Regional variation in resistance artery diameter responses to alpha-adrenergic stimulation during pregnancy.

Whole animal pressor responses are blunted during pregnancy; yet, uterine arteries, paradoxically, become significantly more sensitive to the constrictor effects of phenylephrine (PE). The objectives herein were to investigate 1) the regional variation (uterine vs. mesenteric arteries) in dose-lumen diameter relationship to alpha-adrenergic stimulation during pregnancy, and 2) the selectivity of these sensitivity shifts for this pathway (PE vs. KCl). Lumen diameter was measured in isolated, pressurized (50 mmHg) arterial segments from age-matched virgin (nonpregnant; NP) and late pregnant (LP; days 19-20) Sprague-Dawley rats. Uterine arcuate vs. mesenteric arteries from NP rats were equally sensitive to either vasoconstrictor. Arcuate arteries from LP rats, however, were 4.5-fold more sensitive to PE (P < 0.01) compared with those from NP controls. Furthermore, diameter curves became superimposed at [PE] > or = 0.1 microM, even though initial diameter of arcuate arteries from LP rats was significantly larger (P < 0.001). Conversely, mesenteric arteries from LP rats were three-fold less sensitive to PE (P < 0.02), and the diameter curve displayed a corresponding parallel rightward shift. Pregnancy did not affect the sensitivity to KCl depolarization in either arcuate or mesenteric arteries. The percent reduction in lumen diameter to the maximum [KCl] was significantly decreased only in arcuate arteries from LP rats (P < 0.001). Thus, during pregnancy, divergent constrictor responses specific to alpha-adrenergic stimulation occur in resistance arteries from the uterine vs. splanchnic circulations. Consequently, concentrations of PE that are subthreshold in NP uterine arteries can elicit large changes in lumen diameter and thereby have a pronounced effect on uterine vascular resistance in the pregnant state.