Mechanisms involved in SNP-induced relaxation and [Ca2+](i) reduction in piglet pulmonary and systemic arteries

I We have compared the mechanisms involved in sodium nitroprusside (SNP)-in duced relaxation and [Ca2+](i) reduction in isolated piglet pulmonary (PA) and mesenteric (MA) arteries. 2 SNP (10(-x) M - 3 x 10(-5) M) evoked a concentration-dependent relaxation of PA and MA (pD(2)=6.66+/-0.06 and 6.74+/-0.14, respectively) stimulated by noradrenaline, which was markedly reduced by the guanylate cyclase inhib itor ODQ. In fura 2-incubated PA and MA, SNP produced a parallel reduction in contractile force and in [Ca2+](i), expressed as the ratio of emitted fl uorescence at 340 and 380 nm (F340/F380). 3 The inhibition of the Na+/K+-ATPase after the incubation in a K+-free med ium or the exposure to ouabain (10(-6) M) inhibited SNP-induced relaxation in MA but not in PA. SNP-induced relaxation was not attenuated by 80 mM KCl plus nifedipine (10(-6) M) but was inhibited by thapsigargin (2 x 10 (-6) M; pD(2) = 5.69+/-0.19 and 5.89+/-0.19 for PA and MA, respectively). 4 Pretreatment of PA with thapsigargin and MA with thapsigargin plus ouabai n induced a stronger inhibition on the reduction in [Ca2+](i) than on the r elaxation induced by SNP, indicating the existence of Ca2+-independent mech anisms. 5 The activation of the Na+/K+-ATPase by the addition of KCI after the incu bation in a K+-free medium similarly reduced [Ca2+](i) in PA and MA, wherea s it relaxed with much less efficacy PA than MA. 6 We conclude that SNP red uces [Ca2+]i and causes relaxation through the activation of SERCA in PA an d SERCA and Na+/K+-ATPase in MA. However, Ca2+-independent mechanisms also contribute to SNP-induced effects.