EFFECTS OF DAY OF ESTROUS-CYCLE, TIME OF DAY, LUTEOLYSIS, AND EMBRYO ON UTERINE CONTRACTILITY IN MARES

One-minute continuous ultrasonic scans of longitudinal sections of the uterine body were videotaped, and contractility scores (1 to 5, minim al to maximal contractility) were assigned without knowledge of mare i dentity, day of the estrous cycle or pregnancy status. Contractility w as assessed, and plasma progesterone concentrations were determined fo r each of 3 daily examinations (at 0800, 1600 and 2400 hours) from Day 9 to Day 19 (Day 0 = day of ovulation). For both the nonbred (n=11) a nd pregnant (n=11) mares, there was no effect of hour of scan on the e xtent of uterine contractility. When data for the nonbred mares were n ormalized to the onset of luteolysis (defined for each mare as the fir st greater-than-or-equal-to 25% decrease in plasma progesterone concen trations between successive samples), there was an abrupt increase (P< 0.05) in contractility 24 hours prior to the onset of luteolysis. Cont ractility was also assessed daily in 20 nonbred and 27 pregnant mares from Day 0 to Day 17. For the nonbred mares, a biphasic profile in con tractility occurred during the estrous cycle as indicated by the follo wing significant changes: a decrease between Days 0 and 2, an increase between Days 2 and 4, a plateau between Days 4 and 7, a decrease betw een Days 7 and 11, an increase between Days 11 and 13, and a decrease between Days 14 and 16. For pregnant mares, contractility increased (P <0.05) prior to the late-diestrous increase for nonbred mares. In addi tion, a significant reduction in contractility was detected on Day 5 i n these mares compared with that in the nonbred mares. Contractility i n the uterine body in 7 mares was assessed every 5 minutes after depar ture of the embryonic vesicle from the uterine body. Levels of contrac tility in the uterine body were lower (P<0.05) 55 minutes after the ve sicle had exited the body than less-than-or-equal-to 5 minutes after e xit. Mean contractility scores in the body decreased in a linear fashi on (r2=0.77; P<0.0001) over the 60 minutes following vesicle exit. Con tractility was enhanced in the uterine body when the embryonic vesicle was present and then progressively decreased with time after the vesi cle departed from the uterine body.