Regional changes in constitutive nitric oxide synthase and the hemodynamicconsequences of its inhibition in lipopolysaccharide-treated pigs

The role of constitutive nitric oxide synthases (cNOS) in sepsis remains co ntroversial. Part of the problem is that many of the studies have been perf ormed in rats, which respond differently than larger animals. Our objective , therefore, was to determine whether cNOS, i.e. ecNOS (NOS-3) and nNOS (NO S-1) are still active in vessels of pigs treated with lipopolysaccharide (L PS) from Escherichia coli. We also characterized the dose-response relation ship of the NOS inhibitor N-G-nitro-L-arginine-methyl-ester (L-NAME) in the arterial, venous, and pulmonary circuits as a reflection of NO production. We anesthetized and ventilated 14 pigs, which were instrumented for hemody namic measurements. We measured mean circulatory filling pressure and resis tance to venous return by transiently arresting the circulation with a ball oon in the right atrium. Animals were given 20 mug/kg of LPS (n = 8) or sal ine (n = 6) over 2 h. They were then given progressively increasing doses o f L-NAME (0.5 to 16 mug/kg). We injected 20 pg boluses of norepinephrine at baseline, after 2 h, and after 0.5, 4, and 16 pg of L-NAME to test the pre ssor response. Tissue was obtained from six control animals followed for 2 h, eight animals treated with LIPS for 2 h and then sacrificed, and four an imals treated for 2 h and sacrificed after 2 more h. Cardiac output did not change, and the systemic vascular resistance fell in LPS animals. By Weste rn analysis, ecNOS was increased in LPS animals at 2 and 4 h in the aorta a nd vena cava, and this was paralleled by changes in nNOS in the vena cava. In contrast, ecNOS decreased in the pulmonary artery and nNOS did not chang e. Calcium-dependent NOS activity increased with LPS in the aorta and vena cava but decreased in pulmonary artery at 4 h. The dose-response relationsh ips to L-NAME for systemic vascular resistance, resistance to venous return , and cardiac output were shifted to the left after LPS in support of incre ased sensitivity supporting increased NO. The pressor response to norepinep hrine was depressed after LPS and was partially restored with 4 mg/kg of L- NAME, but this dose produced 90% of the fall in cardiac output. In conclusi on, in contrast to rats, cNOS activity is present in the systemic vessels o f LPS-treated pigs and could play a role in the pathophysiology of sepsis.