Nω-nitro-L-arginine methyl ester (L-NAME) amplifies the pulmonary hypertensive response to endotoxin in broilers

The pulmonary hypertensive response to bacterial lipopolysaccharide (LPS, endotoxin) varies widely among individual broilers, leading to the suggestion that innate variability may exist in the proportions or profiles of chemical mediators released during the ensuing inflammatory cascade. LPS induces the expression of nitric oxide synthase (iNOS), which produces the vasodilator nitric oxide (NO) to modulate the responses to concurrently produced vasoconstrictors. In experiment 1, broilers were given the NOS inhibitor N-w-nitro-L-arginine methyl ester (L-NAME), followed by a supra-maximal dose of LPS while the pulmonary arterial pressure was recorded. In experiment 2 the cardiac output also was recorded before and following the i.v. injection Of L-NAME. In both experiments, injection with L-NAME modestly increased the pulmonary arterial pressure when compared with control values, confirming previous reports that tonic/basal NO synthesis is required to promote flow-dependent pulmonary vasodilation in chickens. This response to L-NAME occurred in spite of a tendency for cardiac output and stroke volume to decline and, therefore, can be attributed to pulmonary vasoconstriction (an increase in the pulmonary vascular resistance) rather than an increase in pulmonary blood flow. When L-NAME was used to block NO synthesis induced by LPS, an early peak of pulmonary hypertension was revealed that rarely develops in broilers in the absence of L-NAME, and that has been correlated with the release of platelet activating factor and thromboxane A(2) in mammals. The control group responded to LPS with a delayed-onset pulmonary hypertension that was typical in timing, amplitude, and duration of the responses previously observed in broilers and that has been attributed to endothelin-mediated thromboxane A(2) synthesis in mammals. This delayed-onset pulmonary hypertensive response to LPS was longer in duration and higher in amplitude in the L-NAME group when compared with the control group. These observations are consistent with the hypothesis that NO modulates the responses to vasoconstrictors released concurrently during the LPS-mediated inflammatory cascade. Inhibition of NOS by L-NAME apparently reduced the modulatory influence of NO and exposed a more dramatic pulmonary hypertensive response to LPS.