Interaction of myogenic and adrenergic mechanisms in isolated, pressurized uterine radial arteries from late-pregnant and nonpregnant rats. 
The purpose of this study was to examine how myogenic and adrenergic mechanisms interact in controlling the lumen diameter of small uterine arteries from nonpregnant and late-pregnant rats.  Radial arteries (150 to 250 microns lumen diameter) from nonpregnant (n = 28) and late-pregnant (n = 18) rats were studied in vitro under conditions of varying transmural pressure and agonist concentrations. ( 1) Arteries from late-pregnant rats were significantly (p < 0.05) larger in diameter and, unlike nonpregnant vessels, developed a stable intrinsic tone at transmural pressures > 25 mm Hg. ( 2) Vessels from late-pregnant rats displayed a threefold increase in sensitivity to the constrictor effects of phenylephrine: 50% of maximal constriction, nonpregnant = 691 +/- 148 nmol/L and late-pregnant = 229 +/- 32 nmol/L (p < 0.01). ( 3) There was no difference in sensitivity to potassium depolarization. ( 4) Arteries from late-pregnant rats actively constricted to changes in transmural pressure, whereas those from nonpregnant did not unless preactivated beforehand with phenylephrine or K+. ( 5) After preconstriction the autoregulatory effectiveness of late-pregnant arteries in physiologic saline solution versus phenylephrine or K+, or of nonpregnant in K+ and phenylephrine, appeared to be equal in terms of absolute micrometers but not relative percent change in lumen diameter.  Pregnancy is associated with significant changes in the active contractile properties of uterine resistance artery function, specifically heightened alpha-adrenergic sensitivity, intrinsic (pressure-dependent) tone, and myogenic reactivity.