Binding and bioactivity of insulin in primary cultures of carp (Cyprinus carpio) hepatocytes

Isolated carp hepatocytes cultured in serum-free, chemically defined medium were used to investigate within the same cell preparation characteristics of the binding of insulin as well as effects of insulin on cellular metabolism. The binding of human [(125)I]-insulin to carp hepatocytes was studied in kinetic, saturation and displacement experiments. A dependency of insulin binding on the collagenase used for cell isolation was demonstrated. Insulin binding decreased during the first 12h of culture but remained constant during the following 12h. The kinetic experiments revealed that [(125)I]-insulin binding reached a steady state within 20-30 min of incubation. The mathematical analysis of the saturation experiments demonstrated the existence of two populations of binding sites, one with high affinity (Kd1 = 5.5 pM) and low capacity (Bmax1 = 0.14 fmol/mg protein or 77 binding sites/cell) and one with low affinity (Kd2 = 2.4 nM) and high capacity (Bmax2 = 17.6 fmol/mg protein or 9623 binding sites/cell). In competition experiments, 312 pM [(125)I]-insulin was displaced by cold insulin, IGF-I and IGF-II with IC50 values of 2.2, 7.9 and 20.3 nM, respectively. Glucagon was without effect. Binding of insulin to carp hepatocytes resulted in a significant reduction of glucose release and a significant increase of protein synthesis as of de novo fatty acid synthesis.