1,25(OH)2D3 binding along the rat nephron: autoradiographic study in isolated tubular segments

1,25-dihydroxyvitamin D3 [1,25(OH)2D3] binding sites were studied along the nephron of rats. The animals were pretreated with the diphosphonate EHDP at doses that inhibit the endogenous production of 1,25(OH)2D3. A dry film autoradiographic technique was applied to tubular segments isolated by microdissection from kidneys incubated in vitro with various concentrations (0.2-12 nM) of [3H]1,25(OH)2D3 in the presence or absence of an excess unlabeled hormone (X200) in order to determine specific binding. Total, nonspecific, and specific labeling were quantified by silver grain counting over cytoplasmic and nuclear areas. Specific nuclear labeling appeared in the cortical ascending limb and papillary collecting tubule at 1 nM. In the distal tubule and, to a lesser extent, in the cortical collecting tubule a specific nuclear labeling was also present, but only at higher concentrations. No specific nuclear labeling was detected in the proximal tubule. All along the nephron, a significant and nonspecific labeling was observed in the cytoplasm, either alone or superimposed over the specific nuclear labeling. In conclusion 1,25(OH)2D3 specific binding sites appear to be localized mainly in the cortical ascending limb of the loop of Henle, in the distal and cortical collecting duct, and in the papillary collecting duct.