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

Abstract

The reaction of CO2 with H2O to form bicarbonate (HCO3−) and H+ controls sperm motility and fertilization via HCO3−-stimulated cAMP synthesis. A complex network of signaling proteins participates in this reaction. Here, we identify key players that regulate intracellular pH (pHi) and HCO3− in human sperm by quantitative mass spectrometry (MS) and kinetic patch-clamp fluorometry. The resting pHi is set by amiloride-sensitive Na+/H+ exchange. The sperm-specific putative Na+/H+ exchanger SLC9C1, unlike its sea urchin homologue, is not gated by voltage or cAMP. Transporters and channels implied in HCO3− transport are not detected, and may be present at copy numbers < 10 molecules/sperm cell. Instead, HCO3− is produced by diffusion of CO2 into cells and readjustment of the CO2/HCO3−/H+ equilibrium. The proton channel Hv1 may serve as a unidirectional valve that blunts the acidification ensuing from HCO3− synthesis. This work provides a new framework for the study of male infertility.