The response of the unanesthetized sheep fetus to sympathomimetic amines and adrenergic blocking agents

Abstract

The response of the unanesthetized intrauterine sheep fetus to sympathomimetic amines was investigated before and after administration of adrenergic blocking agents. Epinephrine, norepinephrine, and methoxamine elicited a prompt increase in systolic and diastolic blood pressures and a decrease in heart rate which tended to be proportional to the rise in mean blood pressure. Isoproterenol, in contrast, produced a fall in arterial blood pressure and a rise in fetal heart rate. The relative changes in blood pressure and in heart rate following the administration of sympathomimetic amines were unaffected by increasing maturity although there was a continuous increase in fetal blood pressure throughout the last trimester of gestation. Phenoxybenzamine (an alpha-receptor-blocking agent) or propranolol (a beta-receptor-blocking agent) exerted only a minor effect on fetal heart rate and blood pressure and did not alter the oxygenation and acid-base state of the fetus. The response to sympathomimetic amines was obliterated. In contrast to the adult of most species, alpha blockade of the fetal sheep did not lead to epinephrine reversal.