Effect on Sperm-Induced Activation Current and Increase of Cytosolic Ca2+by Agents That Modify the Mobilization of [Ca2+]i: I. Heparin and Pentosan Polysulfate

The mechanism of the elevation of intracellular free Ca2+([Ca2+]i) induced by a single sperm in eggs of the sea urchinLytechinus variegatuswas investigated. Simultaneous measurements of [Ca2+]i, and of the activation current, were carried out on eggs microinjected with Ca Green-1 or Ca Green dextran, and voltage clamped at −20 mV. The microinjection of 0.5 to 1.0 mg/ml heparin (MW 6000) or pentosan polysulfate (MW 3000), final intracellular concentration, causes a concentration-dependent inhibition in all parameters of the sperm-induced elevation of [Ca2+]iand the phase 2 calcium-activated cation current (Ip). For each: (1) the onset is delayed; (2) the rate of change is slowed; and (3) the peak amplitude attained is diminished. In some experiments at the higher concentrations, the microinjected polysulfates cause the complete suppression of the sperm-induced elevation of [Ca2+]iandIp. The entry of multiple sperm overcomes the inhibitory effects of the polysulfates. Our data suggest that inositol 1,4,5-trisphosphate is the primary mechanism responsible for the sperm-induced release of Ca2+from intracellular stores.