LOCALIZATION OF BINDING-SITES FOR EPIDERMAL GROWTH-FACTOR (EGF) IN RAT-KIDNEY - EVIDENCE FOR THE EXISTENCE OF LOW AFFINITY EGF BINDING-SITES ON THE BRUSH-BORDER MEMBRANE

We investigated the renal distribution of I-125-EGF in the filtering perfused rat kidney using an acid washing technique. Trichloroacetic acid-precipitable I-125-EGF radioactivity was eluted from both the renal vein and the urinary cannulae, the former regarded as representing the antiluminal, and the latter the luminal, cell surface bound I-125-radioactivity. The addition of excess unlabeled EGF (20 nM) to the perfusate completely inhibited the binding of I-125-EGF to the antiluminal membrane but did not inhibit that of I-125-EGF to the luminal membrane. On the other hand, the order of relative density of I-125-EGF binding sites in the in vivo kidney determined by auto-radiography was cortex > inner medulla > outer medulla. After the i.v. administration of excess unlabeled EGF together with I-125-EGF, the renal uptake of I-125-EGF was inhibited completely in the inner medulla, but only by 50% in the cortex and outer medulla, suggesting the presence of nonsaturable luminal uptake of EGF in the cortex and outer medulla. After i.v. administration of I-125-EGF, a change in position of silver grains from the luminal cell surface membrane to the intracellular space was observed in the proximal convoluted tubules. In conclusion, in addition to the previously identified uptake mechanism of circulating EGF through high-affinity binding sites on the antiluminal cell surface membrane, the reabsorption mechanism of filtered EGF through low-affinity binding sites on the luminal cell surface membrane was demonstrated. In vivo autoradiography showed the gradual internalization of EGF from the luminal cell surface membrane to the intracellular space of the proximal convoluted tubule.