Volume 168, Number 2 Am J Obstet Gynecol 12. Nakamura T, Tomita Y, Ichihara A. Density-dependent growth control of adult rat hepatocytes in primary culture. J Biochem 1983;94: 1029-35. 13. Shiota K, Nakamura T, Ichihara A. Distinct effects of transforming growth factor-B, on EGF receptors and EGF- induced DNA synthesis in primary cultured rat hepato- cytes. Biochem Int 1986;13:893-901. 14. Lowry OH, Rosebrough NJ, Farr AL, Randall RJ. Protein measurement with the Folin phenol reagent. J Bioi Chern 1951;193:265-75. 15. Brenstad GO, Sand TE, Christoffersen T. Bidirectional concentration-dependent effects of glucagon and dibu- tyryl cyclic AMP on DNA synthesis in cultured adult rat hepatocytes. Biochim Biophys Acta 1983;763:58-83. Osol and Cipolla 16. Katz M, Block BS, Heymann MA, et al. Fetal organ weight changes after intravenous terbutaline (I') administration in the rabbit [Abstract 10]. In: Proceedings of the twenty- ninth annual meeting of the Society for Gynecologic In- vestigation, Dallas, Texas, March 24-27, 1982. Dallas: Society for Gynecologic Investigation, 1982:7. 17. Johnson JWC, Beck JC, Lee P, et al. Ritodrine hydrochlo- ride effects on the rhesus fetus: carbohydrate metabolism and organ growth [Abstract 324]. In: Proceedings of the thirtieth annual meeting of the Society for Gynecologic Investigation, Washington, DC, March 17-20, 1983. Washington: Society for Gynecologic Investigation, 1983: 176. Interaction of myogenic and adrenergic mechanisms in isolated, pressurized uterine radial arteries from late- pregnant and nonpregnant rats George 0501, PhD, and Marilyn Cipolla, BS Burlington, Vermont 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 urn 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 nrnol/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 a-adrenergic sensitivity, intrinsic (pressure-dependent) tone, and myogenic reactivity. (AM J OBSTET GVNECOL 1993;168:697-705.) Key words: Adrenergic, myogenic, pregnancy, radial arteries, rats, uterine circulation From the Division of Research, Department of Obstetrics and Gyne- cology, University of Vermont College of Medicine. Supported by American Heart Association Grant-In-Aid 523421. G.O. is an Established Investigator of the American Heart Association. Received for publication February 6, 1992; revised September 22, 1992; accepted September 25, 1992. Reprint requests: George 0.101, PhD, Department of Obstetrics and Gynecology, Given Building, Room C-213, University of Vermont College of Medicine, Burlington, IT 05405. Copyright © 1993 Mosby-Year Book, Inc. ()()D2-9378/93 $I.DO + .20 6/1/42965 Systemic arterial pressure normally decreases during pregnancy. By definition, gestational increments in uterine flow can therefore only be achieved by reducing the overall resistance of the vascular bed. This is accom- plished in part by vascular growth and remodeling, which increases arterial diameter and alters the passive mechanical properties of the vascular wall. 1-3 The uter- ine circulation normally operates with a substantial 697