Activity of the Na,K-ATPase α4 isoform is regulated during sperm capacitation to support sperm motility

The α4 polypeptide is a testis-specific isoform of the catalytic subunit of the Na,K-ATPase, which is essential for sperm motility and fertility. In the present study, we have investigated the regulation of activity of the α4 isoform and the relevance of this event for sperm capacitation. We have performed this by taking advantage of the selective high affinity of α4 for the inhibitor ouabain. Our results show that ouabain-sensitive hydrolysis of ATP and uptake of (86)Rb, corresponding to the enzymatic and ion transport activities of α4, respectively, increased during sperm capacitation in a time-dependent manner. Specific labeling of α4 with the fluorescent indicator bodipy-ouabain and immunoblot analysis of biotinylated and streptavidin-precipitated sperm plasma membrane proteins indicated a capacitation- and time-dependent rise in levels of active α4 isoform at the sperm surface. Ouabain inhibition of α4 blocked the increase in total sperm motility and the hyperactive motility pattern characteristic of sperm capacitation. Moreover, interference of α4 activity with ouabain partially prevented the intracellular decrease in Na(+) and the plasma membrane hyperpolarization that typically accompany sperm capacitation. In contrast, ouabain inhibition of α4 did not affect the spontaneous sperm acrosomal reaction following capacitation. Together, these results demonstrate that Na,K-ATPase α4 activity is up-regulated during sperm capacitation through mechanisms that involve both increases in molecular activity and levels of α4 at the sperm plasma membrane. This increase in α4 activity helps maintain the changes in motility that are associated with sperm capacitation, emphasizing the biologic relevance of the Na,K-ATPase α4 isoform in sustaining sperm function.