Involvement of soluble guanylate cyclase and calcium-activated potassium channels in the long-lasting hyporesponsiveness to phenylephrine induced by nitric oxide in rat aorta

Excessive nitric oxide (NO) production by inducible NO synthase has been im plicated in the hyporesponsiveness to vasoconstrictors present in septic sh ock. Here we show that a brief incubation (30 min) of rat aorta rings with NO donors renders the vessels hyporesponsive to phenylephrine for several h ours. Contraction of rings without endothelium by phenylephrine (0.1 nM to 100 mu M) was decreased by 50-60% after incubation (30 min) with sodium nit roprusside (3-300 mu M) or S-nitroso-acetyl-D,L-penicillamine (SNAP; 70-200 mu M). This decrease was characterized by reductions in maximal response a nd rightwards shifts of phenylephrine concentration/response curves, presen t even 130 min after NO donor removal. Soluble guanylate cyclase inhibitors methylene blue (10 mu M) and 1H-(1,2,4)-oxadiazol-(4,3-a)-quinoxalin-1-one (ODQ, 1 mu M) or the potassium channel blockers TEA (tetraethylammonium; 1 0 mM) and charybdotoxin (100 nM) inhibited the hyporesponsiveness to phenyl ephrine induced by the NO donors. In contrast, 4-aminopyridine (1 mM) and g libenclamide (10 mu M) had no effect. Our results show that incubation with NO donors reproduces the hyporesponsiveness to phenylephrine and that NO a lone accounts for most, if not all, the refractoriness to vasoconstrictors present in septic shock. In addition, soluble guanylate cyclase activation and opening of potassium channels, more specifically the calcium-activated subtype, play a predominant role in this NO-induced hyporesponsiveness to p henylephrine in the rat aorta.