Role of potassium channels in relaxations of canine middle cerebral arteries induced by nitric oxide donors

Background and Purpose: The mechanisms underlying smooth muscle relaxations of cerebral arteries in response to nitric oxide (NO) and cyclic GMP (cGMP) are still not completely understood. The present study was designed to determine the role of potassium channels in the relaxations to NO donors 3-morpholinosydnonimine (SIN-1) and sodium nitroprusside (SNP), as well as 8-bromo-3',5'-cGMP (a synthetic analogue of cGMP) and zaprinast (a selective cGMP phosphodiesterase inhibitor). Methods: Rings of canine middle cerebral arteries without endothelium were suspended to Krebs-Ringer bicarbonate solution for isometric tension recording. The levels of cGMP were measured by radioimmunoassay. Relaxations to NO donors 8-bromo-cGMP and zaprinast were studied in the presence and in the absence of K⁺ channel blockers charybdotoxin (large-conductance Ca²⁺-activated K⁺ channels), glyburide (ATP-sensitive K⁺ channels), 4-aminopyridine (delayed rectifier K⁺ channels), and BaCl₂ (multiple types of K⁺ channels). Results: Concentration-dependent relaxations caused by NO donors (SIN-1 and SNP) were significantly reduced in arteries treated with BaCl₂ (3x10^-4 mol/L) or charybdotoxin (3x10^-8 mol/L). Relaxations to 8-bromo-cGMP were not affected by the same concentrations of BaCl₂ and charybdotoxin; however, they were reduced by higher concentrations of BaCl₂ (3x10^-3 mol/L) and charybdotoxin (10^-7 mol/L). Zaprinast-induced relaxations were significantly reduced by BaCl₂ (3x10^-4 mol/L) or charybdotoxin (3x10^-8 mol/L). Glyburide (10^-3 mol/L) and 4-aminopyridine (10^-3 mol/L) did not alter the relaxations to SIN-1 or SNP. The production of cGMP stimulated by SIN-1 in the vascular smooth muscle was not affected by BaCl₂ (3x10^-3 mol/L) or charybdotoxin (10^-3 mol/L). Conclusions: These results indicate that in canine middle cerebral arteries, a significant portion of relaxations to NO liberated from nitrovasodilators is mediated by large-conductance Ca²⁺-activated K⁺ channels. Other K⁺ channels, sensitive to BaCl₂ may also be involved in the mechanism of relaxations induced by NO.