Interaction of myogenic and adrenergic mechanisms in isolated, pressurized uterine radial arteries from late-pregnant and nonpregnant rats.

OBJECTIVE: 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. STUDY
DESIGN: 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.
RESULTS: (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.
CONCLUSION: 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.