Drugs that block smooth muscle contraction inhibit in vivo ovulation in hamsters

We have demonstrated previously that smooth muscle cells (SMC) in the base of hamster follicles contract minutes before ovulation. The contraction of these cells correlates well in time with the development of a constriction in the base of the follicle. The purpose of this study was to determine whether contraction of SMC (1) produces this constriction and (2) is necessary for ovulation. We treated preovulatory hamster ovaries with six classes of drugs (lanthanides, calcium antagonists, local anesthetics, prostaglandins, cAMP modulators, and cytochalasin B) known to inhibit SMC contraction in other tissues. Drugs were injected into the experimental bursal cavity of a hamster 3 hours before ovulation; the contralateral ovary received no treatment and served as a control. Three hours after ovulation, the number of ruptured follicles on experimental and control ovaries were compared by light and electron microscopy. All the smooth muscle (SM) inhibitors, except those that affect cAMP levels and cytochalasin B, prevented (1) contraction of follicular SM, (2) constriction of the base of the follicle, and (3) ovulation. These results support the idea that contraction of follicular SMC constricts the follicle and is required for ovulation. The results further show that follicular SMC are activated by an influx of extracellular calcium and that prostaglandin F2 alpha may be involved in promoting this contraction.