Regulation of Ca2+ -dependent K+ -channel activity in tracheal myocytes by phosphorylation

Abstract

ISOPRENALINE is a (β-adrenergic agonist of clinical importance as a remedy for asthma. In airway smooth muscle its relaxant action is accompanied by hyperpolarization of the membrane^1,2 and elevation of the level of intracellular cyclic AMP³. Hyper-polarization and relaxation are also induced by drugs such as forskolin, theophylline and dibutyryl cAMP, indicating that cAMP-dependent phosphorylation is involved in producing the electrical response¹. Cyclic AMP-dependent protein kinase (pro-tein kinase A) has been reported to activate Ca²⁺-dependent K⁺ channels in cultured aortic smooth muscle cells⁴ and snail neurons^5,6. The membrane of tracheal smooth-muscle cells is characterized by a dense distribution of Ca²⁺-dependent K⁺-channels⁷. We have now examined the effect of isoprenaline and protein kinase A on Ca²⁺-dependent K⁺-channels in isolated smooth muscle cells of rabbit trachea, using the patch-clamp technique⁸. Our results show that the open-state probability of Ca²⁺-dependent K⁺-channel of tracheal myocytes is reversibly increased by either extracellular application of isoprenaline or intracellar application of protein kinase A. We also show that this effect is significantly enhanced and prolonged in the presence of a potent protein phos-phatase inhibitor, okadaic acid^9–13.