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 a nd basolateral binding and uptake of I-125-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 b asolateral membrane vesicles from rabbit proximal tubules. I-125-IGF-I was added to isolated perfused proximal tubules for 30 min in concent rations of 1.6-3.9 mug/l to either the perfusate or the bath. The lumi nal and basolateral uptake in 30 min averaged 447 and 410 fg/mm, respe ctively. About 20% of the luminally absorbed IGF-I was digested. Addit ion of excess unlabeled IGF-I (10(-7) M) to the bath produced complete inhibition of the basolateral binding/uptake, whereas no inhibition o f the luminal uptake was seen. Electron microscope autoradiography sho wed that IGF-I after luminal endocytic uptake to a large extent was tr ansported 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 baso lateral membrane vesicles, single-class, high-affinity binding sites f or IGF-I were found with dissociation constants of 6.3 and 5.7 nM, res pectively. 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 recept ors in the latter. The extensive basolateral endocytic binding/uptake of IGF-I compared with that of the luminal in isolated perfused tubule s differs considerably from the processing of other peptide hormones.