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

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 (pH_i) and HCO₃^- in human sperm by quantitative mass spectrometry (MS) and kinetic patch-clamp fluorometry. The resting pH_i 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 H_v1 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.