Further characterization of the electromyographic activity of the myometrium and mesometrium in nonpregnant sheep under estrogen supplementation.

The effects of estrogen administration on uterine contractility varies with animal species. In nonpregnant ovariectomized sheep, estrogen administration has been reported either to inhibit, inhibit then stimulate, or only stimulate uterine contractility. The aim of the present study was to determine the effects of prolonged estrogen administration in the electromyographic (EMG) activity recorded from the myometrium and mesometrium in nonpregnant ovariectomized sheep after estrous synchronization by inserting vaginal progesterone sponges 14 days before surgery. Surgery was performed on four ewes under halothane anesthesia. Bilateral oophorectomy was performed, and stainless steel EMG electrodes were sewn to the mesometrium and myometrium in both left and right horns of the uterus. Blood samples were taken at 1000 h from the uterine vein for 13,14-dihydro-15-keto-prostaglandin F2 alpha determination, and from the femoral artery for estradiol determination. Starting on Day 7 after surgery, estradiol 17 beta (50 micrograms/24 h) was infused continuously into the jugular vein. Estrogen administration had a different effect on the EMG activity recorded from myometrium and mesometrium. The myometrial response to estrogen was an increase in the frequency of short EMG events from 19.0 +/- 8.7 to 57.0 +/- 5.0 (p less than 0.05) for events less than 60 sec, and from 2.70 +/- 0.83 to 10.30 +/- 1.36 (p less than 0.05) for events lasting greater than 60 sec but less than less than 180 sec. In contrast, there was no stimulatory effect of estrogen on mesometrial EMG for both types of short events less than 60 sec, and greater than 60 but less than less than 180 sec.(ABSTRACT TRUNCATED AT 250 WORDS)