Luminal and basolateral uptake and receptor binding of IGF-I in rabbit renal proximal tubules.
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.
Flyvbjerg
A
A
Institute of Experimental Clinical Research, Aarhus Kommunehospital, Denmark.
Nielsen
S
S
Sheikh
M I
MI
Jacobsen
C
C
Orskov
H
H
Christensen
E I
EI
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*
D007457:Iodine Radioisotopes; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1