Internalization of EGF in perfused rat liver is independent of the degree of receptor occupancy.

The internalization rate of epidermal growth factor (EGF) by the perfused rat liver was evaluated to determine whether the internalization rate constant depends on the degree of receptor occupancy. A tracer concentration of 125I-labeled EGF (30 pM) alone or 125I-EGF plus unlabeled EGF (20 nM) was infused into the liver in the single-pass perfusion system at 37 degrees C. At various times (2-20 min), the perfusion medium was switched to medium of pH 3.0, and the radioactivity of 125I-EGF emerging into the outflow (surface-bound EGF) and remaining in the liver (internalized EGF) was determined. At the tracer condition, less than 0.1% of the surface receptor was occupied by 125I-EGF for 20 min perfusion. When excess unlabeled EGF (20 nM) was present in the perfusate and the bulk of the cell surface receptors was occupied, the density of the cell surface EGF receptor after 20 min dropped to 14% of the initial value. The internalization rate constant, defined as the probability of an occupied receptor being internalized per minute, was calculated from the slope of a plot of the amount of internalized EGF vs. the integrated amount of surface receptor-bound EGF with time. The internalization rate constants calculated from the plots with and without unlabeled EGF in the perfusate were 0.21 and 0.33 min-1, respectively, and did not differ significantly (P greater than 0.1). These results indicated that the internalization rate constant of the EGF receptor complex is independent of the degree of receptor occupancy in the liver.