Control of intracellular pH and bicarbonate by CO 2 diffusion into human sperm.

The reaction of CO 2 with H 2 O to form bicarbonate (HCO 3 - ) and H + controls sperm motility and fertilization via HCO 3 - -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 HCO 3 - 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 HCO 3 - transport are not detected, and may be present at copy numbers < 10 molecules/sperm cell. Instead, HCO 3 - is produced by diffusion of CO 2 into cells and readjustment of the CO 2 /HCO 3 - /H + equilibrium. The proton channel H v 1 may serve as a unidirectional valve that blunts the acidification ensuing from HCO 3 - synthesis. This work provides a new framework for the study of male infertility.