Responses of fetal ovine systemic and umbilical arteries to angiotensin II

Angiotensin II (ANG II) contracts umbilical arteries and has been hypothesized to regulate fetal blood pressure primarily by altering umbilical vascular resistance. To determine whether systemic arteries in term fetal sheep are sensitive to ANG II, isometric contraction of endothelium-intact isolated fetal renal, mesenteric, and umbilical arteries in response to ANG II was studied. ANG II (10(-7) M) elicited contractile responses in all three vessels (43 +/- 8%, 99 +/- 21%, and 105 +/- 5% of the maximal response seen with 90 mM KCl for renal, mesenteric, and umbilical arteries, respectively). The time course of the contractile responses differed among the vessels: renal and mesenteric arteries exhibited rapid transient contraction followed by relaxation, whereas umbilical artery displayed a more slowly developing but sustained contraction (1 +/- 0%, 3 +/- 1%,and 93 +/- 4% of maximal contractile response at 5 min, for renal, mesenteric, and umbilical arteries, respectively). The AT1 receptor antagonist, losartan (10(-6) M), abolished contractile responses in renal and mesenteric arteries but only slowed the contraction in umbilical artery in response to ANG II and had no effect on maximal tension. AT2 receptor blockade (PD 123319, 10(-7) M) had no significant effect on the response to ANG II in any vessel. Indomethacin (10(-6) M) significantly potentiated contraction to ANG II in renal and mesenteric but not umbilical arteries. Northern and Western blot analyses demonstrated the presence of AT1 mRNA and protein in all three vessels. Immunostaining for the AT1 receptor was present in endothelium and the tunica media. These findings demonstrate the AT1 receptor is present and functionally active in fetal systemic arteries and are consistent with previous findings that the umbilical circulation displays a greater responsiveness to ANG II than the systemic vasculature.