Lateral motion of beta receptors in membranes of cultured liver cells.

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Abstract
The lateral mobility and distribution of .beta. receptors on Chang human liver cells were studied by fluorescence photobleaching recovery and video intensification microscopy. The .beta. receptors were labeled with the fluorescent antagonist 7-(2-allylphenoxy)-2,2-dimethyl-6-hydroxy-1-(4-nitrobenzo-2-oxa-1,3-diazolyl)-1,4-diazaheptane (Alp-NBD). Of the staining, 60-75% was specific (displaceable by unlabeled antagonists). Most of the antagonist-occupied .beta. receptors were immobile, because only 15-25% of their fluorescence recovered on the experimental time scale at 23.degree. C. This immobility correlates with the clustered distribution of Alp-NBD-.beta.-receptor complexes at 4.degree. and 37.degree. C. The .beta. receptors appear to be aggregated prior to antagonist binding, because visible patches were observed immediately after labeling for 30 s at 4.degree. C. Preincubation at 37.degree. C with (-)-isoproterenol, a .beta. agonist, prior to Alp-NBD labeling induced a time-dependent release of the .beta. receptors to a more homogeneous distribution and increased the mobile fraction to 70-80% (lateral diffusion coefficient = 1.4 .times. 10-9 cm2/s at 23.degree. C). This is not due to an effect on membrane fluidity, because the diffusion coefficient of a lipid probe was not altered. The time course of agonist-induced .beta.-receptor mobilization correlates with receptor loss and adenylate cyclase desensitization but is much slower than adenylate cyclase activation. Apparently, adenylate cyclase activation by .beta. receptors does not require macroscopic lateral mobility of the majority of the .beta. receptors.