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Summary. This study examined the effects of progesterone and intrauterine injection of ovine conceptus secretory proteins (oCSP) on endometrial responsiveness to oxytocin. Twelve ewes were ovariectomized on day 4 of the cycle (oestrus = day 0) and assigned in a 2 x 2 factorial arrangement, to receive either 1·5 mg ovine serum proteins (SP) or oCSP containing 25 µg ovine trophoblast protein 1 (oTP-1) (by radioimmunoassay) in 1·5 mg total protein into each uterine horn, via catheters, twice a day on days 11, 12, 13 and 14. Ewes received 200 mg progesterone per day (i.m.) from day 4 to day 10 or 15. Oxytocin-induced prostaglandin F2
was measured as 13,14-dihydro-15-keto-prostaglandin F2 (PGFM) on days 11, 12, 13 and 14 in plasma from three integrated, 10 min (10 ml) blood samples (0–10, 10–20, 20–30 min) obtained after intravenous injection of 20 iu oxytocin, and in a pre-oxytocin (– 10 to 0 min) sample collected via an indwelling jugular catheter. The pre-oxytocin samples were also assayed for progesterone. Oxytocin-induced turnover of inositol phosphate was determined in endometrium on day 15 after hysterectomy. In ewes receiving progesterone to day 10, plasma progesterone decreased from about 12 to 2 ng ml–1 (SEM ± 2·6) during the treatment period (days 11–14), but remained high (12–20 ± 2·6 ng ml–1) in ewes that received progesterone to day 15. Intrauterine injection of oCSP resulted in high basal concentrations of PGFM on days 12 and 13 compared with SP-treated ewes (P < 0·01). Treatments with progesterone did not affect basal PGFM concentrations. Treatment with oCSP abolished oxytocin-induced endometrial secretion of prostaglandin only if progesterone was maintained to day 15 (P < 0·01); in ewes receiving such treatment, oCSP inhibited (P < 0·01), but SP did not inhibit, oxytocin-induced endometrial turnover of inositol phosphate (P < 0·06), which was greater in ewes treated with progesterone to day 10 than in those treated to day 15 (P < 0·05). Ewes that responded to oxytocin with increased PGFM exhibited increased oxytocin-stimulated turnover of inositol phosphate on day 15. These results indicate that the antiluteolytic action oTP-1 exerts on the endometrium requires progesterone and that this mechanism involves inhibition of oxytocin-stimulated turnover of inositol phosphate.
Keywords: sheep; conceptus; prostaglandin F2; progesterone; oxytocin
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