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 f rom 1 (flaccid) to 6 (turgid) by an independent operator who had no kn owledge of reproductive status, day, or group. The uterine-tone profil e 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 occu rred 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 earl y pregnancy (Days 11 to 35) was studied. Three groups were used: 1) no nbred mates with luteolysis induced by PGF(2 alpha) on Day 11 followed by daily treatment with 100 mg of progesterone (n=6); 2) pregnant mar es 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 da y and the group-by-day interaction were significant (P<0.0001). The in teraction resulted primarily from lower mean tone in the nonbred proge sterone-treated mates during Days 11 to 29 than in the 2 pregnant grou ps. Uterine tone scores on Days 30 to 35 were not different (P>0.05) a mong the 3 groups. Results indicated the following: 1) progesterone wa s the only factor produced by the corpus luteum that was needed for th e development of uterine turgidity and 2) continued exposure of nonbre d mares to progesterone caused uterine turgidity equivalent to that in pregnant mares by approximately Day 30. Apparently, continued exposur e of the uterus to luteal progesterone in pregnant mates accounts for the eventual attainment of maximal uterine turgidity, exceeding the re ported transient increase in tone resulting from local stimulation by the conceptus during the mobility phase.