Changes in the activity of sperm nitric oxide synthase in the oviductal reservoir during ovulation.

The oviductal isthmus is known to act as a sperm reservoir in several mammalian species including mice, but it is still unclear how sperm are released from the reservoir after ovulation. Recently, nitric oxide (NO) was reported to have important roles as a mediator in various sperm functions, including hyperactivation and capacitation. Therefore, we have investigated the change of the activity of nitric oxide synthase (NOS) of sperm of the isthmus in relation to ovulation under fertilization conditions. The sperm were collected from the isthmus and uterus of the female mated before or after ovulation. The NOS activity change was evaluated by using the β-nicotinamide adenine dinucleotide phosphate-diaphorase staining method, and sperm NOS activity was quantified by using NIH image software. The results showed that, in the reservoir, the peak intensity of sperm NOS activity was higher after ovulation (135.5 ± 22.4) than before ovulation (102.7 ± 15.5;  ≤ 0.05). After ovulation, the number of free sperm in the isthmus increased, and these sperm expressed strong NOS activity. The change of sperm NOS activity is related to their release from the epithelium of the oviductal reservoir. (Reprod Med Biol 2003; : 75-81).