DEVELOPMENT OF UTERINE TONE IN NONBRED AND PREGNANT MARES

Uterine tone was monitored daily in nonbred (n=6) and pregnant mares (n=7) on Days 0 to 20. Uterine tone was assessed digitally and scored from 1 (flaccid) to 6 (turgid) by an independent operator who had no knowledge of reproductive status, day, or group. The uterine-tone profile was bimodal in nonbred mates with nadirs (P<0.05) on Days 0 and 10 (P<0.05) and peaks (P<0.05) on Days 6 and 13. A divergence in tone occurred after Day 13 with decreasing tone in nonbred mares and increasing tone in pregnant mares. In another experiment, the role of the embryo and corpus luteum in the stimulation of uterine turgidity during early pregnancy (Days 11 to 35) was studied. Three groups were used: 1) nonbred mates with luteolysis induced by PGF(2 alpha) on Day 11 followed by daily treatment with 100 mg of progesterone (n=6); 2) pregnant mares with induced luteolysis and daily replacement of progesterone (n=5 to 6); and 3) control pregnant mates (n=4 to 6). The main effect of day and the group-by-day interaction were significant (P<0.0001). The interaction resulted primarily from lower mean tone in the nonbred progesterone-treated mates during Days 11 to 29 than in the 2 pregnant groups. Uterine tone scores on Days 30 to 35 were not different (P>0.05) among the 3 groups. Results indicated the following: 1) progesterone was the only factor produced by the corpus luteum that was needed for the development of uterine turgidity and 2) continued exposure of nonbred mares to progesterone caused uterine turgidity equivalent to that in pregnant mares by approximately Day 30. Apparently, continued exposure of the uterus to luteal progesterone in pregnant mates accounts for the eventual attainment of maximal uterine turgidity, exceeding the reported transient increase in tone resulting from local stimulation by the conceptus during the mobility phase.