Changes in bovine luteal progesterone metabolism in response to exogenous prostaglandin F(2alpha)

An experiment was conducted to determine the effect of prostaglandin F2alpha (PGF2alpha) on luteal synthesis of progesterone (P4) and related progestins. Sixteen beef heifers were assigned in equal numbers to four groups in a 2x2 factorial arrangement of treatments. The experiment consisted of two levels of PGF2alpha analog (0 and 500 microg) and two levels of time (4 and 24 h after injection) of corpus luteum collection. All heifers were injected intravenously with saline (2 ml) or PGF2alpha (cloprostenol) on day 8 of the estrous cycle (estrus=day 0). Jugular blood was collected at 0, 1, 2, 3, 4 and 20, 21, 22, 23, and 24 h after injection. Resulting sera were analyzed for P4 by use of radioimmunoassay. Luteal tissue was analyzed by gas chromatography/mass spectrometry for P4, 20beta-hydroxyprogesterone, pregnenolone, and allopregnanolone (3beta-hydroxy-5alpha-pregnan-20-one). Treatment with PGF2alpha reduced serum concentrations of P4 as early as 1 h after injection (P<0.005) and steroid levels remained low over 24 h. Similarly, administration of PGF2alpha caused a decline in luteal P4 (P<0.005), 20beta-hydroxyprogesterone (P<0.10), and pregnenolone (P<0.05). In contrast, treatment with PGF(2alpha) caused an increase in luteal allopregnanolone over time (time x treatment interaction; P<0.05). These data are interpreted to suggest that PGF2alpha promotes conversion of P4 to the metabolite allopregnanolone.