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.