Inwardly rectifying K+ channels in spermatogenic cells:: Functional expression and implication in sperm capacitation

To fertilize, mammalian sperm must complete a maturational process called capacitation. It is thought that the membrane potential of sperm hyperpolarizes during capacitation, possibly due to the opening of K+ channels, but electrophysiological evidence is lacking. In this report, using patch-clamp recordings obtained from isolated mouse spermatogenic cells we document the presence of a novel K+-selective inwardly rectifying current. Macroscopic current activated at membrane potentials below the equilibrium potential for K+ and its magnitude was dependent on the external K+ concentration. The channels selected K+ over other monovalent cations, Current was virtually absent when external K+ was replaced with Na+ or N-methyl-D-glucamine. Addition of Cs+ or Ba2+ (IC50 of similar to 15 muM) to the external solution effectively blocked K+ current. Dialyzing the cells with a Mg2+-free solution did not affect channel activity. Cytosolic acidification reversibly inhibited the current. We verified that the resting membrane potential of mouse sperm changed from -52 +/- 6 to -66 +/- 9 mV during capacitation in vitro. Notably, application of 0.3-1 mM Ba2+ during capacitation prevented this hyperpolarization and decreased the subsequent exocytotic response to zona pellucida. A mechanism is proposed whereby opening of inwardly rectifying K+ channels may produce hyperpolarization under physiological conditions and contribute to the cellular changes that give rise to the capacitated state in mature sperm. (C) 2001 Academic Press.