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

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(2) receptors. However, in the higher concentrations (from 100 nM to 10 muM), anglotensin II showed an inhibitory effect via AT(2) receptors. Captopril (1 muM) inhibited the biosynthesis of angiotensin II in the internal anal sphincter, antagonized the contractile effects of anglotensin I, and, importantly, caused a decrease in the basal tone. Amastatin inhibited the effects of anglotensin 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.