RESUMO
The synthetic nitro-alcohol 2-nitro-1-phenyl-1-propanol (NPP) has endothelium-independent relaxing properties in isolated preparations of rat aorta and mesenteric artery. In this study, we investigated whether the vasodilator effects occur in coronary vessels and explored whether hyperpolarization is involved in the underlying mechanism of NPP-induced smooth muscle relaxation. The relaxing responses were studied in isolated preparations of the left anterior descending coronary (ADC) and the septal coronary (SC) arteries, which had been previously maintained under sustained contraction induced by the thromboxane A2 analogue U-46619. Administered cumulatively, NPP elicited concentration-dependent vasorelaxation with similar potency in both vessels. The relaxant effect remained unaffected by the nitric oxide synthase inhibitor L-NAME, the protein kinase C inhibitor bisindolylmaleimide IV and the Rho-associated protein kinase inhibitor Y-27632. However, it was significantly diminished by the adenylyl cyclase inhibitor MDL-12,330A, the guanylyl cyclase inhibitor ODQ, as well as the K+ channel inhibitors tetraethylammonium and CsCl. In ADC preparations impaled with intracellular micropipettes, NPP hyperpolarized the vascular preparation. When the isolated preparation was precontracted by 5-hydroxytryptamine or 80 mM KCl, NPP-induced relaxation with lower pharmacological potency compared to the vessels contracted by U-46619. In conclusion, NPP exhibits vasorelaxant effects on rat coronary arteries, likely involving pathways that include cyclic nucleotide production and membrane hyperpolarization.
RESUMO
2-Nitro-1-phenyl-1-propanol (NPP) is a nitro alcohol with vasodilator activity in the rat aorta. The present study investigated the vasodilator properties of NPP in small vessels of the mesenteric bed, which, contrary to the aorta, contains resistance vessels. Using myography, isometric contractions were recorded in rings of second- and third-order branches from the rat mesenteric artery. NPP relaxed mesenteric ring preparations that were contracted with phenylephrine, U-46619, and KCl (40mM), resulting in significantly different EC50 values (0.41µM [0.31-0.55µM], 0.16µM [0.10-0.24µM], and 4.50µM [1.86-10.81µM], respectively). NPP-induced vasodilation decreased as the extracellular K+ concentration increased. The pharmacological blockade of K+ channels with tetraethylammonium, BaCl2, CsCl, and apamin also blunted NPP-induced vasorelaxation. In contrast, NPP-induced vasodilation was resistant to indomethacin, L-NG-nitroarginine methyl ester (L-NAME), and endothelium removal, indicating that neither prostaglandins nor the endothelial release of nitric oxide is involved in the relaxant effects of NPP. Conversely, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine hydrochloride (MDL-12,330A), and H-89 reduced NPP-induced vasodilation. Under Ca2+-free conditions, NPP did not alter transient contractions that were evoked by caffeine, but it reduced transient contractions that were evoked by phenylephrine. In mesenteric rings that were loaded with the fluorescent Ca2+ indicator Fluo-4 AM and stimulated with phenylephrine, NPP blunted both contractions and fluorescence signals that were related to cytosolic Ca2+ levels. In conclusion, the vasodilatory actions NPP on mesenteric vessel resistance involved the participation of cyclic nucleotides and the opening of K+ channels.