Potassium ions Improve the Progesterone-Induced [Ca2+]I Increase and Acrosome Reaction as well as the Potassium-Induced [Ca2+]I Increase in Human Spermatozoa

Abstract

Ejaculated sperm undergo a series of physiological changes in the female reproductive tract to become capable of fertilization. These changes, called capacitation can be induced in vitro in appropriate media and culminate in hyperactivated motility and the ability of the sperm to acro-some-react. These downstream effects of capacitation are mediated by increased intracellular Ca2+ concentration ([Ca2+]i). Exposure of sperm to progesterone (P4) and potassium (K+) induce an increase in [Ca2+]i. The effects of incubating sperm in media containing different potassium concentrations on the P4-induced [Ca2+]i increase and AR as well as the effect of K+ on [Ca2+]i increase were investigated. Swum-up spermatozoa were capacitated for about 6 h in culture me-dia containing either 5.4 (control), 25 or 116.4 mM K+ before being stimulated with P4. P4 evoked a biphasic [Ca2+]i response in spermatozoa incubated in each of the media. The amplitude of the transient P4-induced increase in [Ca2+]i and the percentage of P4-induced AR were significantly lower in sperm incubated in media with very low or very high K+ concentration. Absence of K+ in the incubating medium significantly inhibited the amplitude of both the P4- and K+-induced [Ca2+]i increases. Application of K+ after P4 application evoked a second biphasic [Ca2+]i re-sponse. The results suggest that i) the presence of K+ in the incubating medium is important in the K+-induced [Ca2+]i increase as well as the P4-induced [Ca2+]i increase and AR, ii) human sperm capacitates better in an incubating medium with a K+ concentration around that of ovi-ductal fluid and iii) P4- and K+ utilize different channels for inducing Ca2+ influx.