Effect of oxytocin, prostaglandin F2 alpha, and clenbuterol on uterine dynamics in mares

The effects of oxytocin, prostaglandin F2 alpha (PGF2 alpha), and clenbuterol on uterine contractility and tone during anestrus and diestrus, and during mobility and postfixation of the embryonic vesicle were studied in 51 pony mares. Contractility was assessed by scoring real-time ultrasound images, and tone was assessed by transrectal digital compression. Scoring was done by an operator who had no knowledge of treatment assignments. In anovulatory mares primed with progesterone for 16 d, oxytocin did not significantly alter contractility but did stimulate an increase in tone, whereas clenbuterol depressed both contractility and tone. The PGF2 alpha given on Days 12, 15, and 18 did not significantly alter uterine contractility in pregnant mares, but it increased contractility on all days in nonpregnant mares. Clenbuterol decreased both tone and contractility when given to pregnant mares on the day of embryonic-vesicle fixation, while it decreased tone but not contractility when given on Day 19. Clenbuterol treatment was associated with dislodgment of the fixed embryo in only 1 of 5 mares. However, on Day 19, clenbuterol treatment was associated with a change in shape of the conceptus when viewed in a cross section of the uterine horn. The conceptus shape became more circular rather than irregular or triangular, as indicated by a significant decrease in the variation in the distances between adjacent walls measured in 4 different directions. Results indicated that: 1) oxytocin increased uterine tone but did not alter contractility in progesterone-primed anestrous mares; 2) on Days 12, 15 and 18, PGF2 alpha increased uterine contractility in nonpregnant mares but not in pregnant mares; 3) clenbuterol decreased both tone and contractility at all reproductive states except for a lack of a decrease in contractility on Day 19 of pregnancy; and 4) reduction in uterine tone from clenbuterol treatment on Day 19 was associated with a change in the two-dimensional shape of the in situ conceptus from irregular to a more circular form.