INFLUENCE OF LOW PLASMA PROGESTERONE CONCENTRATIONS ON THE RELEASE OFPROSTAGLANDIN-F2-ALPHA DURING LUTEOLYSIS AND ESTRUS IN HEIFERS

A study was conducted to determine the effect of suprabasal plasma con centrations of progesterone on the release of prostaglandin F-2 alpha (PGF(2 alpha)) at luteolysis and oestrus, Heifers received silicone im plants 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 fre quently between Days 14 and 28 and assayed for progesterone and 15-ket odihydroprostaglandin 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 i n all three groups. In the heifers treated with 6 - 15 g of progestero ne, basal concentrations were maintained at higher (P < 0.05) levels c ompared with those in the controls, ranging from 0.8 to 1.6 nmol 1-'. The effect of these elevated progesterone levels was to delay ovulatio n 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 D ays 15.3 and 16.3 (P > 0.05) and was associated with the onset of a de crease in progesterone concentrations, which had reached levels below 3 nmol l(-1) by Days 17.4 - 19.1. PGF(2 alpha) metabolite peaks associ ated with luteolysis were frequent until Day 20. In the period from Da y 20 until implant removal, sporadic peaks were observed, ranging in n umber 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 h igher (P < 0.05) in heifers treated with 10 and 15 g than in controls. A positive correlation was found between the basal concentration of p rogesterone and the number of PGF(2 alpha) peaks after luteolysis (r = 0.54; P < 0.01). Plasma progesterone concentrations above approximate ly 1.4 nmol l(-1) were able to maintain the release of PGF(2 alpha) un til the progesterone implants were removed and plasma levels decreased to basal values. These heifers had a preovulatory PGF(2 alpha) releas e pattern resembling that found in repeat breeder heifers.