Progesterone-induced increase of sperm cytosolic calcium is enhanced by previous fusion of spermatozoa to prostasomes

Ejaculated spermatozoa must undergo a number of modifications before becoming able to fertilize the oocyte. The interaction of sperm with other semen components may influence these phenomena; human semen contains vesicles of prostatic origin, called prostasomes that may fuse to sperm at slightly acidic to neutral pH values.Prostasomes contain calcium and it has been demonstrated that their fusion with spermatozoa produces a transient increase (wave) of Ca(2+)](i) in these cells. The fusion process also transfers protein and lipid to spermatozoa. These phenomena may induce long-lasting changes of sperm properties. We test the hypothesis that spermatozoa, as modified by fusion, change their ability to undergo the progesterone-induced increase of Ca(2+)](i) and we find that the increase of Ca(2+)](i) produced by the fusion with prostasomes and by the stimulation with progesterone are independent and additive phenomena. We also find that spermatozoa present a stronger response to the progesterone-induced increase of Ca(2+)](i) if they are previously made to fuse with prostasomes. This effect does not depend directly on the Ca(2+)](i) increase due to fusion, since it is still present after the Ca(2+)](i) has returned to resting values.