Sodium nitroprusside stimulates Ca2+ -activated K+ channels in porcine tracheal smooth muscle cells

M. Yamakage, C. A. Hirshman and T. L. Croxton Department of Anesthesiology, The Johns Hopkins Medical Institutions, Baltimore, Maryland 21205, USA. To directly investigate the possible role of large-conductance Ca2+ -activated K+ (KCa) channels in nitro-vasodilator-induced relaxation of airway smooth muscle, we used cell-attached patch-clamp techniques to test the effects of sodium nitroprusside (SNP) on KCa channels in freshly dispersed porcine tracheal smooth muscle cells. Channel open-state probability (nPo) increased approximately 13-fold with exposure to 10(-5) M SNP, and this was partially reversed by addition of the guanylate cyclase inhibitors methylene blue (3 X 10(-4) M) or LY-83583 (5 X 10(-5) M). Pretreatment with the guanosine 3',5' -cyclic monophosphate (cGMP)-dependent protein kinase (G kinase) inhibitor Rp-8-(p-chlorophenylthio) cGMP-phosphorothioate (2 X 10(-5) M) prevented activation of KCa channels by SNP. We also tested the ability of G kinase to directly activate KCa channels in inside-out patches. G kinase (2.5 U/microliter) with ATP (0.5 mM) and cGMP (0.1 mM), but not ATP and cGMP alone, increased nPo approximately 23-fold. We conclude that SNP activates KCa channels in airway smooth muscle via guanylate cyclase and G kinase. Phosphorylation of the channel protein by G kinase may account for this response. Consequent membrane hyperpolarization and inhibition of Ca2+ entry through voltage-dependent channels may contribute to SNP-induced relaxation of airway smooth muscle.