Protein kinase C potentiates homologous desensitization of the β2-adrenoceptor in bovine tracheal smooth muscle

Preincubation (30 min) of bovine tracheal smooth muscle with various concentrations (0.1, 1 and 10 μM) of fenoterol decreased isoprenaline-induced maximal relaxation (Emax) of methacholine-contracted preparations in a concentration dependent fashion, indicating desensitization of the β2-adrenoceptor. Preincubation with 1 μM of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) caused a small but significant decrease in isoprenaline-induced Emax, indicating activated PKC-mediated heterologous β2-adrenoceptor desensitization. To investigate the capacity of activated PKC to regulate homologous desensitization, we incubated the smooth muscle strips with the combination of both 1 μM PMA and 1 μM fenoterol. This combined treatment synergistically decreased the isoprenaline-induced maximal relaxation, as compared to the individual effects of PMA and fenoterol alone, indicating a common pathway for heterologous and homologous desensitization. Moreover, the specific PKC-inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl) maleimide (GF 109203X) markedly increased the potency and Emax of isoprenaline for all conditions used, including control conditions, and the synergistic effects of PMA and fenoterol were completely prevented. In conclusion, the present study demonstrates that homologous desensitization of the β2-adrenergic receptor can be enhanced by PKC activation. For the first time we have provided evidence that this concept is functionally operative in airway smooth muscle, and it may explain the reduced bronchodilator response to β2-adrenoceptor agonists in patients with asthma during a severe exacerbation.