The rate at which human sperm are immobilized and killed by mild acidity

Objective: To determine the rate at which mild acidity immobilizes and kills human sperm and to evaluate an acidic microbicide, BufferGel, for sperm immobilization. Design: Controlled in vitro study. Setting: An academic research university and hospital andrology lab. Patient(s): Eight volunteer male sperm donors. Intervention(s): Semen samples were treated with hydrochloric acid (HCl) or BufferGel. Main Outcome Measure(s): Sperm motility was measured by using a computerized automated semen analyzer and video microscopy. Sperm membrane permeability and intracellular pH were measured by using fluorescent techniques. Result(s): In semen acidified with HCl to pH 4.0, sperm were rapidly immobilized (within 1 min) and were irreversibly immobilized (killed) within 10 minutes. The speed of immobilization and of killing were both linearly proportional to hydrogen ion activity over a pH range of 7.5–4.0. Across the same range, the intracellular pH of human sperm equilibrated to within 0.5 pH units of extracellular pH within 1–2 minutes. BufferGel immobilized sperm significantly faster than HCl from pH 4.0–6.0. Conclusion(s): Exposure to mild acidity rapidly acidifies the intracellular pH of human sperm and is rapidly spermicidal. BufferGel accelerates acid immobilization of sperm.