Differential involvement of guanylate cyclase and potassium channels in nitric oxide-induced hyporesponsiveness to phenylephrine in endotoxemic rats

This study evaluated the involvement of nitric oxide (NO), guanylate cyclase, and potassium channels in the long-lasting vascular hyporesponsiveness to phenylephrine induced by Escherichia coli lipopolysaccharide (LPS) in vitro and in vivo. Experiments in rat aorta rings with endothelium incubated with LPS (10 mug/mL) for 12 h showed that the hyporesponsiveness depends on guanylate cyclase activity and tetraethylammonium-sensitive, but not voltage- or ATP-dependent, potassium channels. Pressor responses to phenylephrine were reduced by 50% in rats injected 8 and 24 h before with LPS (10 mg/kg, intraperitoneally), Pretreatment with NO synthase inhibitors (NOS; N-omega-nitro-L-arginine methyl ester [L-NAME], 55 mu mol/kg or aminoguanidine, 244 mu mol/kg, intraperitoneally) fully prevented LPS-incluced hyporesponsiveness. When administered just before phenylephrine, L-NAME (11 mu mol/kg, intravenously) reversed the hyporesponsiveness in rats injected 8 h, but not in those injected 24 h before with LPS, whereas 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1 (ODQ, 11 mu mol/kg, intravenously) reversed the hyporesponsiveness in animals injected 24 h, but not in those injected 8 h before with LPS. Tetraethylammonium (360 mu mol/kg, intravenously) reestablished normal responses to phenylephrine in rats injected 8 and 24 h before with LPS. Again, neither voltage- nor ATP-dependent potassium channels appears to be involved. Western blot showed that NOS expression peaked at 8 h, decreasing to low levels 24 h after LPS injection. Therefore, NO is important in initiating LPS-induced hyporesponsiveness to vasoconstrictors, but not in maintaining it for long periods. Once NO has exerted its effects and even when NOS expression is minimal, the long-lasting hyporesponsiveness appears to depend on a complex interplay between guanylate cyclase and potassium channel activation.