Effect of phentolamine methanesulfonate upon uterine contractility induced by l-norepinephrine pregnancy

Twelve women with normal pregnancies at term were studied by means of continuous records of amniotic fluid pressure during advanced prelabor. Patients received continuous infusions of l-norepinephrine at two different concentrations, 0.05 μg per kilogram per minute and 0.08 μg per kilogram per minute. Phentolamine methanesulfonate was administered in a single intravenous injection of 5 mg. followed by a continuous infusion of the same drug. Uterine activity increased when norepinephrine was administered in any dose. When phentolamine was infused before norepinephrine, uterine activity also increased, but the increments were less marked than in the preceding group. When phentolamine was administered after norepinephrine effects were established, previous increments in uterine activity diminished. The effects of phentolamine, an alpha blocking agent, suggest that the action of norepinephrine on uterine contractility may be mediated through alpha receptors.