Fig. 1. Comparison of agonist-mediated internalization of receptor subtypes. Cells stably expressing ß₁AR () or ß₂AR () were exposed to 1 µM ISO for the indicated times or 30 minutes, washed and assayed for [³H]CGP-12177 binding to surface receptors as described in Materials and Methods. (A) Time course of internalization in BHK-hß₁ and -hß₂ cells expressing 978±47 (n=6) and 1010±84 (n=5) fmol of ßAR/mg protein. (B) Effect of ßAR density on internalization. Different clonal lines of BHK-hß₁ and -hß₂ cells were assayed for the amount of internalization after 30 minutes of agonist treatment. Some were induced with zinc sulfate for 24 hours. Internalization is plotted as a function of initial surface receptor density. The slopes of the linear regression lines are 196 and 454 fmol/30 minutes/pmol, a difference of 2.3-fold. (C) Same as A except for HEK 293-hß₁ and -hß₂ cells expressing 1790±42 (n=3) and 1580±31 (n=4) fmol of ßAR/mg protein. (D) Same as A except for Arr2-BHK-hß₁ and -hß₂ expressing 1680±204 (n=6) and 1310±169 (n=5) fmol of ßAR/mg protein.