L-citrulline increases arginase II protein levels and arginase activity in hypoxic piglet pulmonary artery endothelial cells

The L-arginine precursor, L-citrulline, re-couples endothelial nitric oxide synthase, increases nitric oxide production, and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs. L-arginine can induce arginase, which, in turn, may diminish nitric oxide production. Our major purpose was to determine if L-citrulline increases arginase activity in hypoxic piglet pulmonary arterial endothelial cells, and if so, concomitantly impacts the ability to increase endothelial nitric oxide synthase re-coupling and nitric oxide production. Piglet pulmonary arterial endothelial cells were cultured in hypoxic conditions with L-citrulline (0–3 mM) and/or the arginase inhibitor S-(2-boronoethyl)-L-cysteine. We measured arginase activity and nitric oxide production. We assessed endothelial nitric oxide synthase coupling by measuring endothelial nitric oxide synthase dimers and monomers. L-citrulline concentrations ≥0.5 mM increased arginase activity in hypoxic pulmonary arterial endothelial cells. L-citrulline concentrations ≥0.1 mM increased nitric oxide production and concentrations ≥0.5 mM elevated endothelial nitric oxide synthase dimer-to-monomer ratios. Co-treatment with L-citrulline and S-(2-boronoethyl)-L-cysteine elevated endothelial nitric oxide synthase dimer-to-monomer ratios more than sole treatment. Despite inducing arginase, L-citrulline increased nitric oxide production and endothelial nitric oxide synthase coupling in hypoxic piglet pulmonary arterial endothelial cells. However, these dose-dependent findings raise the possibility that there could be L-citrulline concentrations that elevate arginase to levels that negate improvements in endothelial nitric oxide synthase dysfunction. Moreover, our findings suggest that combining an arginase inhibitor with L-citrulline merits evaluation as a treatment for chronic hypoxia-induced pulmonary hypertension.