AJP - Endocrinology and Metabolism, Vol 255, Issue 3 E357-E365, Copyright © 1988 by American Physiological Society
In vivo characterization of insulin uptake by dog renal cortical epithelium
C. I. Whiteside, C. J. Lumsden and M. Silverman
Department of Medicine, University of Toronto, Ontario, Canada.
In vivo 125I-labeled insulin uptake by dog renal tubular epithelium was
studied using the single-pass multiple indicator dilution (MID) method and
analyzed by a computer-assisted model of transcapillary exchange and
substrate-cell interaction. Anesthetized dogs received an intrarenal
arterial bolus of multiple tracers: [3H]dextran greater than 70 kDa (plasma
reference), [14C]inulin (extracellular reference), and 125I-insulin. Rapid
serial sampling of the renal venous and urine outflows was performed. The
renal venous outflow curves of 125I-insulin fell below [14C]inulin implying
postglomerular extraction and antiluminal membrane (ALM) uptake. The
fractional urine recovery of 125I-insulin was less than 0.03, indicating
luminal tubular uptake of filtered hormone. After intravenous infusion of
unlabeled insulin, repeat MID runs with tracer revealed saturable ALM
uptake as evidenced by the 125I-insulin renal venous outflow curves
approaching [14C]inulin. Luminal tubular uptake was unchanged and therefore
unsaturable. The 125I-insulin renal venous data were studied using three
mathematical models, incorporating postglomerular reversible binding,
irreversible binding or transport. The best fit was obtained using the
transport model. The modeling analysis is consistent with either uptake
into a virtual epithelial membrane space (i.e., insulin never enters the
cell but binds to or is distributed along the ALM) or insulin actually
enters the intracellular compartment. In vivo uptake of 125I-insulin ALM is
characterized by a Km of 15.44 nM.
