that produce various levels of polyspermy are associated with IP3 increases similar to those noted after monospermic fertilization. These data suggest a specific order of fertilization events: sperm utilize an initial production of inositol 1,4,5-trisphosphate to produce the [Ca2+](i) increase at fertilization. (C) 1994 Academic Press,Inc.