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

To fertilize, mammalian sperm must complete a maturational process called c apacitation. It is thought that the membrane potential of sperm hyperpolari zes during capacitation, possibly due to the opening of K+ channels, but el ectrophysiological evidence is lacking. In this report, using patch-clamp r ecordings obtained from isolated mouse spermatogenic cells we document the presence of a novel K+-selective inwardly rectifying current. Macroscopic c urrent activated at membrane potentials below the equilibrium potential for K+ and its magnitude was dependent on the external K+ concentration. The c hannels selected K+ over other monovalent cations, Current was virtually ab sent when external K+ was replaced with Na+ or N-methyl-D-glucamine. Additi on of Cs+ or Ba2+ (IC50 of similar to 15 muM) to the external solution effe ctively blocked K+ current. Dialyzing the cells with a Mg2+-free solution d id not affect channel activity. Cytosolic acidification reversibly inhibite d the current. We verified that the resting membrane potential of mouse spe rm changed from -52 +/- 6 to -66 +/- 9 mV during capacitation in vitro. Not ably, application of 0.3-1 mM Ba2+ during capacitation prevented this hyper polarization and decreased the subsequent exocytotic response to zona pellu cida. A mechanism is proposed whereby opening of inwardly rectifying K+ cha nnels may produce hyperpolarization under physiological conditions and cont ribute to the cellular changes that give rise to the capacitated state in m ature sperm. (C) 2001 Academic Press.