Kinetic analysis of clearance of epidermal growth factor in isolated perfused rat kidney

Our previous in vivo studies identified the saturable uptake of epidermal growth factor (EGF) by rat kidney (D.C. Kim, Y. Sugiyama, H. Sato, T. Fuwa, T. Iga, and M. Hanano, J. Pharm. Sci. 77: 200-207, 1988). In the present study, renal handling of EGF in filtering and nonfiltering isolated perfused rat kidneys was investigated. At designated times after the recirculatory perfusion of 125I-EGF in the nonfiltering kidney, the surface-bound and internalized EGF were separately determined by an acid-washing technique. Time profiles of cell-surface-bound and internalized EGF obtained at the perfusion of a tracer 125I-EGF were fitted to the pharmacokinetic model, and kinetic parameters obtained were as follows: konRs = 0.49 ml.min-1.g-1, koff = 0.87 min-1, kint = 0.20 min-1 where konRs is the binding clearance of EGF with its receptor and koff and kint represent the dissociation and internalization rate constants of the EGF-receptor complex, respectively. The Scatchard analysis of the concentration-dependent EGF binding in the nonfiltering kidney suggests the presence of two binding components, one with high affinity [the apparent dissociation constant (Kd1 = 0.1 nM) and the other with low affinity (Kd2 = 30 nM]. By comparing the internalization clearance (CLi) with filtering and nonfiltering kidneys, we concluded that the renal uptake of EGF occurs mainly from the antiluminal side via receptor-mediated endocytosis in a saturable manner and that the nonsaturable reabsorption of EGF from the luminal membrane after glomerular filtration is relatively small. The contribution of reabsorption, however, becomes larger with the increase in EGF concentration.