Increase of human spermatozoa intracellular Ca2+ concentration after fusion with prostasomes.

Prostasomes are membranous vesicles (150-200 nm diameter) present in human semen. They are secreted by the prostate gland and contain large amounts of cholesterol, sphingomyelin and calcium, and some of their proteins are enzymes. Prostasomes are involved in a number of biological functions. In previous work, we discovered that prostasomes may fuse to sperm at neutral or at slightly acidic pH values. This mechanism may deliver calcium to sperm, thereby influencing sperm functions. We measured sperm Ca2+]i with the fura-2 AM method and found that it increased after mixing prostasomes and sperm at pH values allowing fusion (pH 5-7). The increase of Ca2+]i was proportional to the extent of fusion as measured through the relief of R18 self-quenching. We also examined the increase of sperm Ca2+]i and the extent of fusion as a function of sperm to prostasome ratio and, also in this case, there was proportionality between the extent of fusion and the increase of Ca2+]i that reached its maximal values in about 10-20 min. However, a detectable increase of Ca2+]i was attained after 2 min of fusion. This would represent a new mechanism to influence sperm Ca2+]i besides ion-exchange systems and ATP-dependent pumps. The value of Ca2+]i remained elevated, unless Na+ was also present in the external medium. Therefore, the mechanism of fusion might influence deeply the physiology of sperm by producing a transient increase of Ca2+]i.