Possible stimulation of Na+-K+-ATPase by NO produced from sodium nitrite by ultraviolet light in mouse gastric fundal strip.

1. In the present study, we investigated the roles of Na+-K+-ATPase and extracellular Na+ or Ca2+ ions in ultraviolet (UV) light-induced photorelaxation of methacholine-contracted mouse isolated gastric fundus in the presence of NaNO2 (50 microM). 2. Ouabain (1-500 microM), sodium vanadate (10 microM to 3 mM) and amiloride (1-100 microM) completely inhibited the photorelaxation in a concentration-dependent manner. 3. Metabolic inhibitors, sodium azide (10-100 microM), 2,4-dinitrophenol (100 microM to 1 mM) and sodium fluoride (100 microM to 1 mM) significantly reduced photorelaxation. 4. Substitution of sucrose, lithium or KCl with extracellular Na+ completely abolished the photorelaxant responses. 5. Replacement of all extracellular CaCl2 with BaCl2 also completely inhibited UV-induced relaxation. 6. Verapamil (1-10 microM) decreased UV-induced relaxation significantly. 7. These results suggest that nitric oxide produced from NaNO2 by UV-light in mouse gastric fundus probably stimulates Na+-K+-ATPase activity, and photorelaxation of gastric smooth muscle is dependent on extracellular Na+ and Ca2+ ions.