Basolateral binding and uptake of 125-I-insulin in proximal tubular cells after peritubular extraction in the avian kidney.

Several studies have suggested the existence of a renal peritubular extraction of insulin. So far, however, no conclusive evidence, as to whether insulin enters the proximal tubular cell following peritubular extraction, has been presented. In the present study we injected iodine labelled porcine insulin into the renal portal system on hens prepared according to a modified Sperber technique and followed cellular handling of the extracted 125-I-insulin using electron microscope autoradiography at 1 and 7 min after injection. The results showed that at 1 min after injection, peritubular extraction of 125-I-insulin accounted for as much as 30% of total proximal tubular accumulation of grains. Of these grains about 40% were located over basal vesicles, lysosomers or other cell organelles with the remaining 60% located in the intercellular space, probably bound to the basolateral membrane. At 7 min after injection the distribution of grains subsequent to peritubular extraction of 125-I-insulin was unchanged. In contrast, very few grains were located over distal tubules at 1 or 7 min. Thus, the results demonstrate a sizable peritubular extraction of 125-I-insulin in the avian kidney with subsequent uptake into the proximal tubular cells of about one third of the extracted insulin.