HETEROGENEITY IN BETA-ADRENERGIC-RECEPTOR KINASE EXPRESSION IN THE LUNG ACCOUNTS FOR CELL-SPECIFIC DESENSITIZATION OF THE BETA(2)-ADRENERGIC RECEPTOR

The principal mechanism of homologous desensitization of the beta-adre nergic receptor (beta(2)AR) is phosphorylation of the receptor by the beta AR kinase (beta ARK) or other closely related G protein-coupled r eceptor kinases (GRKs), However, within a single organ such as the lun g where many cell types express the receptor, the presence or extent o f beta(2)AR desensitization in different cells has been noted to be hi ghly variable, We hypothesized that such variability in desensitizatio n is due to significant cell-type differences in PARK expression and/o r function, To approach this, in situ hybridization was carried out in the lung and indeed revealed heterogeneity in beta ARK gene expressio n, Quantitative studies using ribonuclease protection assays with cell lines revealed that the level of beta ARK mRNA in airway smooth muscl e cells was similar to 20% of that in bronchial epithelial cells and s imilar to 11% of that in mast cells (6.65 +/- 0.96 versus 32.6 +/- 4.0 and 60.7 +/- 1.5 relative units, respectively, p < 0.001). beta ARK2 gene expression was not detected in any of these cells, At the protein level, beta ARK expression in airway smooth muscle cells was nearly u ndetectable, being similar to 10-fold less than that expressed on mast cells, The activities of the GRKs in cell extracts were assessed in v itro by quantitating their ability to phosphorylate rhodopsin in the p resence of light, Consistent with the gene and protein expression resu lts, a marked discrepancy in activities was observed between extracts derived from mast cells (90.7 +/- 0.5 relative units) as compared to a irway smooth muscle cells (9.28 +/- 0.6 relative units, p < 0.001), In contrast, the activities of protein kinase A (the other kinase that p hosphorylates beta(2)AR) in these extracts were not different, We pred icted, then, that airway smooth muscle beta(2)AR would undergo minimal short-term (5 min) agonist-promoted desensitization as compared to th e beta(2)AR expressed on mast cells, Mast cell cAMP reached maximal le vels after 90 s and did not further increase over time, indicative of receptor desensitization in this cell, In contrast, cAMP levels of air way smooth muscle cells did not plateau, increasing at a rate of 103 /- 9% per min, consistent with little desensitization over the study p eriod, We conclude that there is significant cell-type variation in ex pression of beta ARK and that such variation is directly related to th e extent of short-term agonist-promoted desensitization of the beta(2) AR.