Photoaffinity labeling of the beta-adrenergic receptor from cultured lymphoma cells with [125I]iodoazidobenzylpindolol: loss of the label with desensitization.

The beta-adrenergic antagonist [125I]iodoazidobenzylpindolol ( [125I]IABP) specifically photolabeled two polypeptides in membrane preparations from wild-type (WT) and coupling protein-deficient cyc- cultured lymphoma cells. The molecular weights of the two polypeptides determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis were 65,000 and 55,000. They were labeled in a ratio of approximately 1:1. Pretreatment of intact WT or cyc- cells with 1.0 microM epinephrine for 15 min (desensitization) resulted in a greater loss of the 55,000 Mr polypeptide (40-60%) relative to the 65,000 Mr peptide (10-30% loss). An 18- to 24-hr pretreatment of WT cells with terbutaline (down-regulation) led to a greater than 90% reduction of the photolabeling of both polypeptides, whereas a similar pretreatment of cyc- cells resulted in no further loss of labeled receptor than that observed after only a 15-min pretreatment with epinephrine. There was no indication of a change in the electrophoretic mobility of the [125I]IABP-labeled receptors after either short- or long-term agonist pretreatment. These data provide direct evidence for heterogeneity of the beta-adrenergic receptor in lymphoma cells. The differential loss of the [125I]IABP labeling in the two polypeptides suggests a functional heterogeneity as well.