The alpha 4 isoform of the Na+,K+-ATPase is tuned for changing extracellular environments

In their journey from the male to the female reproductive tract, spermatozoa are confronted with a constantly changing environment. To cope with the associated challenges, spermatozoa express a distinct set of transporters, channels and pumps. One of the membrane proteins unique to spermatozoa is the alpha 4 isoform of the Na+,K+-ATPase. In addition to alpha 4, spermatozoa express the ubiquous alpha 1 variant. To get a detailed understanding of how alpha 1 and alpha 4 differ, and why spermatozoa need an additional Na+,K+-ATPase, we have conducted an electrophysiological comparison of the rodent isoforms (rat alpha 4 versus mouse alpha 1-3) using the Xenopus oocyte expression system. We demonstrate isoform-specific differences in the voltage sensitivity of steady-state turnover, with alpha 2 being the more sensitive, and alpha 1 and alpha 2 having faster Na+ release kinetics than alpha 3 and alpha 4. Our data further show that the alpha 1 and alpha 2 turnover rates are fast compared with those of alpha 3 and alpha 4. Finally, alpha 4 is less influenced by changes in extracellular Na+ and temperature than alpha 1. Based on these findings, we discuss the possibility that evolution has selected robust activity rather than rapid turnover for alpha 4.