title: Luminal and basolateral uptake and receptor binding of IGF-I in rabbit renal proximal tubules.
abstract: The aim of the present study was to quantify and compare the luminal and basolateral binding and uptake of 125I-labeled insulin-like growth factor I (IGF-I) by means of 1) isolated, perfused, proximal tubules combined with electron microscope autoradiography and 2) luminal and basolateral membrane vesicles from rabbit proximal tubules. 125I-IGF-I was added to isolated perfused proximal tubules for 30 min in concentrations of 1.6-3.9 micrograms/l to either the perfusate or the bath. The luminal and basolateral uptake in 30 min averaged 447 and 410 fg/mm, respectively. About 20% of the luminally absorbed IGF-I was digested. Addition of excess unlabeled IGF-I (10(-7) M) to the bath produced complete inhibition of the basolateral binding/uptake, whereas no inhibition of the luminal uptake was seen. Electron microscope autoradiography showed that IGF-I after luminal endocytic uptake to a large extent was transported into lysosomes. After basolateral exposure the major portion of the grains was found over the basolateral cell membrane; however, a significant amount was located over endocytic vacuoles and lysosomes in both apical and basal parts of the cells. In both luminal and basolateral membrane vesicles, single-class, high-affinity binding sites for IGF-I were found with dissociation constants of 6.3 and 5.7 nM, respectively. Specific binding capacities averaged 2.7 and 25.7 pmol IGF-I/mg protein in luminal and basolateral vesicles. The biochemical data suggest an asymmetric distribution of specific IGF-I receptors in the luminal and basolateral membranes, with a greater abundance of receptors in the latter. The extensive basolateral endocytic binding/uptake of IGF-I compared with that of the luminal in isolated perfused tubules differs considerably from the processing of other peptide hormones.
authors.affiliation: Institute of Experimental Clinical Research, Aarhus Kommunehospital, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Flyvbjerg
authors.forename: S
authors.initials: S
authors.lastname: Nielsen
authors.forename: M I
authors.initials: MI
authors.lastname: Sheikh
authors.forename: C
authors.initials: C
authors.lastname: Jacobsen
authors.forename: H
authors.initials: H
authors.lastname: Orskov
authors.forename: E I
authors.initials: EI
authors.lastname: Christensen
mesh_terms: D000818:Animals; D001345:Autoradiography; D001667:Binding, Competitive; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D005260:Female; D066298:In Vitro Techniques; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007457:Iodine Radioisotopes; D007687:Kidney Tubules, Proximal / Q000378:metabolism* / Q000648:ultrastructure; D007700:Kinetics; D008854:Microscopy, Electron; D011817:Rabbits; D017526:Receptor, IGF Type 1 / Q000378:metabolism*
chemical_list: D007457:Iodine Radioisotopes; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1