Control of intracellular pH and bicarbonate by CO diffusion into human sperm.

The reaction of CO with HO to form bicarbonate (HCO) and H controls sperm motility and fertilization via HCO-stimulated cAMP synthesis. A complex network of signaling proteins participates in this reaction. Here, we identify key players that regulate intracellular pH (pH) and HCO in human sperm by quantitative mass spectrometry (MS) and kinetic patch-clamp fluorometry. The resting pH 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 HCO transport are not detected, and may be present at copy numbers < 10 molecules/sperm cell. Instead, HCO is produced by diffusion of CO into cells and readjustment of the CO/HCO/H equilibrium. The proton channel H1 may serve as a unidirectional valve that blunts the acidification ensuing from HCO synthesis. This work provides a new framework for the study of male infertility.