Modification by propylene glycol of ovulation rate in ewes in response to a single injection of FSH

Five experiments were conducted with the objective of developing a method to induce superovulation in ewes with a single i.m. injection of FSH. This was achieved by injection of 10 mg FSH-P in propylene glycol at the same time as luteolysis was induced by cloprostenol on day 13 of the oestrous cycle (day 0 = oestrus). Experiments 1, 2, 3 and 5 were conducted in a single flock of Manchega ewes in Spain during the breeding season. Ovulation rates were determined at laparoscopy. In Expt 1, FSH-P was diluted in saline, and neither 5 mg FSH on day 1 nor 5 or 10 mg FSH-P on day 13 changed the ovulation rate after cloprostenol treatment on day 13. In Expt 2, FSH-P was diluted in propylene glycol and data were collected over 2 years. Ten milligrams FSH-P, on day 13 only, increased (P < 0.01) the mean number of corpora lutea to 5.5 compared with a control value of 1.5. Five milligrams FSH-P on day 13 only had no effect; however, 5 mg FSH-P on day 1 reduced the mean number of corpora lutea formed in ewes receiving 10 mg FSH-P on day 13 to 2.6 (P < 0.01). Saline and propylene glycol, as vehicles for 10 mg FSH-P, were compared directly at two times of injection in Expt 3. FSH-P increased the mean number of corpora lutea when injected on day 13 in propylene glycol (4.7) but not in saline (2.5; P < 0.5). Ovulation rate did not differ between diluents when FSH-P was injected 24 h before cloprostenol (1.3; day 12). Experiments 4 and 5 were conducted to examine possible mechanisms by which propylene glycol improved the response to FSH. In Expt 4, conducted during anoestrus in a crossbred flock in West Virginia, concentrations of FSH in plasma were measured for 58 h after injection of 10 mg FSH-P in saline or propylene glycol. Propylene glycol did not delay the time of maximum concentration of FSH in plasma after i.m. injection (2.7 \m=+-\0.7 h) when compared with saline injection (3.6 \m=+-\0.5 h). Maximum concentration was reached later when FSH-P was injected s.c. in propylene glycol (7.6 \m=+-\0.7 h; P < 0.05). In Expt 5, ovulation rate was greater (P < 0.05) in ewes treated with 10 mg FSH-P in propylene glycol than in ewes treated with FSH-P in saline and an injection of propylene glycol at a separate site. The number of corpora lutea did not differ in ewes treated with FSH-P in saline and in ewes treated with FSH-P in saline and propylene glycol at a separate site. Thus neither delayed absorption nor an augmentation effect could account for the benefit of propylene glycol as a vehicle for delivery of FSH to superovulate ewes.