Different mechanisms underlying the stimulation of KCa channels by nitric oxide and carbon monoxide

The molecular mechanisms underlying the effects of nitric oxide (NO) and carbon monoxide (CO), individually and collectively, on large-conductance calcium-activated K+ (KCa) channels were investigated in rat vascular smooth muscle cells (SMCs). Both NO and CO increased the activity of native KCa channels. Dehydrosoyasaponin-I, a specific agonist for β subunit of KCa channels, increased the open probability of native KCa channels only when it was delivered to the cytoplasmic surface of membrane. CO, but not NO, further increased the activity of native KCa channels that had been maximally stimulated by dehydrosoyasaponin-I. After treatment of SMCs with anti–KCa,β subunit antisense oligodeoxynucleotides, the stimulatory effect of NO, but not of CO, on KCa channels was nullified. CO, but not NO, enhanced the KCa current densities of heterologously expressed cloned KCa,α subunit, showing that the presence of KCa,β subunit is not a necessity for the effect of CO but essential for that of NO. Finally, pretreatment of SMCs with NO abolished the effects of subsequently applied CO or diethyl pyrocarbonate on KCa channels. In summary, the stimulatory effects of CO and NO on KCa channels rely on the specific interactions of these gases with KCa,α and KCa,β subunits.