Absorption of I 125-labeled homologous albumin by rat kidney proximal tubule cells : A study of microperfused single proximal tubules by electron microscopic autoradiography and histochemistry

Abstract

Small amounts of I125-labeled rat albumin were injected through micropipettes into single proximal tubules of the rat kidney. After different time-intervals the injected tubules were microperfusion-fixed with glutaraldehyde and the tissue analyzed by electron microscopic autoradiography. No ultrastructural alterations were observed in the proximal tubule cells during the absorption of the labeled protein. Six to 10 minutes after start of I125-albumin perfusion most of the absorbed protein was located in the large apical vacuoles in the proximal tubule cells but some label was also associated with small apical vacuoles. The observations suggest that the labeled albumin was transferred to the large apical vacuoles via small apical vacuoles which most likely represented pinched-off apical cell membrane invaginations. After 30 minutes most of the label was present in cytoplasmic bodies, which were limited by a triple-layered membrane about 90 Å in thickness and had an electron-dense content, and which were located in the middle or apical regions of the cells. After 60 minutes the label was confined to similar cytoplasmic bodies located in all regions of the cells. Combined electron microscopic autoradiography and histochemistry demonstrated that the labeled cytoplasmic bodies were acid phosphatase positive. The latter observation indicates that the cytoplasmic bodies, in which the I125-albumin was located, were lysosomes. It is suggested that at least some of the absorbed albumin was degraded in these lysosomes. There was no autoradiographic or morphologic evidence that the labeled albumin crossed the wall of the proximal tubule by passing between the cells or that vacuoles or cytoplasmic bodies containing absorbed albumin emptied into the peritubular space.