AGONIST-INDUCED DESENSITIZATION OF BETA-ADRENOCEPTORS OF BOVINE BRONCHIAL EPITHELIAL-CELLS

1. Cultured bovine bronchial epithelial cells express a large number o f beta-adrenoceptors of the beta2 subtype which are coupled to activat ion of adenylate cyclase and synthesis of cyclic AMP. The mechanisms i nvolved in agonist-induced desensitization of beta-adrenoceptor functi on in these cells were investigated. 2. Preincubation of cells with th e beta-adrenoceptor agonist isoprenaline induced a rapid desensitizati on of adenylate cyclase activity stimulated by isoprenaline (40% at 30 min) with little or no decrease in forskolin stimulation or in the nu mber of beta-adrenoceptors detected by radioligand binding, indicating that uncoupling of beta-adrenoceptors from adenylate cyclase had occu rred. 3. Uncoupling of beta-adrenoceptors from the stimulatory guanine -nucleotide-binding protein G(s) was indicated by the loss of GTP effe cts on agonist competition for radioligand binding. Results from subce llular fractionation by sucrose density gradient centrifugation sugges t that rapid internalization or sequestration of beta-adrenoceptors oc curred along with the desensitization. 4. Preincubation with isoprenal ine for longer times led to a marked decrease in total receptor number (85% at 18 h), indicating that receptor down-regulation also occurs. Recovery of receptors after down-regulation was prevented by cyclohexi mide and by actinomycin D, suggesting that degradation of both recepto r protein and receptor mRNA may be involved in down-regulation.