Evidence that the anti‐spasmogenic effect of the <i>β</i>‐adrenoceptor agonist, isoprenaline, on guinea‐pig trachealis is not mediated by cyclic AMP‐dependent protein kinase

The spasmolytic and anti‐spasmogenic activity of β ‐adrenoceptor agonists on airways smooth muscle is thought to involve activation of the cyclic AMP/cyclic AMP‐dependent protein kinase (PKA) cascade. Here we have tested the hypothesis that PKA mediates the anti‐spasmogenic activity of isoprenaline and other cyclic AMP‐elevating agents in guinea‐pig isolated trachea by utilizing a number of cell permeant cyclic AMP analogues that act as competitive ‘antagonists’ of PKA. Anion‐exchange chromatography of guinea‐pig tracheae resolved two peaks of PKA activity that corresponded to the type I (∼5%) and type II (∼93%) isoenzymes. Pre‐treatment of tracheae with zardaverine (30 μ M ), vasoactive intestinal peptide (VIP) (1 μ M ) and the non‐selective activator of PKA, Sp ‐8‐CPT‐cAMPS (10 μ M ), produced a non‐parallel rightwards shift in the concentration‐response curves that described acetylcholine (ACh)‐induced tension generation. The type II‐selective PKA inhibitor, Rp ‐8‐CPT‐cAMPS (300 μ M ), abolished this effect. Pre‐treatment of tracheae with Sp ‐8‐Br‐PET‐cGMPS (30 μ M ) produced a non‐parallel rightwards shift of the concentration‐response curves that described ACh‐induced tension generation. The selective cyclic GMP‐dependent protein kinase (PKG) inhibitor, Rp ‐8‐pCPT‐cGMPS (300 μ M ), abolished this effect. Pre‐treatment of tracheae with isoprenaline (1 μ M ) produced a 10 fold shift to the right of the ACh concentration‐response curve by a mechanism that was unaffected by Rp ‐8‐Br‐cAMPS (300 μ M , selective inhibitor of type I PKA), Rp ‐8‐CPT‐cAMPS (300 μ M ) and Rp ‐8‐pCPT‐cGMPS (300 μ M ). We conclude that the anti‐spasmogenic activity of Sp ‐8‐CPT‐cAMPS, zardaverine and VIP in guinea‐pig trachea is attributable to activation of the cyclic AMP/PKA cascade whereas isoprenaline suppresses ACh‐induced contractions by a mechanism(s) that is independent of PKA and PKG. British Journal of Pharmacology (2001) 133 , 1201–1212; doi: 10.1038/sj.bjp.0704213