Sperm Increase Inositol 1,4,5-Trisphosphate Mass in Xenopus laevis Eggs Preinjected with Calcium Buffers or Heparin

Abstract

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.