Inhibition of endogenous nitric oxide synthesis potentiates the effects of sodium nitroprusside but not of adenosine in experimental pulmonary hypertension

This study examined the systemic and pulmonary vasodilator effects of sodium nitroprusside (SNP) and adenosine during experimental pulmonary hypertension with and without inhibition of endogenous NO synthesis. Male New Zealand White rabbits were anesthetized and mechanically ventilated. The NO synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) was administered to 15 of the 28 rabbits. Pulmonary hypertension was then produced in all rabbits by U46619, a thromboxane A₂ mimetic. SNP was infused in 14 rabbits (7 L-NAME, 7 control) at doses of 0.5-20 μg/kg/min; adenosine was infused in the other 14 rabbits (8 L-NAME, 6 control) at doses of 12.5-300 μg/kg/min. The U46619 dose required to produce pulmonary hypertension was significantly lower in the L-NAME group. SNP dose-dependently decreased pulmonary (Ppa) and systemic (Psa) artery pressures and systemic vascular resistance (SVR). Both Ppa and Psa were decreased more with SNP in the L-NAME than in the no L-NAME group. The SNP ED₅₀ for the decrease in PVR was almost threefold lower in the L-NAME group. Adenosine dose-dependently decreased Ppa, Psa, PVR and SVR. The adenosine ED₅₀ for the decreases in PVR and SVR were similar in the L-NAME group and the control group. We conclude that inhibition of endogenous NO synthesis shifts the dose-response curves for both the pulmonary and systemic vasodilator effects to the left for the nitrovasodilator SNP but not for the non-nitrovasodilator adenosine.