Demonstration of postovulatory sphincter action by the isthmus of the rabbit oviduct.

Doughnut-shaped, carbon-impregnated rubber microtransducers calibrated to measure changes in diameter are used to demonstrate the occurrence of a sphincter action in the isthmus of the rabbit oviduct following human chorionic gonadotropin (HCG)-induced ovulation. The postovulatory constricture of the isthmus is temporal and coincides with the period when ova are retained in the oviduct. The mean change in diameter of transducers positioned within the lumen of the isthmus in rabbits treated with HCG are significantly different from mean changes observed in nontreated control rabbits when tested at 8-hour intervals over a 104-hour period. The difference is due to a statistically significant decrease in transducer diameter which occurs between 16 and 56 hours post-HCG injection. Treatment with exogenous estrogen in combination with HCG enhances both the magnitude and duration of this constricture. Significant differences occur between 8 and 104 hours. Pretreatment with progesterone prior to HCG-induced ovulation completely blocks the constricture of the isthmus. Both oviducts exhibit identical responses following HCG-induced ovulation. Sphincter action is not confirmed to a small localized segment of the isthmus but occurs over a wide area.