Contractile Response of the Uterus of the Estrous Ewe to Adrenergic Stimulation

Abstract

Anesthetized estrous ewes were infused with varying doses of norepinephrine, epinephrine, isoproterenol and phenylephrine in 3 experiments to evaluate the adrenergic response of the ovine uterus at estrus. Contractions in anterior and posterior segments of the uterus were studied by open-ended catheters attached to pressure transducers. During their infusion, norepinephrine, epinephrine and phenylephrine induced contractions that could be blocked by the .alpha.-adrenergic receptor blocker, phenoxybenzamine. During infusion, the .beta. adrenergic agonist, isoproterenol, relaxed the uterus. This effect could be blocked by the .beta.-adrenergic receptor blocker, propranolol. Anterior segments of the uterus had stronger contractions, but the duration of contractions was greater in posterior segments of the uterus. In the postinfusion interval, the uterus relaxed if norepinephrine or epinephrine had been infused. This relaxation was blocked by phenoxybenzamine, but not by propranolol and may result from the accumulation of some inhibitor though nonmyometrial changes, such as in the vascular system, could be involved. The frequency of contractions increased in response to both norepinephrine and epinephrine in both early and late estrus. Relaxation in the postinfusion interval increased in posterior segments from early to late estrus for norepinephrine and decreased for epinephrine. Longer relaxation intervals for posterior segments of the uterus at late estrus, if they occur in vivo, may afford anterior segments of the uterus to dominate in originating contractions.