Relaxing action of adrenergic β₂-agonists on guinea-pig skinned tracheal muscle

Abstract

Although adrenergic β₂-agonist-induced smooth muscle relaxation has been attributed to increased intracellular cyclic AMP (cAMP), a relaxation response has been observed at low β₂-agonist concentrations that do not cause increased cAMP. To elucidate the mechanism of tracheal muscle relaxation induced by low concentrations of β₂-agonists, we used a guinea-pig skinned tracheal smooth muscle preparation to examine the effects on the contractile protein system. The isotonic contraction of β-escin-treated skinned tracheal muscle from guinea-pig was measured. When the intracellular Ca²⁺ concentration was maintained at 1 µmol/L in the presence of guanosine 5'-triphosphate (GTP; 100 µmol/L), neither isoproterenol (10 nmol/L) nor salbutamol (60 nmol/L) affected Ca²⁺ sensitivity, but a significant decrease in Ca²⁺ sensitivity was observed in the presence of okadaic acid (1 µmol/L). The decrease in Ca²⁺ sensitivity was a slow response and was blocked by pretreatment with propranolol (1 µmol/L). Forskolin (1 µmol/L) did not affect Ca²⁺ sensitivity. These results suggest that adrenergic β₂-agonists may activate protein phosphatase through an unknown pathway involving the β₂-receptor, which enhances dephosphorylation of the myosin light chain and/or thin filament proteins, resulting in relaxation of the tracheal smooth muscle.