Calcium channel antagonists inhibit the acrosome reaction and bind to plasma membranes of sea urchin sperm

Abstract

As a prerequisite to fertilization, sea urchin sperm undergo an acrosome reaction that is mediated in part by increased permeability to Ca2+, with an attendant rapid, massive intracellular Ca2+ accumulation. The acrosome reaction is inhibited by Ca2+ channel antagonists, including verapamil, D600, and dihydropyridines such as nitrendipine, nimodipine, and nisoldipine. To examine the interaction of Ca2+ antagonists with sperm, a plasma membrane preparation enriched for Na+,K+-ATPase was isolated from sea urchin sperm. These plasma membranes specifically bound [3H]nitrendipine and [3H]verapamil at concentrations similar to those that inhibit the acrosome reaction. The binding of verapamil was sigmoidal and half-maximal at 1 microM. There was a high specificity in the binding interaction, since by competition binding verapamil, (-)-D600, and (+)-D600 had different relative Kd values, 11, 2.5, and 0.5 microM, respectively. These data suggest that sperm mediate the Ca2+ influx required for induction of the acrosome reaction via Ca2+ channels with properties similar, but not identical, to those of other excitable tissues.