Systemic and uterine responses to alpha-adrenergic stimulation in pregnant and nonpregnant ewes

Attenuated systemic pressor responses to infused angiotensin II characterize normal human and ovine pregnancy; moreover, uterine vascular refractoriness is greater than that of the systemic vasculature overall. It remains unclear whether this generalized refractoriness also pertains to other vasoconstrictors; therefore we studied simultaneous systemic and uterine responses to alpha-agonists in pregnant (n = 6) and nonpregnant (n = 6) sheep. Mean arterial pressure, heart rate, uterine blood flow, and cardiac output were measured before and during infusions of norepinephrine (0.456 to 45.84 micrograms/min) and phenylephrine (1.29 to 129 micrograms/min). Both alpha-agonists caused dose-dependent increases (p less than 0.01) in mean arterial pressure and systemic vascular resistance and decreases in cardiac output (p less than 0.01) in nonpregnant and pregnant animals; however, nonpregnant pressor responses exceeded pregnant ones. Nonpregnant ewes also had greater decreases in uterine blood flow (p less than 0.05) and increases in uterine vascular resistance (p less than 0.05); furthermore, increases in uterine vascular resistance exceeded those of systemic vascular resistance in both groups (p less than 0.01). Attenuated uterine and systemic responses to alpha-agonists characterize normal ovine pregnancy; however, in contrast to the results with angiotensin II, the uterine vascular bed is substantially more responsive to alpha-agonists than the systemic vasculature overall.