EFFECT OF OXYTOCIN, PROSTAGLANDIN-F2-ALPHA, AND CLENBUTEROL ON UTERINE DYNAMICS IN MARES

The effects of oxytocin, prostaglandin F2 alpha (PGF2 alpha), and clen buterol on uterine contractility and tone during anestrus and diestrus , and during mobility and postfixation of the embryonic vesicle were s tudied in 51 pony mares. Contractility was assessed by scoring real-ti me ultrasound images, and tone was assessed by transrectal digital com pression. Scoring was done by an operator who had no knowledge of trea tment assigments. In anovulatory mares primed with progesterone for 16 d, oxytocin did not significantly alter contractility but did stimula te an increase in tone, whereas clenbuterol depressed both contractili ty and tone. The PGF2 alpha given on Days 12, 15, and 18 did not signi ficantly alter uterine contractility in pregnant mares, but it increas ed contractility on all days in nonpregnant mares. Clenbuterol decreas ed both tone and contractility when given to pregnant mares on the day of embryonic-vesicle fixation, while it decreased tone but not contra ctility when given on Day 19. Clenbuterol treatment was associated wit h dislodgment of the fixed embryo in only 1 of 5 mares. However, on Da y 19, clenbuterol treatment was associated with a change in shape of t he conceptus when viewed in a cross section of the uterine hem. The co nceptus shape became more circular rather than irregular or triangular , as indicated by a significant decrease in the variation in the dista nces between adjacent walls measured in 4 different directions. Result s 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 m ares but not in pregnant mares; 3) clenbuterol decreased both tone and contractility at all reproductive states except for a lack of a decre ase in contractility on Day 19 of pregnancy; and 4) reduction in uteri ne tone from clenbuterol treatment on Day 19 was associated with a cha nge in the two-dimensional shape of the in situ conceptus from irregul ar to a more circular form. (C) 1998 by Elsevier Science Inc.