Abstract
Clenbuterol was used to inhibit myometrial contractions in mares ( n=10) on Days 67–84 of pregnancy (ovulation = Day 0). Transrectal ultrasonography was used at 5-min intervals for 1 h to monitor the diameter of the allantoic fluid compartment and fetal intrauterine location (umbilical cord horn, non-cord horn, uterine body), presentation, and recumbency. The percentage of ultrasound scans in which the cornual segments were fully constricted (no detectable allantoic fluid) was less ( P<0.0001) after clenbuterol treatment (2.2±1.1%) than before treatment (14.0±2.6%). The diameter of the allantoic fluid compartment was greater ( P<0.0001) after treatment (45.8±2.7 mm) than before treatment (36.8±2.7 mm). There were no differences between pre- vs. post-treatment examinations in the mean number of changes per hour in fetal location, presentation or recumbency. Transcervical videoendoscopy was used on Days 74–84 to monitor fetal activity and mobility and diameter of the allantoic fluid compartment at the cornual entrances in two groups (fetus treated with succinylcholine, n=5; controls, n=6). The mean number of fetal changes per 30 min was reduced ( P<0.05) in the treated group for location (0.6±0.4 vs. 2.8±0.7), presentation (1.0±0.5 vs. 5.3±1.3), and recumbency (1.0±0.6 vs. 13.6±0.7). There was no difference between groups in the number of changes in diameter of the cornual entrances. Results indicated that during the equine early fetal stage: (1) the uterus is extremely active, resulting in pronounced and dynamic shifts in allantoic fluid distribution; (2) fetal mobility and changes in fetal presentation and recumbency are dependent primarily on vigorous movements by the fetus rather than on uterine contractions; (3) allantoic fluid shifts and fetal activity and positional changes are primarily independent phenomena.
Published Version
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