AJP - Endocrinology and Metabolism, Vol 256, Issue 1 E55-E63, Copyright © 1989 by American Physiological Society
Absorption of epidermal growth factor and insulin in rabbit renal proximal tubules
S. Nielsen, E. Nexo and E. I. Christensen
Department of Cell Biology, University of Aarhus, Denmark.
Epidermal growth factor (EGF), which stimulates the growth of a variety of
tissues, was originally isolated from mouse submandibular glands. Human EGF
(hEGF) has been isolated from the urine, and cDNA encoding for EGF has been
isolated from the human kidney. Thus the kidney may represent an alternate
source of EGF. Another potential explanation for the urinary content of EGF
is a limited reabsorption of filtered EGF and/or a transtubular transport
of EGF from the peritubular compartment to the lumen. Therefore, we exposed
isolated and perfused rabbit proximal tubules to 125I-hEGF either in the
perfusate or in the bath fluid. Luminal uptake of hEGF was compared with
uptake of 125I-porcine insulin, which is known to be taken up with large
efficiency. The results demonstrate that only 4% of the perfused 125I-hEGF
load was taken up per millimeter of tubule length compared with 73% for
insulin. Furthermore, hEGF is subject to a small transtubular transport
from bath to lumen. Renal clearance of endogenous rabbit EGF was also
measured and was comparable with that of creatinine. Thus in conclusion
this study strongly suggests that filtered EGF to a large extent remains in
the ultrafiltrate, unlike insulin, which is removed within the first few
millimeters of proximal tubule. In addition filtered EGF in preurine is
probably supplied by EGF transported intact across the proximal tubular
epithelium.
