Localization and degradation of calcitonin in renal proximal tubule of rat visualized by immunocytochemistry.

Immunocytochemical and cryoultramicrotomy techniques were applied to label endogenous as well as intravenously injected calcitonin (CT) in rat kidney. Endogenous CT was located to the membranes of the brush border, endocytotic invaginations and vacuoles and dense apical tubules suggesting degradation at the brush border and/or within endocytotic vacuoles after glomerular filtration of endogenous CT. There was no labeling at the basolateral membranes indicating that possible basolateral binding sites are of minor quantitative importance in degradation. After injection of human CT there was an additional labeling of lysosomes whereas injection of insulin did not change the labeling pattern for endogenous CT indicating that the uptake of CT involves specific binding sites. Quantitative immunocytochemistry after injection of h-CT applying monoclonal antibodies against two different fragments of CT showed that the relative amounts of these fragments differed in vacuoles and lysosomes from that observed in the brush border, but no difference was seen in dense apical tubules. This suggests that injected CT may be degraded both at the brush border, within endocytotic vacuoles and lysosomes and that some fragments are more sensitive to hydrolysis in lysosomes than others. Furthermore, it indicates an early formation of dense apical tubules before detectable degradation of CT occurs in endocytotic vacuoles.