Nifedipine reveals the existence of two discrete components of the progesterone-induced [Ca2+]i transient in human spermatozoa

The steroid progesterone, an agonist of acrosome reaction, induces a biphasic [Ca(2+)](i)-signal in human sperm comprising an initial transient [Ca(2+)](i) elevation, and a subsequent ramp or plateau. Nifedipine, an inhibitor of voltage-operated Ca(2+) channels, inhibits progesterone-induced acrosome reaction in human sperm, but fluorimetric studies have detected no effect of this compound on the progesterone-induced [Ca(2+)](i) signal. We have used single-cell imaging to study the effects of nifedipine on [Ca(2+)](i) signalling in human sperm. Analysis of mean responses from large numbers of cells showed that treatment with nifedipine reduced the duration but not the amplitude of the progesterone-induced [Ca(2+)](i) transient. In control cells, the latency of the transient peak (maximum fluorescence) fell within the range of 30-105 s. In the presence of nifedipine, very few cells peaked "late" (>60 s after application of progesterone). Analysis of transient responses in control cells revealed characteristic "early" and "late" responses, most cells showing both "early" and "late" transients, whereas "late" transients were rare and smaller in the presence of nifedipine. Sustained responses showed strong association with late transients, and occurrence and amplitude of sustained responses were significantly reduced in nifedipine pretreated cells. These findings are consistent with the occurrence of a discrete, nifedipine-sensitive component of the progesterone-induced [Ca(2+)](i) transient that peaks 1-2 min after exposure to the hormone and is crucial for the induction of the sustained [Ca(2+)](i) signal.