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Renal handling of 125I-insulin was studied using a modification of the Sperber technique. Results showed 125I-insulin to be extracted at the peritubular side of the nephron in a process that was competitively inhibited by increasing amounts of unlabelled insulin, but not ACTH, in the injection mixture. When unlabelled insulin instead was injected 30 sec after the labelled insulin it showed significantly less interference with peritubular extraction of 125I-insulin, indicating strong attachment to the cell membrane or possible internalization of 125I-insulin into proximal tubular cells. Light microscope autoradiography 1 min after injection of 125I-insulin showed grains over proximal tubules only. On the ligated side localization was preferably peritubular while on the control side it was luminal. Electron microscope autoradiography showed sparsely distributed grains, however, frequently located over basal parts of proximal tubular cells. Pretreatment with lysine hydrochloride lowered renal extraction of 125I-insulin and increased urinary recovery of iodine label bilaterally. 125I-glucagon and 125I-C-peptide were not extracted from the peritubular circulation. In conclusion, the model has provided evidence of a rapid and significant peritubular extraction of 125I-insulin by proximal tubular cells in a process probably involving specific insulin receptors. Following receptor binding probably only minor amounts of 125I-insulin enters the proximal tubular cells, while the greater part is degraded at the cell surface or released into the circulation.
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