L-ARGININE RESTORES ENDOTHELIUM-DEPENDENT RELAXATION IN PULMONARY CIRCULATION OF CHRONICALLY HYPOXIC RATS

We investigated whether loss of endothelial-derived relaxing factor (EDRF) activity in the pulmonary vessels of chronically hypoxic rats could be restored by pretreatment with L-arginine. We measured vasodilation to acetylcholine (ACh), calcium ionophore A23187, or linsidomine (Sin-1) under conditions of increased vascular tone induced by U-46619 (50 pmol/min), as well as vasoconstriction to endothelin-1 (ET) in isolated lungs pretreated with meclofenamate (3-mu-M). In lungs from normoxic (N) rats, in vitro L- or D-arginine (10(-3) M) did not alter vasodilation to the endothelium-dependent agents ACh (10(-9)-10(-6) M) and A23187 (10(-9)-10(-7) M), but N(G)-monomethyl-L-arginine (10(-3) M) completely abolished it. In lungs from rats exposed to 3 wk of hypoxia (H), vasodilation to ACh or A23187 was fully restored after in vitro L-arginine (10(-3) M) or N(alpha)-benzoyl-L-arginine (5 x 10(-5) M) but remained abolished after D-arginine, L-Citrulline, L-ornithine, or L-argininosuccinic acid. In vivo pretreatment of H rats with L-arginine (300 mg/kg iv) 30 min before isolating the lung also restored vasodilation to A23187. Vasodilation to the endothelium-independent agent Sin-1 was similar in both groups of lungs and was not altered by in vitro L-arginine. L-arginine attenuated the increased pressor response to ET (300 pmol) of H rat lungs but had no effect in N rats. Our results demonstrate that loss of EDRF activity associated with hypoxic pulmonary hypertension may be reversed by supplying L-arginine.