Activation of Ca

When sperm of the sea urchin, Hemicentrotus pulcherrimus, were exposed to high pH (9.0) sea water, they showed large increases in intracellular Ca2+ ([Ca2+ ]i) and pH (pHi) and underwent the acrosome reaction (AR) without the aid of the egg jelly. Not only [Ca2+ ]i increase but also pHi rise did not occur under Ca2+ -free conditions. Both the increases in [Ca2+ ]i and pHi and the AR by high pH were inhibited by a Ca2+ channel blockers, verapamil and nisoldipine, and by a lectin, wheat germ agglutinin (WGA) which interacts with a 220 kD membrane glycoprotein of sperm. These reagents inhibited also the AR by the egg jelly. The inhibitory effects of WGA were immediately canceled by the addition of N-acetyl-D-glucosamine, a sugar which is known to remove WGA from its binding site. These results suggest that 1) the same Ca2+ transport system is activated by high external pH and the egg jelly, 2) increase in [Ca2+ ]i is prerequisite for the stimulation of the H+ -efflux system(s) and 3) the 220 kD WGA-binding membrane protein functions as a regulator protein of Ca2+ transport system.