Angiotensin-(1-7) downregulates the angiotensin II type 1 receptor in vascular smooth muscle cells.
Angiotensin (Ang)-(1-7) is a biologically active peptide of the renin-angiotensin system that has both vasodilatory and antiproliferative activities that are opposite the constrictive and proliferative effects of angiotensin II (Ang II). We studied the actions of Ang-(1-7) on the Ang II type 1 (AT(1)) receptor in cultured rat aortic vascular smooth muscle cells to determine whether the effects of Ang-(1-7) are due to its regulation of the AT(1) receptor. Ang-(1-7) competed poorly for [(125)I]Ang II binding to the AT(1) receptor on vascular smooth muscle cells, with an IC(50) of 2.0 micromol/L compared with 1.9 nmol/L for Ang II. The pretreatment of vascular smooth muscle cells with Ang-(1-7) followed by treatment with acidic glycine to remove surface-bound peptide resulted in a significant decrease in [(125)I]Ang II binding; however, reduced Ang II binding was observed only at micromolar concentrations of Ang-(1-7). Scatchard analysis of vascular smooth muscle cells pretreated with 1 micromol/L Ang-(1-7) showed that the reduction in Ang II binding resulted from a loss of the total number of binding sites [B(max) 437.7+/-261.5 fmol/mg protein in Ang-(1-7)-pretreated cells compared with 607.5+/-301.2 fmol/mg protein in untreated cells, n=5, P<0.05] with no significant effect on the affinity of Ang II for the AT(1) receptor. Pretreatment with the AT(1) receptor antagonist L-158,809 blocked the reduction in [(125)I]Ang II binding by Ang-(1-7) or Ang II. Pretreatment of vascular smooth muscle cells with increasing concentrations of Ang-(1-7) reduced Ang II-stimulated phospholipase C activity; however, the decrease was significant (81.2+/-6.4%, P<0.01, n=5) only at 1 micromol/L Ang-(1-7). These results demonstrate that pharmacological concentrations of Ang-(1-7) in the micromolar range cause a modest downregulation of the AT(1) receptor on vascular cells and a reduction in Ang II-stimulated phospholipase C activity. Because the antiproliferative and vasodilatory effects of Ang-(1-7) are observed at nanomolar concentrations of the heptapeptide, these responses to Ang-(1-7) cannot be explained by competition of Ang-(1-7) at the AT(1) receptor or Ang-(1-7)-mediated downregulation of the vascular AT(1) receptor.
Clark
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Hypertension and Vascular Disease Center, Wake Forest University School of Medicine, Winston-Salem, NC 27157-1032, USA.
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D000704:Analysis of Variance; D000803:Angiotensin I / Q000008:administration & dosage / Q000378:metabolism*; D000804:Angiotensin II / Q000037:antagonists & inhibitors / Q000378:metabolism; D057911:Angiotensin Receptor Antagonists; D000818:Animals; D001667:Binding, Competitive; D002478:Cells, Cultured; D015536:Down-Regulation; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D010446:Peptide Fragments / Q000008:administration & dosage / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011945:Receptors, Angiotensin / Q000187:drug effects / Q000378:metabolism*; D012044:Regression Analysis; D013777:Tetrazoles / Q000494:pharmacology; D010738:Type C Phospholipases / Q000378:metabolism
D057911:Angiotensin Receptor Antagonists; D007093:Imidazoles; D010446:Peptide Fragments; D011945:Receptors, Angiotensin; D013777:Tetrazoles; D000804:Angiotensin II; C070967:L 158809; D000803:Angiotensin I; D010738:Type C Phospholipases; C118790:angiotensin I (1-7)