Modulation of spontaneous myometrial activity in chronically instrumented ovariectomized sheep

The uterine electromyogram (EMG) and intrauterine pressure curves (IUP) were investigated as indicators of myometrial activity in chronically instrumented, ovariectomized ewes. Spontaneous electrical activity was characterized by rhythmic patterns of trains of bursts accompanied by IUP waves. Administration of adrenergic (propranolol or phentolamine) or cholinergic (atropine) blocking agents had no effect on spontaneous uterine activity. Both oxytocin and PGF 2α appeared to stimulate spontaneous myometrial activity. 17β-Estradiol temporarily depressed uterine activity in a dose-dependent fashion. The period of relaxation was followed by a pronounced increase in activity. Progesterone treatment resulted in long-term suppression of myometrial activity. Oxytocin and PGF 2α increased EMG and IUP activity during estradiol suppression but not after progesterone treatment. These results indicate that (1) the myometrium is active in chronically instrumented, ovariectomized ewes, (2) the autonomic nervous system or its receptors do not play a role in the maintenance of spontaneous myometrial activity, (3) estradiol and progesterone suppress myometrical activity but by different mechanisms.