Rises in intracellular Ca2 concentration ([Ca2 ]i) caused by progesterone, an inducer of the acrosome reaction, or by cyclic nucleotides, possible second messengers, were investigated by Ca2 imaging of the head of individual mouse sperm. Progesterone induced a [Ca2 ]i rise in a dose-dependent manner (4–40 μM), primarily in the postacrosomal region. For 20-μM progesterone, Ca2 responses occurred in 42% of sperm, separated into two types: transient type (60% of responding cells; duration, 1–1.5 min; mean amplitude, 335 nM) and prolonged type (40%; >3 min; 730 nM). Prolonged responses required higher doses of progesterone, and their occurrence was enhanced significantly by preincubation for 2–4 h as compared with transient responses. 8-Bromo-cGMP (0.3–3 mM) induced a [Ca2 ]i rise more effectively than did 8-bromo-cAMP. For 1-mM 8-bromo-cGMP, 90% of cells exhibited transient Ca2 responses (∼1 min; 220 nM), independently of the preincubation time. In Ca2 -free medium, most sperm showed no Ca2 response to progesterone and 8-bromo-cGMP. Pimozide, a Ca2 channel blocker, completely blocked prolonged responses and partially inhibited transient responses. These results suggest that progesterone activates at least two distinct Ca2 influx pathways, with fast or slow inactivation kinetics, and some sperm show both types of response. A cyclic nucleotide-mediated process could participate in the progesterone-induced [Ca2 ]i rise.
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