Fig. 7. Endocytosis and recycling of a cell surface receptor. EcR293 pIND-Hygro/EGFP-rab5aQ79L cells were stably transfected with pcDNA3.1/ß₂AR, then incubated with either vehicle () or 5 µM ponasterone () for 72 hours. The cells were treated with 10 µM isoproterenol for varying times, then chilled, washed and kept on ice with 6 nM [³H]CGP12177 for 90 minutes to selectively label surface receptors. The fraction of receptors left on the surface was plotted versus time of agonist exposure, and the curves modeled as described (Morrison et al., 1996) to obtain estimates for the first-order rate constants of endocytosis (k_e) and recycling (k_r). The rate constant k_e for control cells was 0.199±0.010 minutes^–1 and for ponasterone-treated cells, 0.151±0.005 minutes^–1 (significantly different, P=0.029). The rate constant k_r was 0.053±0.005 minutes^–1, and for ponasterone-treated cells, 0.038±0.010 minutes^–1 (not significant, P=0.1005) (n=3).