Endotoxin-induced Nonendothelial Nitric Oxide Activities the Ca2+-activated K+ Channel in Cultured Vascular Smooth Muscle Cells

Endotoxin induces an enzyme that synthesizes nitric oxide (NO) from L-arginine (NO synthase) in vascular smooth muscle cells, resulting in nonendothelial NO release. In this study, we measured the NO release and its intracellular action on the Ca2+-activated K+ channel (KCa channel) in cultured smooth muscle cells of porcine coronary artery using a newly-developed porphyrinic-based microsensor and the patch-clamp technique. In smooth muscle cells pretreated with endotoxin, extracellular application of 10-4M L-arginine increased NO release, which induced rapid and prolonged activation of the KCa channel. This activation was only partially blocked by application of 10-5M 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-oxyl 3-oxide, which neutralizes NO. NO formation and activation of the KCa channel were suppressed by pretreatment with 10-3M NG-methyl-L-arginine or 10-3M Nω-nitro-L-arginine methyl ester, each of which is a specific antagonist of the L-arginine-NO pathway. One micromolar methylene blue, a blocker of guanylate cyclase, inhibited L-arginine-induced activation of the KCa channel. The effect of nitroprusside in opening the KCa channel was transient, although it induced production of larger amounts of NO in the bath. These results suggest that the endotoxin-induced and L-arginine pathway generates NO and directly modulates the KCa channel intracellularly in an autocrine manner.