AUTORADIOGRAPHIC CHARACTERIZATION OF ANGIOTENSIN-II RECEPTOR SUBTYPES IN RAT INTESTINE

Angiotensin II is known to regulate motility and ion an water absorption in the intestine. These effects are presumed to be mediated by angiotensin II (ANG II) receptors that are present in both mucosal and muscular layers throughout the intestine. To evaluate tissue density and distribution of ANG II receptor subtypes (AT1 and AT2), we performed an in situ autoradiographic study on jejunum, ileum, and colon of Sprague-Dawley rats. Tissue sections (10 mum) were incubated with 500 pM I-125-[Sar1,Ile8]ANG II, fixed with paraformaldehyde vapors, and coated with photographic emulsion. Binding specificity was verified by competition with unlabeled [Sar1]ANG II (10 muM). AT1 and AT2 receptor distribution was characterized by competition with the nonpeptide antagonists losartan (10 muM) and PD123177 (10 muM), respectively, and the density of receptors was quantified by counting the silver grains overlying the different layers of intestinal wall. Specific binding was moderately abundant in the mucosa and the muscularis of both jejunum and ileum, whereas no binding was present in the submucosa and the serosa. Losartan inhibited 86% of radioligand binding to the mucosa in both jejunum and ileum, whereas PD123177 inhibited only 10%. The combination of the two compounds inhibited 96% of specific binding. In the colon, binding was significantly more abundant in the muscularis than in the mucosa. In this segment, losartan inhibited 90% and PD123177 16% of specific binding to muscularis. The combination of these compounds reduced binding by 97%. Thus the predominant ANG II receptor in all intestinal segments is AT1, but a small population of AT2 receptors also seems to be present. Neither intraperitoneal infusion of ANG II (200 ng.kg-1.min-1) for 7 days nor treatment with an angiotensin-converting enzyme inhibitor (enalapril, 3.75 mg.kg-1.day-1 for 10 days) caused a significant change in ANG II receptor density in jejunum, ileum, or colon, indicating that these receptors are not regulated by plasma ANG II levels.