Boar sperm plasma membrane Ca(2+)-selective channels in planar lipid bilayers

Entry of Ca2+ through Ca2+ channels is thought to trigger the acrosome reaction of spermatozoa during fertilization. Antagonists of the L-type Ca2+ channel are known to prevent the intracellular Ca2+ (Ca2+) increase and inhibit acrosomal exocytosis in mammalian sperm. Planar bilayer recordings were used to study Ca2- channels incorporated from partially purified boar sperm plasma membranes. With symmetrical 50 mM NaCl and 100 mM BaCl2 on the cis side, single-channel events consistent with Ba2+ flux from cis to trans were observed. These channels were activated by the dihydropyridine agonist (+/-)BAY K 8644 and blocked by the antagonist nitrendipine. Sperm Ca2- channels did not require depolarization for activation and did not inactivate. The (+/-)BAY K 8644 and (S-)BAY K 8644 enantiomers increased apparent open time in a dose-dependent [half-maximal activity constant (K0.5) = 0.9 and 0.3 microM, respectively] manner. Dihydropyridine antagonists nitrendipine (K0.5 = 0.5 microM) and (R+)BAY K 8644 (K0.5 = 2.8 microM) decreased apparent open times. The channels described in this report share some properties with brain, cardiac, and skeletal muscle t tubule Ca2+ channels and may be involved in increasing Cai2+ before the acrosome reaction.