Localization of immunoreactive vitamin D-dependent calcium binding protein in chick nephron.

Immunoreactive vitamin D-dependent calcium binding protein (iCaBP) has been localized in the chicken nephron, using microdissection and subsequent radioimmunoassay (RIA). Six to ten weeks old vitamin D-replete chicks were sacrificed and the kidneys removed. The various segments of the nephron, i.e. the proximal convoluted tubule (PCT), the thick part of the hairpin loops (thick loops), the thin part of the hairpin loops (thin loops), the distal convoluted tubules (DCT) and the cortical collecting tubules (CCT) were microdissected and grouped according to their morphological characteristics. Because the transition between DCT and CCT was not evident, these two segments were gathered together. The samples were assayed for iCaBP by a microradioimmunoassay, using antisera against the vitamin D-dependent chicken intestinal CaBP. iCaBP was localized in the thin loops and in the DCT-CCT segments (24.7 +/- 5 and 20.8 +/- 3 ng/mg protein, respectively). No iCaBP was detected either in the PCT using up to 32 micrograms protein, or in fragments of medullary papilla, assaying up to 4 micrograms protein.