Receptor-mediated endocytosis of A₁₄-¹²⁵I-insulin by the nonfiltering perfused rat kidney

The mechanism of insulin uptake and/or degradation in the peritubular circulation of the kidney was investigated using nonfiltering perfused rat kidneys, in which glomerular filtration was sufficiently reduced. After perfusion of A₁₄-¹²⁵I-insulin in the nonfiltering kidney for designated intervals, the acid-wash technique was employed to separately measure the acid-extractable and acid-resistant A₁₄-¹²⁵I-insulin, which were quantitated by HPLC and TCA-precipitability. HPLC profiles showed that the nonfiltering kidney metabolizes A₁₄-¹²⁵I-insulin only to a small extent during 1-h perfusion, suggesting that the peritubular clearance of A₁₄-¹²⁵I-insulin was not due to extracellular degradation but for the most part to uptake by the kidney. Acid-extractable A₁₄-¹²⁵I-insulin rapidly increased with time and reached pseudo-equilibrium with perfusate at approx. 10 min, whereas acid-resistant A₁₄-¹²⁵I-insulin increased continuously. An endocytosis inhibitor, phenylarsine oxide, inhibited significantly the acid-resistant A₁₄-¹²⁵I-insulin with no change in acid-extractable A₁₄-¹²⁵I-insulin, suggesting that the peritubular uptake of A₁₄-¹²⁵I-insulin largely represents endocytosis of the peptide into the intracellular space. Moreover, both the acid-extractable and acid-resistant A₁₄-¹²⁵I-insulin were significantly decreased in the presence of unlabeled insulin (1 μM). These lines of evidence suggest that insulin is taken up by the nonfiltering perfused kidney via receptor-mediated endocytosis (RME), which possibly occurs at the basolateral side of renal tubular cells, and that the peritubular clearance of insulin is largely accounted for by this mechanism.