Fig. 8. Degradation and recycling of endocytosed FGF1. HeLa cells transfected with FGFR1 (R1) or FGFR4 (R4) were incubated in growth medium with 100 ng/ml [¹²⁵I]FGF1 and 40 U/ml heparin for 20 minutes at 37°C. Excess and surface-bound [¹²⁵I]FGF1 was removed by acid/salt wash of the cells, which were incubated further in growth medium at 37°C for the indicated periods of time. (A) Degradation of internalized [¹²⁵I]FGF1 was measured for each time point as the amount of radioactivity in the TCA-soluble fractions and expressed as a percentage of the total radioactivity in the culture. (B) Recycling of [¹²⁵I]FGF1 was measured for each time point as the amount of radioactivity in the TCA-insoluble fraction of the medium and of the high salt/low pH buffer and expressed as a percentage of the total radioactivity in the culture. The experiment was carried out twice with similar results.