Failure of Exogenous Progestogens to Block Dexamethasone-Induced Prostaglandin F Release from the Uterus of the Late-Pregnant Cow

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Abstract
The effect of exogenous progesterone or megestrol acetate (MGA) on cervical extensibility and on the concentrations of prostaglandin(PG)F in utero-ovarian plasma and progesterone, estrone sulfate and unconjugated estradiol-17.beta. concentrations in jugular plasma was studied over Days 238-242 in pregnant cows given intrafetal infusions of dexamethasone to induce premature delivery. All 11 cows used in the study were given continuous intrafetal infusions of dexamethasone at a rate of 3.3 mg/day to induce premature labor. Progesterone was infused continuously at a rate of 400 mg/day for 10 days into the jugular vein of 3 of the 11 cows while another 3 were given MGA at a rate of 20 mg/day. In the cows given dexamethasone alone premature parturition was induced in 2.5-4.5 days and all calves were born alive and well. In the progestogen-treated cows, dexamethasone failed to induce premature delivery. The cervix of these cows remained firm and unyielding during the time of the progesterone infusion. Following the withdrawal of exogenous progestogens the cervix became soft and compliant and the cows delivered within 0.5-1.5 days, but only 2 of the 6 calves were alive at birth. Hormone changes in the cows given intrafetal infusions of dexamethasone included a marked rise in utero-ovarian PGF and a concomitant decline in jugular progesterone concentrations. Plasma concentrations of estrone sulfate and estradiol-17.beta. showed a small increase before delivery. An increase in utero-ovarian PGF concentrations was also observed in all of the progestogen-treated cows, although there was considerable individual variation in the timing of the PGF surge. The plasma concentrations of estrone sulfate and unconjugated estradiol-17.beta. levels in the progestogen-treated cows showed the same general trend, with the levels increasing within 1-2 days of starting the dexamethasone infusion and then falling before delivery. Utero-ovarian PGF concentrations were inversely related to plasma progesterone concentrations in both control and the MGA-treated cows over the time of declining plasma progesterone concentrations at luteolysis. A decline in plasma progesterone concentrations apparently is not obligatory for the dexamethasone-induced rise in uterine PGF production, although the exogenous progesterone can block cervical dilation and compliance.