Effect of dose and day of treatment on uterine response to oxytocin in mares

To determine the effect of dose and day of oxytocin treatment on intrauteri ne pressure, 6 normal mares were treated with 10 or 25 IU oxytocin 2 days b efore ovulation, on the day of ovulation and 2 days after ovulation. Intrau terine pressure (IUP) was measured using micro-tip-catheters (one placed in trauterine, a second and third serving as reference sensors in the vagina a nd external to the mare) and transmitted by telemetry for 30 min to establi sh a baseline before saline was administered, iv, and for an additional 30 min after saline administration. Oxytocin was then given, iv, and IUP was r ecorded for 60 min. No change in IUP was observed after saline injection. T he administration of both 10 (n=16) and 25 (n=10) IU oxytocin induced a res ponse (P<0.01). The intensity of response depended on the day of administra tion (P<0.01) and the dose of oxytocin (P<0.001). The variation of response was significantly greater after 10 IU oxytocin (CV 15.78%) compared with 2 5 IU oxytocin (CV 6.42%). The uterine response was greatest on Day 2 prior to ovulation and lowest on Day 2 after ovulation. The response was negative ly correlated to increasing plasma progesterone (10 IU oxytocin: r = -0.435 , 25 IU oxytocin: r = -0.265). There was no correlation between the uterine response and plasma estradiol-17 beta concentration (P<0.01). in conclusio n the results of this study show that oxytocin administration to mares befo re ovulation provides a greater response than after ovulation. A decline in the intensity of response after ovulation can be compensated for with a hi gher dose of oxytocin. Furthermore, the use of the multiple catheter techni que is an effective method for assessing changes in uterine pressure. (C) 2 000 by Elsevier Science Inc.