Autocrine regulation of internal anal sphincter tone by renin-angiotensin system: comparison with phasic smooth muscle

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Abstract
The myogenic control mechanisms that govern the basal tone in the internal anal sphincter (IAS) are not known. The present studies determined the autocrine regulation of ANG II in the IAS. The studies were performed in the freshly isolated smooth muscle cells (SMC) of the IAS. We determined the presence of ANG II precursor angiotensinogen (Angen), and the enzymes that convert it into ANG II, using functional, molecular biology, and immunocytochemical studies in rats. ANG II levels in the SMC were determined using ELISA. The IAS SMC generate ANG II at a rate severalfold higher than those from the adjoining smooth muscle of rectum (RSM). RT-PCR data show that IAS exclusively expresses significant higher levels of renin, Angen, and angiotensin-converting enzyme (ACE). These data were confirmed using Western blot analyses and immunocytochemistry. In the IAS SMC, H-77 (10 μM; renin inhibitor) and captopril (1 μM; ACE inhibitor) decreased the basal as well as Angen-increased levels of ANG II. The following functional data corroborate the role of renin-angiotensin system (RAS) in the IAS tone. Angen produced concentration-dependent shortening of the IAS SMC that was inhibited by H-77 and captopril. In addition, H-77 or captopril caused a concentration-dependent fall in the IAS tone vs. nontonic tissues. Basal tone in IAS is partially under the autocrine control of cellular RAS evident by the expression of mRNA coding Angen, renin, and ACE and translation to the respective proteins in the SMC.