Variation in the Pulmonary Hypertensive Responsiveness of Broilers to Lipopolysaccharide and Innate Variation in Nitric Oxide Production by Mononuclear Cells

Abstract Variability among broilers in their pulmonary hypertensive (PH) responsiveness to lipopolysaccharide (LPS) appears to reflect innate variation in the types or proportions of vasodilators and vasoconstrictors released by leukocytes and endothelial cells. Two experiments were designed to evaluate possible correlations between the PH responsiveness to LPS in vivo and the quantities of nitric oxide (NO; a potent pulmonary vasodilator) produced by mononuclear cells in vitro. In Experiment 1, blood samples were collected from male broilers from a base population (control group) and from survivors of a 60% lethal dose i.v. injection of cellulose microparticles (MP survivor group). In Experiment 2, blood samples were collected from male broilers from a relaxed line and from lines known to be susceptible or resistant to pulmonary hypertension syndrome. Peripheral mononuclear cells (PMNC) from each blood sample were cultured at 2 million cells per well, remained unstimulated, or were stimulated with LPS to elicit the expression of inducible NO synthase, and the 24-h production of NO was measured. In both experiments, unstimulated PMNC cultures did not produce consistently detectable levels of NO, whereas LPS-stimulated cultures produced quantities of NO that varied widely among individuals. Nitric oxide production by cultured PMNC also was evaluated by flow cytometry, demonstrating that LPS-stimulated PMNC produced substantially more NO than did unstimulated cells in all of the groups evaluated. Moreover, NO-producing PMNC were identified to be monocytes. The same broilers from which PMNC had been isolated were catheterized subsequently to record pulmonary arterial pressure, LPS was injected i.v. to assess the amplitudes of peak and postpeak PH responses, then Nω-nitro- l -arginine methyl ester was injected to inhibit ongoing NO production. In Experiment 1, the amplitude of the peak and postpeak PH responses to LPS were correlated with the quantity of NO produced by LPS-stimulated cultured PMNC from broilers in the control group but not for MP survivors. In Experiment 2, the postpeak PH response to LPS was correlated with the quantity of NO produced by LPS-stimulated PMNC from broilers in the relaxed line, but not in the susceptible or resistant lines. In all groups, Nω-nitro- l -arginine methyl ester injections triggered substantial increases in pulmonary arterial pressure (≥8 mm Hg), thereby revealing a significant ongoing modulation by NO of the PH response to LPS. We concluded that most of the modulatory NO generated in vivo during the acute PH response to LPS (within 60 min postinjection) likely is produced by constitutive NO synthase in the vascular endothelium. In addition, the NO produced by inducible NO synthase in PMNC appeared to have modulated the LPS-stimulated PH responses of unselected broilers having the broadest range of pulmonary vascular capacities (control broilers and relaxed line), but not in broilers whose pulmonary vascular capacities had been selected to represent the higher (MP survivors, resistant line) or lower (susceptible line) extremes of the population.