AJP - Renal Physiology, Vol 253, Issue 5 857-F867, Copyright © 1987 by American Physiological Society
Luminal and basolateral uptake of insulin in isolated, perfused, proximal tubules
S. Nielsen, J. T. Nielsen and E. I. Christensen
Department of Cell Biology, University of Aarhus, Denmark.
The present study was performed to quantitate and compare the luminal and
the peritubular uptake of 125I-labeled insulin in isolated, perfused,
proximal tubules from rabbit kidneys. 125I-insulin was added in
physiological concentrations of 3.0-7.0 ng/ml or 59.0-89.5 ng/ml (high
insulin 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 less than 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. Addition of excess unlabeled
insulin (10(-5) M) to the bath produced significant inhibition of binding
(53.7%) at 7.0 ng/ml, but no inhibition at 89.5 ng/ml labeled insulin in
the bath. This 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.
