Plasma concentrations of progesterone in ovariectomized and prepubertal heifers following intravaginal and intramuscular administration of progesterone

Abstract

The peripheral plasma concentrations of progesterone resulting from the intravaginal administration of progesterone (PRID, Ceva, Australia) were observed in prepubertal and ovariectomized heifers. Prior to intravaginal treatment, six of 12 ovariectomized heifers were given an intramuscular injection of 400 μg oestradiol benzoate, followed 3 days later by a series of ten daily intramuscular injections, each of 40 mg progesterone. Peak concentrations of progesterone of about 2 ng/ml plasma were observed 2 h after the first injection but by 12 h progesterone was undetectable in the plasma of most heifers. Continued intramuscular treatment resulted in a non-significant increase in peak plasma concentrations and tended to prolong the period of high plasma concentrations. Treatment of ovariectomized heifers with intramuscular oestradiol benzoate and progesterone prior to PRID insertion increased plasma concentrations of progesterone for the first 2–3 days after insertion. Thereafter there was no effect of previous treatment on plasma progesterone concentrations. In ovariectomized heifers, progesterone concentrations of 6–8 ng/ml plasma were maintained for the first 6–7 days of PRID insertion after which concentrations declined rapidly to about 2 ng/ml whereas in prepubertal heifers concentrations of 6–8 ng/ml were maintained for 14 days or longer. It is suggested that previous experience of ovarian steroid hormones can exert both long and short term effects on the concentrations of peripheral plasma progesterone which result from treatment with exogenous sources of progesterone.