Luminal and basolateral uptake of insulin in isolated perfused, proximal tubules

The present study was performed to quantitate compare the luminal and the peritubular uptake of 125I-insulin in isolated, perfused, proximal tubules from rabbit kidneys. 125I-insulin was added in physiological concentrations to either the perfusate or the bath fluid for 30 min. The luminal uptake in 30 min averaged 0.76 pg/mm at physiological concentrations and 18.0 pg/mm at high insulin concentrations. About 15-41% of the absorbed insulin was digested and <5% was transported from the lumen to the peritubular space as intact insulin. The peritubular binding/uptake of 125I-insulin at physiological and high concentrations in the bath was 0.136 and 0.318 pg, respectively. The data indicates that insulin is bound/absorbed at the basolateral membranes both by a saturable specific mechanism and a nonspecific, nonsaturable mechanism. The basolateral absorption constituted 15.2 and 1.8% of the total tubular extraction of insulin at physiological and high insulin concentrations, respectively. Electron microscope autoradiography showed that, after luminal as well as basolateral endocytosis, insulin was exclusively accumulated in endocytic vacuoles and lysosomes