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-adrenergic receptor (beta(2)AR) is phosphorylation of the receptor by the beta AR kinase (beta ARK) or other closely related G protein-coupled receptor kinases (GRKs), However, within a single organ such as the lung where many cell types express the receptor, the presence or extent of beta(2)AR desensitization in different cells has been noted to be highly variable, We hypothesized that such variability in desensitization is due to significant cell-type differences in PARK expression and/or function, To approach this, in situ hybridization was carried out in the lung and indeed revealed heterogeneity in beta ARK gene expression, Quantitative studies using ribonuclease protection assays with cell lines revealed that the level of beta ARK mRNA in airway smooth muscle cells was similar to 20% of that in bronchial epithelial cells and similar 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 undetectable, being similar to 10-fold less than that expressed on mast cells, The activities of the GRKs in cell extracts were assessed in vitro by quantitating their ability to phosphorylate rhodopsin in the presence of light, Consistent with the gene and protein expression results, a marked discrepancy in activities was observed between extracts derived from mast cells (90.7 +/- 0.5 relative units) as compared to airway smooth muscle cells (9.28 +/- 0.6 relative units, p < 0.001), In contrast, the activities of protein kinase A (the other kinase that phosphorylates beta(2)AR) in these extracts were not different, We predicted, then, that airway smooth muscle beta(2)AR would undergo minimal short-term (5 min) agonist-promoted desensitization as compared to the beta(2)AR expressed on mast cells, Mast cell cAMP reached maximal levels after 90 s and did not further increase over time, indicative of receptor desensitization in this cell, In contrast, cAMP levels of airway smooth muscle cells did not plateau, increasing at a rate of 103 +/- 9% per min, consistent with little desensitization over the study period, We conclude that there is significant cell-type variation in expression of beta ARK and that such variation is directly related to the extent of short-term agonist-promoted desensitization of the beta(2)AR.