Effect and distribution of intravenously injected ¹²⁵I-guanylin in rat kidney examined by high-resolution light microscopic radioautography

Abstract

 ¹²⁵I-guanylin was injected intravenously into rats, and their kidney and intestinal tract were processed for light microscopic radioautography using semithin sections to examine the binding sites. Various doses of unlabeled guanylin were also injected to examine the morphological effects of guanylin on the kidney. Dense labeling of silver grains due to ¹²⁵I-guanylin were observed only in the kidney. In the cortex, silver grains were localized on the luminal side of the proximal tubules at 5–30 min. In the medulla, silver grains appeared at the basal side of the collecting ducts, capillaries and loops of Henle after 5 min. Silver grains then accumulated in the cytoplasm of the collecting ducts after 10 min, and disappeared after 30 min. The cell height of the inner medullary collecting ducts (IMCD) decreased and their luminal spaces increased dose-dependently 5 min after the injection of both labeled and unlabeled guanylin. These structural changes returned to control levels within 30 min. These results indicate a high density localization of guanylin receptors on the luminal surface of proximal tubules in the renal cortex and also rapid excretion of guanylin through the IMCD. The morphological changes of the IMCD suggest a diuretic effect of guanylin.