IGF-II receptors in luminal and basolateral membranes isolated from pars convoluta and pars recta of rabbit proximal tubule

Abstract

The binding of 125I-labeled insulin-like growth factor-II ( 125I-IGF-II) to luminal and basolateral membrane vesicles isolated from pars convoluta and the straight part (pars recta) of rabbit proximal tubule was investigated. Analyses of the binding data by use of the general stoichiometric binding equation revealed, that in all preparations IGF-II was bound to one high-affinity binding site and other sites with lower affinities. The specificity of the high-affinity 125I-IGF-II binding to the membrane vesicles assessed by displacement by unlabeled IGF-II, IGF-I and insulin showed that IGF-I displaced 125I-IGF-II in the range 22.5-47.9 nM (IC5o) whereas insulin did not effect 125I-IGF-II binding at all. /gb-Galactosidase inhibited the 125I-IGF-II binding with half-maximal inhibition of 20-30 nM β-galactosidase. D-Mannose 6-phosphate increased the binding of 125I-IGF-II and reversed the inhibitory effect of β-galactosidase. Analyses of 125I-IGF-II binding curves in the presence of /β-galactosidase or D-mannose 6-phosphate demonstrated that none of these compounds changed the binding affinity of 125I-IGF-II for the membrane vesicles. The IGF-II/M6P receptor content in the luminal membranes was in the range 0.21–0.34 pmol IGF-II/M6P receptor per mg protein and very low compared to 2.27–2.86 pmol IGF-II/M6P receptor per mg protein in basolateral membranes.