Critical dependence of the NO-mediated component of cyclic AMP-induced vasorelaxation on extracellular L-arginine in pulmonary arteries of the rat

1 A component of isoprenaline-mediated vasorelaxation in pulmonary arteries is mediated by nitric oxide (NO). We examined the effects of physiological concentrations (less than or equal to 400 mu M) of L-arginine on isoprenal ine-induced relaxation in rat pulmonary arteries, and following inhibition of L-arginine uptake with L-lysine. In addition, we examined the role of th e endothelium, and whether L-arginine affected acetylcholine (ACh)-induced relaxation. 2 Isoprenaline-induced relaxation was potentiated by 400 mu M L-arginine in pulmonary arteries; maximum relaxation was increased from 83+/-4% of initi al tone to 94+/-4% (P<0.05). L-lysine (10 mM) not only abolished the potent iation by L-arginine, but suppressed relaxation compared to control (70+/-4 %, P<0.05), even in the absence of L-arginine added to the bath. Blockade o f NO synthase with 100 mu M L-NMMA or removal of the endothelium inhibited isoprenaline-induced relaxation to the same extent as L-lysine, and under t hese conditions the presence or absence of 400 mu M L-arginine made no diff erence. L-lysine had no additional effect when applied in combination with L-NMMA. 3 The effect of extracellular L-arginine was concentration dependent, with an apparent EC50 of similar to 1-7 mu M. 4 Relaxation to the membrane permeant cyclic AMP analogue CPT cyclic AMP wa s also potentiated by L-arginine and inhibited by L-lysine. There was howev er no difference in relaxation induced by acetylcholine (ACh) in the presen ce of L-arginine or L-lysine, and isoprenaline-induced relaxation of mesent eric arteries was unaffected by L-arginine or L-lysine. 5 These results strongly suggest that extracellular L-arginine is criticall y important for development of the NO- and endothelium-dependent component of cyclic AMP-induced vasorelaxation in rat pulmonary arteries, but is not required for ACh-induced relaxation. As the apparent EC50 for this effect i s in the low micromolar range it is likely to be fully activated in vivo, a s plasma L-arginine is >150 mu M.