Increase of intracellular free [Ca2+] in single human motile spermatozoa treated with human follicular fluid.

The intracellular free [Ca2+] concentration ([Ca2+]i) in individual human sperm was measured using a fluorescent Ca2+ indicator. In 18 of 23 motile sperm (78.3%), [Ca2+]i increased significantly and promptly after addition of 20% human follicular fluid (hFF), but in the others it did not increase. The mean resting [Ca2+]i level of sperm in which [Ca2+]i increased after addition of 20% hFF (the influx group) was significantly lower than those in which it did not increase (112.8 +/- 40.1 nM vs. 156.9 +/- 13.5 nM, p < .05). After addition of 20% hFF, the mean [Ca2+]i in the influx group reached a peak value of 210.7 +/- 24.7 nM within 30 s and then decreased slowly; the mean [Ca2+]i values 1, 5, 10, and 15 min after addition of 20% hFF were 179.3 +/- 31.4, 174.3 +/- 30.2, 172.5 +/- 27.8, and 175.1 +/- 27.2 nM, and all values were significantly higher than the resting level (p < .01). The frequency distribution of [Ca2+]i after addition of 20% hFF was shifted toward higher concentrations (p < .01). However, the addition of 20% hFF did not increase the percentage of live acrosome reaction (before 3.8 +/- 0.9% vs. after 2.9 +/- 0.5%, respectively). Thus, hFF increased [Ca2+]i in about 80% of the motile sperm. Relatively high [Ca2+]i levels persisted for at least 10-15 min after its addition. However, hFF did not trigger a rapid response in acrosome reaction.