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

A component of isoprenaline-mediated vasorelaxation in pulmonary arteries is mediated by nitric oxide (NO). We examined the effects of physiological concentrations (</=400 microM) of L-arginine on isoprenaline-induced relaxation in rat pulmonary arteries, and following inhibition of L-arginine uptake with L-lysine. In addition, we examined the role of the endothelium, and whether L-arginine affected acetylcholine (ACh)-induced relaxation. Isoprenaline-induced relaxation was potentiated by 400 microM L-arginine in pulmonary arteries; maximum relaxation was increased from 83+/-4% of initial tone to 94+/-4% (P<0.05). L-lysine (10 mM) not only abolished the potentiation 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 of NO synthase with 100 microM L-NMMA or removal of the endothelium inhibited isoprenaline-induced relaxation to the same extent as L-lysine, and under these conditions the presence or absence of 400 microM L-arginine made no difference. L-lysine had no additional effect when applied in combination with L-NMMA. The effect of extracellular L-arginine was concentration dependent, with an apparent EC(50) of approximately 1-7 microM. Relaxation to the membrane permeant cyclic AMP analogue CPT cyclic AMP was also potentiated by L-arginine and inhibited by L-lysine. There was however no difference in relaxation induced by acetylcholine (ACh) in the presence of L-arginine or L-lysine, and isoprenaline-induced relaxation of mesenteric arteries was unaffected by L-arginine or L-lysine. These results strongly suggest that extracellular L-arginine is critically 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 EC(50) for this effect is in the low micromolar range it is likely to be fully activated in vivo, as plasma L-arginine is >150 microM.