Stimulation of human spermatozoa with progesterone gradients to simulate approach to the oocyte -: Induction of [Ca2+]i oscillations and cyclical transitions in flagellar beating

Progesterone is present at micromolar concentrations in the cumulus matrix, which surrounds mammalian oocytes. Exposure of human spermatozoa to a concentration gradient of progesterone (0-3 muM) to simulate approach to the oocyte induced a slowly developing increase in [Ca2+](i) upon which, in many cells, slow oscillations were superimposed. [Ca2+](i) oscillations often started at very low progesterone (<10 nM), and their frequency did not change during the subsequent rise in concentration. Oscillations also occurred, but in a much smaller proportion of cells, in response to stepped application of progesterone (3 mu M). When progesterone was removed, [Ca2+](i) oscillations often persisted or quickly resumed. Superfusion with low-Ca2+ bathing medium (no added Ca2+) did not prevent [Ca2+](i) oscillations, but they could be abolished by addition of EGTA or La3+. Inhibitors of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases or inositol trisphosphate signaling had no effect on [Ca2+](i) oscillations, but pharmacological manipulation of ryanodine receptors affected both their frequency and amplitude. Staining of live spermatozoa with BODIPY FL-X ryanodine showed localization of ryanodine binding primarily to the caudal part of the head and mid-piece. [Ca2+](i) oscillations did not induce acrosome reaction, but in cells generating oscillations, the flagellar beat mode alternated in synchrony with the oscillation cycle. Flagellar bending and lateral movement of the sperm head during [Ca2+](i) peaks were markedly increased compared with during [Ca2+](i) troughs. This alternating pattern of activity is likely to facilitate zona penetration. These observations show that progesterone initiates unusual and complex store-mediated [Ca2+](i) signaling in human spermatozoa and identify a previously unrecognized effect of progesterone in regulating sperm behavior during fertilization.