Influence of low plasma progesterone concentrations on the release of prostaglandin F-2 alpha during luteolysis and oestrus in heifers

A study was conducted to determine the effect of suprabasal plasma concentrations of progesterone on the release of prostaglandin F-2 alpha (PGF(2 alpha)) at luteolysis and oestrus, Heifers received silicone implants containing 2.5 (n = 4), 5 (n = 4), 6 (n = 3), 7.5 (n = 3), 10 (n = 4), or 15 (n = 3) g of progesterone, or an empty implant (controls, n = 4) between Days 8 and 25 post ovulation, Blood was collected frequently between Days 14 and 28 and assayed for progesterone and 15-ketodihydroprostaglandin F-2 alpha. Basal progesterone concentrations in control heifers did not differ from those in heifers with 2.5 - or 5 - g implants and remained around 0.4 - 0.5 nmol l(-1) until ovulation in all three groups. In the heifers treated with 6 - 15 g of progesterone, basal concentrations were maintained at higher (P < 0.05) levels compared with those in the controls, ranging from 0.8 to 1.6 nmol 1-'. The effect of these elevated progesterone levels was to delay ovulation by prolonging the growth of the ovulatory follicle, which continued growing until the implant was removed. In all experimental groups, the first significant increase of the PGF,, metabolite occurred between Days 15.3 and 16.3 (P > 0.05) and was associated with the onset of a decrease in progesterone concentrations, which had reached levels below 3 nmol l(-1) by Days 17.4 - 19.1. PGF(2 alpha) metabolite peaks associated with luteolysis were frequent until Day 20. In the period from Day 20 until implant removal, sporadic peaks were observed, ranging in number from 1.0 +/- 1.2 (mean +/- SEM) in the control group to 3.0 +/- 1.4 peaks in the heifers treated with 7.5 g of progesterone (P > 0.05). The number of PGF(2 alpha) metabolite peaks during that period was higher (P < 0.05) in heifers treated with 10 and 15 g than in controls. A positive correlation was found between the basal concentration of progesterone and the number of PGF(2 alpha) peaks after luteolysis (r = 0.54; P < 0.01). Plasma progesterone concentrations above approximately 1.4 nmol l(-1) were able to maintain the release of PGF(2 alpha) until the progesterone implants were removed and plasma levels decreased to basal values. These heifers had a preovulatory PGF(2 alpha) release pattern resembling that found in repeat breeder heifers.