Uterine motor responses to an alpha-adrenergic agonist (phenylephrine) in the ewe during oestrus and at the end of gestation.

The alpha-adrenergic activity of the myometrium was studied by recording the electromyographic activity (EMG) of the uterus in 4 conscious cyclic ewes during oestrus and in 8 conscious pregnant ewes during the last 6 days of gestation. In the cyclic ewes, changes in intra-uterine pressure were recorded at the same time as the EMG. Motor responses to perfusions of phenylephrine, a specific alpha-adrenergic agonist, were studied at three uterine sites. In the ewes in oestrus, uterine activity was stimulated at the three sites by perfusions with phenylephrine (0.6, 2.5, 5 or 10 micrograms/kg/min). The whole uterine horn was alpha-adrenergic-responsive. The effect of the drug was dose-dependent in that the uterine response increased with the dose and reached a plateau at 5 micrograms/kg/min. In the late pregnant ewes, the motor responses to the perfusion of phenylephrine at 10 micrograms/kg/min were studied at three uterine sites once a day during the last 6 days of gestation. Plasma levels of progesterone and total oestrogens were measured at the same time. Up to 48 h before parturition, the effect of phenylephrine depended on the uterine site. The drug had no or little effect at the tubal end of the horn. Phenylephrine stimulated uterine activity at the cervical end of the horn in at least 50% of the ewes. Thus, during this part of gestation, the alpha-adrenergic sensitivity of the myometrium was greater at the cervical than at the tubal end of the uterine horn. Forty-eight hours before parturition, phenylephrine stimulated uterine activity at all three uterine sites in all the ewes. Thus, alpha-adrenergic sensitivity developed over the uterine horn beginning 48 h before parturition at the same time that the oestrogen level increased.