Quantitative control of oxytocin-induced PGF2 alpha release by progesterone and oestradiol in ewes.

Abstract

The effect of oestradiol and progesterone concentrations on the uterine PGF2 alpha response to oxytocin was investigated by measuring 13,14-dihydro-15-keto PGF2 alpha (PGFM) secretion. One week after ovariectomy, 27 ewes were administered progestagen for 10 days followed by oestradiol for 2 days. Day 0 was designated as the time of the last 'oestrous' oestradiol injection. Six groups of ewes (n = 4) were then treated for 12 days with a high or low dose of progesterone (36 or 12 mg day-1) either alone or with a high or low dose of oestradiol (36 or 12 micrograms day-1) administered (in 1 ml of corn oil by i.m. injection, at intervals of 8 h) in a pattern designed to simulate a natural oestrous cycle profile. A control group (n = 3) was given corn oil alone. Ewes were treated with 1 microgram oxytocin (i.v.) on days 4, 8 and 12 of the simulated cycle and plasma was collected for assay of PGFM. An oxytocin-induced PGFM response occurred only on day 12, when the response was suppressed by high doses of progesterone and stimulated by high oestradiol doses. There was a significant effect of progesterone (P < 0.05) and a highly significant effect of oestradiol (P < 0.01) on the pattern of PGFM release in response to oxytocin. Low progesterone/high oestradiol stimulated the largest and most sustained increase in PGFM following oxytocin. There was a significant relationship between the oestradiol:progesterone ratio and the mean PGFM response on day 12 (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)