Excessive nitric oxide (NO) production by inducible NO synthase has been implicated in the hyporesponsiveness to vasoconstrictors present in septic shock. Here we show that a brief incubation (30 min) of rat aorta rings with NO donors renders the vessels hyporesponsive to phenylephrine for several hours. Contraction of rings without endothelium by phenylephrine (0.1 nM to 100 μM) was decreased by 50-60% after incubation (30 min) with sodium nitroprusside (3-300 μM) or S-nitroso-acetyl-D,L-penicillamine (SNAP; 70-200 μM). This decrease was characterized by reductions in maximal response and rightwards shifts of phenylephrine concentration/response curves, present even 130 min after NO donor removal. Soluble guanylate cyclase inhibitors methylene blue (10 μM) and 1H-(1,2,4)-oxadiazol-(4,3-a)-quinoxalin-1-one (ODQ, 1 μM) or the potassium channel blockers TEA (tetraethylammonium; 10 mM) and charybdotoxin (100 nM) inhibited the hyporesponsiveness to phenylephrine induced by the NO donors. In contrast, 4-aminopyridine (1 mM) and glibenclamide (10 μM) had no effect. Our results show that incubation with NO donors reproduces the hyporesponsiveness to phenylephrine and that NO alone 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 phenylephrine in the rat aorta.
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Acknowledgements We thank Dr. Cristina O. Salgado (FMRP/ USP, Brazil) for the gift of TEA and 4-AP, to Dr. Fernando de Q. Cunha (FMRP/USP, Brazil) for the gift of ODQ and to Dr. João B. Calixto (UFSC, Brazil) by allowing us to use his force transducers and recorder. MRT wishes to thank Drs. M. C. A. Marques and L. Rieck for their support and encouragement. This work was supported partially by fellowships from CNPq (Brazil).
- Guanylate cyclase
- Nitric oxide
- Potassium channel