Potentiation of β-adrenoceptor function in bovine tracheal smooth muscle by inhibition of protein kinase C

To examine the role of contractile agonist-induced activation of protein kinase C (PKC) in functional antagonism of airway smooth muscle contraction by β-adrenoceptor agonists, we examined the effects of the specific PKC-inhibitor GF 109203X (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl) maleimide) on isoprenaline-induced relaxation of bovine tracheal smooth muscle contracted by various concentrations of methacholine and histamine. In the absence of GF 109203X, the potency of isoprenaline (pD2) was gradually reduced at increasing methacholine- and histamine-induced smooth muscle tones, but the maximal relaxation (Emax) was decreased only at higher concentrations of methacholine. In the presence of GF 109203X, pD2 values were significantly increased for both methacholine- and histamine-induced contractions. Moreover, isoprenaline Emax values in the presence of high concentrations of methacholine were also increased. Although both methacholine- and histamine-induced contractions were slightly reduced by GF 109203X, the changes in isoprenaline pD2 could only partially be explained by reduced contractile tone. In contrast to isoprenaline, forskolin-induced relaxations were not affected by GF 109203X. The results indicate that PKC activation contributes to the reduced β-adrenergic responsiveness induced by methacholine and histamine, which may involve uncoupling of the β-adrenoceptor from the effector system. Since many mediators and neurotransmitters in allergic airway inflammation can activate PKC, this cross talk may be important in the reduced bronchodilator response of patients with severe asthma.