Estrous cycle characteristics, luteal function, secretion of oxytocin (OT) and plasma concentrations of 15-keto-13,14-dihydro-PGF2α (PGF2α-metabolite) after administration of low doses of prostaglandin F2α (PGF2α) in pony mares

In the present study, the kinetics of the prostaglandin F2α (PGF2α)-metabolite 15-keto-13,14-dihydro-PGF2α after a single intramuscular application of various doses of the natural PGF2α dinoprost at Day 7 of the cycle in the mare were investigated. Effects of low doses on estrous cycle length and life span of corpus luteum were examined, because release of PGF2α is still under discussion to have detrimental influence on success rates of transcervical transfer of equine embryos. Eight Shetland pony mares were each randomly assigned to each of four treatments: (a) 0.8mg/100kg (group T1), (b) 0.4mg/100kg (group T2), (c) 0.2mg/100kg BM dinoprost i.m. (group T3), and (d) 1ml physiological saline i.m. (group CO). Treatments were administered as single doses on Day 7 of the estrous cycle. Administration of dinoprost caused dose-dependent rises of plasma concentrations of PGF2α-metabolite, although values of individual mares showed great variation within groups. Prostaglandin treatments resulted in a distinct decrease of plasma progesterone concentrations to values between 1.6 and 7.9ng/ml within 24h. Treatment groups had significantly lower progesterone area under the curve (AUC: T1 942.8±175.9, T2 1050±181.2 and T3 1117.9±179.8ng/ml/h) when compared with controls (CO 1601.9±227. 6; t-test, P<0.05). There was a small, but significant negative correlation between AUC of progesterone and of PGF2α-metabolite (R=-0.4; P<0.05). Administration of PGF2α caused secretion of oxytocin in three (T1, T2) and two (T3) mares out of eight ranging from 19.3 to 63.1pg/ml. The AUC of oxytocin was positively correlated with AUC of PGF2α-metabolite (R=0.4, P<0.05) and negatively correlated with AUC of progesterone (R=-0.4, P<0.05). Administration of dinoprost yielded significantly shorter intervals from treatment to estrus and ovulation (values in parentheses), respectively, when compared with controls: T1 3.9±0.7 days (12.1±0.7 days), T2 4.5±0.6 (12.3±0.6), T3 4.9±0.5 (12.3±0.6), and CO 8.9±0.6 days (16.5±0.8 days) (t-test, P<0.01) (Fig. 2). Different doses of PGF2α caused similar effects. Data suggest that progesterone concentrations at applications influence efficacy of treatments more than doses administered, as demonstrated by their high correlation with estrous cycle patterns. It is important to note that differences we achieved are gradual and that all mares responded to treatment by luteolysis and premature estrus, regardless of doses applied.