Angiotensin I converting enzyme in human intestine and kidney. Ultrastructural immunohistochemical localization.

The localization of immunoreactive angiotensin I-converting enzyme (ACE) has been investigated at the optical and ultrastructural level with anti-human ACE antibodies in the human kidney and small intestine. In both tissues ACE was found in blood vessels and in extravascular situation in the absorptive epithelial cells of intestinal mucosa and renal proximal tubules. Ultrastructural immunohistochemistry showed that in intestinal and renal proximal tubular cells ACE was prominent in microvilli and brush borders. In the kidney ACE was also present on the basolateral part of the plasmalemmal membrane, where it may contribute to the regulation of angiotensin II-dependent absorption processes. Intracellular positivities were also observed inside the renal vascular endothelial and proximal tubular cell in endoplasmic reticulum and nuclear envelope reflecting the synthesis and the cellular processing of ACE. The intestinal microvascular endothelium was strongly labeled suggesting that the mesenteric circulation is an important site for the production of angiotensin II. Vascular endothelial ACE was also detected in the peritubular but not glomerular capillaries of the kidney.