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. Bicarbonate (HCO₃⁻) is critical in sperm for stimulation of cAMP synthesis during fertilization, though there is dispute over how HCO₃⁻ is transported into sperm. Here the authors use limit-of-detection LC/MS to characterize sperm protein expression and show that HCO₃⁻ is produced from CO₂ diffusion into sperm rather than active transport. [ABSTRACT FROM AUTHOR]