Inwardly Rectifying K+ Channels in Spermatogenic Cells: Functional Expression and Implication in Sperm Capacitation

Abstract

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 Ba(2+) (IC(50) of approximately 15 microM) to the external solution effectively blocked K(+) current. Dialyzing the cells with a Mg(2+)-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 Ba(2+) 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.

Flagellar ion channels of sperm: similarities and differences between species

“…In living cells, membrane potential is defined by the gradients of K + , Na + and Cl − with potassium channels playing a crucial role in its regulation. Noncapacitated epididymal murine spermatozoa are slightly depolarized at about −40 mV, however they hyperpolarize up to −60 mV upon capacitation [137]. This effect is attributed to potassium permeability and two members of the Slo family of potassium channels have been recently proposed to play a role in this process [21,25,26,29,31,33,[138][139][140][141].…”

KSper, a pH-sensitive K + current that controls sperm membrane potential

“…After in vitro capacitation in bicarbonate-containing media at pH 7.4, pH i increases Ͼ0.3 units (2,8,9). When intracellular alkalinization is prevented by glucose incubation, bovine sperm fail to capacitate (8, 10).Measurements with voltage-sensitive fluorescent dyes indicate that noncapacitated murine sperm are relatively depolarized (approximately Ϫ30 mV) and hyperpolarize to approximately Ϫ60 mV during capacitation (4,11,12). Indirect measurements attribute the capacitation-associated hyperpolarization to an increase in K ϩ permeability (4) and a block of epithelial sodium channels (ENaCs) (13).…”

“…Measurements with voltage-sensitive fluorescent dyes indicate that noncapacitated murine sperm are relatively depolarized (approximately Ϫ30 mV) and hyperpolarize to approximately Ϫ60 mV during capacitation (4,11,12). Indirect measurements attribute the capacitation-associated hyperpolarization to an increase in K ϩ permeability (4) and a block of epithelial sodium channels (ENaCs) (13).…”

Cl− Channels and Transporters in Sperm Physiology

“…Altogether the set of changes required for this maturation process is called capacitation and involves the development of a distinctive sperm motility pattern known as hyperactivation, and the sperm capacity to undergo the AR, an exocytotic event that allows the sperm to fertilize the egg. During sperm capacitation PKA is activated (Harrison 2004 ), leading to tyrosine phosphorylation increases (Visconti et al 1995a , b ); pH i (Zeng et al 1995 ) and [Ca 2+ ] i elevate (Baldi et al 1991 ;Breitbart 2003 ;DasGupta et al 1993 ;Suarez et al 1993 ;Xia and Ren 2009 ); plasma membrane composition and organization are modifi ed (Cross 1998 ;Davis 1981 ;Gadella and Harrison 2000 ;Go and Wolf 1983 ;Travis and Kopf 2002 ;Visconti et al 1999 ); and the cell E m is hyperpolarized in the mouse and other species (Arnoult et al 1999 ;Demarco et al 2003 ;Munoz-Garay et al 2001 ;Zeng et al 1995 ).…”

“…Indeed, hyperpolarization is important in mouse, bovine, and equine sperm capacitation (Arnoult et al 1996 ;Arnoult et al 1999 ;De La Vega-Beltran et al 2012 ;Demarco et al 2003 ;McPartlin et al 2011 ;Munoz-Garay et al 2001 ;Zeng et al 1995 ), although it has not been demonstrated in human sperm. The sperm resting E m is relatively depolarized in most mammalian sperm (between −30 and −40 mV) (De La Vega-Beltran et al 2012 ;Demarco et al 2003 ;Espinosa and Darszon 1995 ;Hernandez-Gonzalez et al 2006 ;McPartlin et al 2011 ;MunozGaray et al 2001 ;Santi et al 2010 ;Zeng et al 1995 ).…”