Abstract
A transient increase in intracellular free calcium is believed to be the signal responsible for the stimulation of the egg metabolism at fertilization and the resumption of the cell cycle. We have studied how the polyphospho-inositides (PP1) turn over at fertilization in sea urchin eggs, in order to determine the relationship between the metabolism of these lipids and the calcium signal. We compare the patterns of PPI turnover that occur during the first minute following fertilization in eggs in which PPI are labelled to steady state with [3H]inositol or [3H]arachidonate with that in which PPI are labelled for a shorter period with [3H]inositol. When eggs are labelled to apparent isotopic equilibrium with either [3H]inositol or [3H]arachidonate, no early increase in [3H]PtdlnsP2occurs while Ptdins decreases slightly. On the contrary, when not labelled to isotopic equilibrium, all [3H]PPI increase during the first 15 seconds following fertilization. We find that, within seconds, fertilization triggers a 600-fold increase in the turnover of PPI, producing an amount of lnsP3 apparently sufficient to trigger calcium release. We suggest that phosphoinositi dase C and PtdlnsP kinase, responsible respectively for the hydrolysis and synthesis of PtdlnsP2, are both stimulated to a comparable degree in the first 30 seconds following fertilization and that net changes in the amount of PtdlnsP2at fertilization are very sensitive to the relative levels of activation of the two enzymes. Activating the eggs with the calcium ionophore A23187 showed that both these enzymes are sensitive to calcium, suggesting that calcium-dependent lnsP3 production might play a role in the initiation and/or the propagation of the fertilization calcium wave. A comparison of the rates of PPI turnover in fertilized and A23187-activated eggs confirms that the fertilizing sperm stimulates an early and possibly calcium-dependent increase in PtdlnsP kinase activity.
