Luteolysis after the intravulvosubmucosal injection of prostaglandin F2α in cattle: Systemic or local mechanism?

The intravulvosubmucosal (IVSM) route has been used to reduce the dose of prostaglandin F2α (PGF2α), a luteolytic agent used in estrus synchronization programs. To validate the effectiveness of PGF2α administered via IVSM, the estrus rate was monitored in 1937 beef cows for 5 days (25.6% of estrus). The cows that did not show signs of estrus by day 5 received 5 mg of dinoprost IVSM (1/5 of the standard dose; n=1440). By day 10, 68.2% of the cows were in estrus. In a second trial, using the same synchronization protocol, the proportion of heifers in estrus after receiving 5 mg dinoprost at day 5 via IVSM (47.4%; n=97) or via an intramuscular injection (IM; 54.7%; n=95) did not differ (P>0.05). The occurrence of luteolysis (serum progesterone concentrations below 1 ng/mL) was dependent on the period of the estrous cycle in which the cows were treated with 5 mg of dinoprost via IM or IVSM. Luteolysis was observed in 5 out of 10 cows treated at day 10 but was not observed in any cows treated at day 5 of the estrous cycle. Luteolysis occurred in all animals treated with 25 mg dinoprost independent of the estrous cycle period (day 5 or 10). After IVSM injection, the PGF2α concentration did not differ in the uterine and jugular veins. This was further confirmed by measuring the concentration of 13,14-dihydro-15-keto-PGF2α (PGFM) after the injection of 5 mg of dinoprost via the IM or IVSM route. Dinoprost IM and IVSM injections resulted in a similar PGFM serum pattern over time, indicating the same rate of absorption into the systemic circulation. Taking the results together, we concluded that PGF2α IVSM injections reached the systemic circulation before reaching the ovary, and the effectiveness of low doses of PGF2α was dependent on the period of the estrous cycle and not on the route of administration.