Evidence for the role of angiotensin II biosynthesis in the rat internal anal sphincter tone

Abstract

Background & Aims: The internal anal sphincter tone is important for anorectal continence. This study examined the role of angiotensin II as a neurohumoral signal for the myogenic tone in the internal anal sphincter. Methods: We determined the effect of angiotensin I, II, III, and IV and angiotensin-(1–7) on the basal tone of the rat internal anal sphincter smooth muscle before and after selective receptor antagonists and biosynthesis inhibitors. Selective pharmacological tools used were losartan (for the AT 1 receptor), PD123,319 (for AT 2), A-779 [for angiotensin-(1–7)], captopril (for angiotensin-converting enzyme), and amastatin (for aminopeptidases A and N). Angiotensins were measured by using high-performance liquid chromatography/UV. Western blot studies were used to determine AT 1 and AT 2 receptors, ACE, and aminopeptidases A and N. Results: Angiotensin I, II, and III produced concentration-dependent contraction in the internal anal sphincter mediated by AT 1 receptors. However, in the higher concentrations (from 100 nM to 10 μM), angiotensin II showed an inhibitory effect via AT 2 receptors. Captopril (1 μM) inhibited the biosynthesis of angiotensin II in the internal anal sphincter, antagonized the contractile effects of angiotensin I, and, importantly, caused a decrease in the basal tone. Amastatin inhibited the effects of angiotensin II while augmenting those of angiotensin III. In contrast, angiotensin-(1–7) and angiotensin IV had only minor effects in the internal anal sphincter. Angiotensin I, II, and III; angiotensin-converting enzyme; aminopeptidase A and aminopeptidase N; AT 1; and AT 2 receptors were shown to be present in the internal anal sphincter. Conclusions: Locally produced angiotensin II may partially regulate basal tone in the internal anal sphincter.