Rho kinase and RND3 regulate the direct effect of estradiol-17β on oviductal tonus

Ensuring the timely transport of gametes and embryos within the oviduct is essential for the successful establishment of pregnancy. This study investigated the direct effect of estradiol-17β (E2) on bovine oviductal contractility and the differences in responsiveness to E2 during the estrous cycle. Bovine isthmic tissues from four estrous stages were analyzed using the Magnus method to assess contractile responses to E2 and related reagents. Protein expression of G-protein-coupled estrogen receptor 1 (GPER1) and components of the RhoA/Rho kinase (ROCK) signaling pathway were also evaluated. E2 and a GPER1 agonist significantly increased oviductal tonus at 1–4 days after ovulation. This effect was significantly suppressed by treatment with a GPER1 antagonist and a ROCK inhibitor. At 1–4 days after ovulation, both ROCK II expression and ROCK activity were elevated. E2 also enhanced phosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) and myosin light chain (MLC), key downstream targets of ROCK. Before ovulation, when endogenous E2 levels peak, the expression of RND3—a ROCK inhibitor—was upregulated. The application of an RNDs inhibitor restored E2 responsiveness in oviductal tonus, ROCK activity, and the phosphorylation of MYPT1 and MLC in oviductal tissues before ovulation. These findings suggest that E2 directly increases oviductal tonus via GPER1 and ROCK/MYPT1/MLC activation at 1–4 days after ovulation. Differences in oviductal responsiveness to E2 during the estrous cycle appear to be mediated by the expression of ROCK and RND3. This mechanism can enable sperm transport within the oviduct at an appropriate time.