Control of intracellular pH and bicarbonate by CO2 diffusion into human sperm

The reaction of CO2 with H2O to form HCO3– and H+ controls sperm motility and fertilization via HCO3--stimulated cAMP synthesis and alkalization. A complex network of signaling proteins has been proposed to participate in this reaction. Here, we identify key players that regulate intracellular pHi and HCO3- in human sperm by quantitative mass spectrometry, kinetic fluorometry, and patch-clamp recording. pHi is set by an amiloride-sensitive Na+/H+ exchanger of the SLC9B type; the sperm-specific putative Na+/H+ exchanger SLC9C1 is not involved in voltage-gated H+ transport. Many transporters and channels previously implied in HCO3- are absent. Instead, HCO3- is produced by rapid CO2 diffusion into the cell and readjustment of the CO2/HCO3-/H+ equilibrium by carbonic anhydrase. The proton channel Hv1 serves as a unidirectional valve that blunts the ensuing acidification. This work calls for substantial revision of signaling concepts in human sperm and provides a new framework for the study of male infertility.