Pregnant ewes were injected with either the antiprogesterone,
RU 486 (4 mg kg-1
body weight, i.m.; n = 5), 3000 iu
relaxin (i.m.; n = 9), or diluent (n = 8) at 12:00 h on days 144 and 145, to determine its effect on
progesterone and
relaxin secretion, and on induction of lambing.
RU 486 induced earlier lambing (P < 0.01) compared with diluent treatment, but
relaxin treatment did not significantly reduce the interval to parturition. Mean injection-lambing intervals were 31 +/- 2, 109 +/- 23 and 121 +/- 27 h for the
RU 486,
relaxin and diluent groups, respectively. There was no incidence of difficult birth (
dystocia); all lambs were vigorous at birth; and placenta delivery was rapid (within 207 min) with
RU 486 and
relaxin treatments compared with diluent treated controls. Plasma
progesterone concentrations averaged 11 ng ml-1 during the pretreatment period for all animals.
RU 486 had a biphasic effect on
progesterone concentrations, causing an initial increase (P < 0.05) within 2 h, and then an abrupt drop (P < 0.01) to 6 ng ml-1 by 18:00 h on day 145.
Progesterone concentrations remained consistently lower (P < 0.05) in
relaxin-treated ewes than in diluent-treated controls from days 144 to 147 and then began a steady decrease to 4 ng ml-1 on the day of parturition (days 149 and 150) in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)