A volatile inhibitor immobilizes sea urchin sperm in semen by depressing the intracellular pH

Sea urchin spermatozoa are normally immotile in semen, but motility can be initiated by increasing gas flow over the semen—for example, by blowing N 2 gas over a thin layer of semen. This result indicates that sperm motility is not O 2 limited and suggests that seminal fluid contains a volatile inhibitor of motility which is responsible for the paralysis of sperm in semen. This inhibitor might be carbon dioxide, which reversibly immobilizes sperm. 31P-NMR measurements of pH show that the sperm intracellular pH (pH i) increases by 0.36 pH unit upon dilution of semen into seawater. Since previous studies have shown that this magnitude of pH increase is sufficient to trigger sperm motility, we suggest that the volatile inhibitor is inhibiting sperm motility in semen by depressing the pH i. A simple hypothesis that explains these observations is that the volatile motility inhibitor is CO 2, which could acidify pH i as a diffusable weak acid. In this regard, sperm diluted into seawater release acid, and this acid release is related to the pH i increase and motility initiation. In fact, nearly half of the acid released by sperm upon dilution is volatile and may therefore be due to CO 2 efflux. Most of the acid, however, cannot be attributed to CO 2 release because it is not volatile. Thus, when sperm are diluted into seawater, they raise their pH i by releasing CO 2 and protons from the cytoplasm into the surrounding seawater.