Effect of adrenergic blockade on dynamics of the pregnant primate uterus (Macaca mulatta)

Unanesthetized rhesus monkeys, conditioned to restraining chairs and monitored continuously by chronically implanted sensors, were used to explore the relationships between catecholamines and uterine dynamics during late gestation. Continuous intra-aortic infusion of phentolamine produced a dose-response decrease in uterine activity and blood flow. With alpha-adrenergic blockade the circadian (Fourier) variations of uterine activity were abolished and those of uterine blood flow were reversed. Placental blood flow (by the microsphere technique) was 86.1% of control values. Beta-adrenergic blockade (propranolol) accentuated the circadian variations of uterine dynamics. Vascular resistance to both the myometrium and the placentas was increased. The data indicate that the maternal placental vasculature is the primary site of uterine vascular resistance and is susceptible to adrenergic response. The data also suggest that the extrinsic resistance produced by the myometrium is of major importance in the distribution of uterine blood flow.