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.