IGF-II RECEPTORS IN LUMINAL AND BASOLATERAL MEMBRANES ISOLATED FROM PARS CONVOLUTA AND PARS RECTA OF RABBIT PROXIMAL TUBULE

The binding of I-125-labeled insulin-like growth factor-II (I-125-IGF- II) to luminal and basolateral membrane vesicles isolated from pars co nvoluta and the straight part (pars recta) of rabbit proximal tubule w as investigated. Analyses of the binding data by use of the general st oichiometric binding equation revealed, that in ail preparations IGF-I I was bound to one high-affinity binding site and other sites with low er affinities. The specificity of the high-affinity I-125-IGF-II bindi ng to the membrane vesicles assessed by displacement by unlabeled IGF- II, IGF-I and insulin showed that IGF-I displaced I-125-IGF-II in the range 22.5-47.9 nM (IC50) whereas insulin did not effect I-125-IGF-II binding at all. beta-Galactosidase inhibited the I-125-IGF-II binding with half-maximal inhibition of 20-30 nM beta-galactosidase. D-Mannose 6-phosphate increased the binding of I-125-IGF-II and reversed the in hibitory effect of beta-galactosidase. Analyses of I-125-IGF-II bindin g curves in the presence of beta-galactosidase or D-mannose 6-phosphat e demonstrated that none of these compounds changed the binding affini ty of I-125-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-I I/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.