Comparison between selective and nonselective nitric oxide synthase inhibition and phenylephrine in normal and endotoxic swine

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Abstract
To compare the cardiopulmonary and peripheral circulatory effects of the nonselective nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) to the more selective inducible NOS inhibitor S-methylisothiourea (SMT) and to phenylephrine (PE) in endotoxic and normal swine. Prospective, randomized, unblinded study. Research laboratory of academic medical center. Nonanesthetized, sedated, mechanically ventilated, minimally invasive swine model. Animals received either lipopolysaccharide (LPS, LPS groups) or equivalent volume of saline (normal groups). LPS animals were further randomized into four groups when mean arterial pressure (MAP) had dropped to <60 mm Hg: the LPS/saline group received saline only; the other groups received either L-NAME, SMT, or PE. These were titrated to elevate MAP by 20–25 mm Hg, and animals were followed for another 3 hrs. Pulmonary artery occlusion pressure was maintained at one to two times baseline with the infusion of saline. Normal groups received the same agents 1 hr after baseline measurements, and drugs were titrated to achieve similar increases in MAP. We measured gastric-arterial Pco2 gradient by tonometry as an index of gastric mucosal perfusion. Left ventricular volumes were determined echocardiographically; right ventricular volumes were determined by a pulmonary arterial catheter equipped with a rapid thermistor. Plasma nitrite/nitrate (NOx) concentrations were measured hourly. In the LPS groups, all agents elevated MAP and systemic vascular resistance similarly. By hr 4, cardiac output had decreased in all groups, but the decrease with L-NAME (35% ± 16%) occurred earlier (at hr 3) and was larger than the decrease with SMT at hrs 3 and 5 and larger than the decrease with saline at hrs 3 to 5. L-NAME resulted in a larger increase in mean pulmonary arterial pressure (MPAP) when compared with saline (130% ± 44% vs. 61% ± 25%;p < .001) and SMT groups (130% vs. 97% ± 80%;p < .007). Only L-NAME had detrimental effects on right ventricular function as indicated by an increase in right ventricular end-systolic volume (54 ± 10 to 87 ± 6 mL;p < .05) and right ventricular end-diastolic volume (90 ± 11 to 128 ± 18 mL;p < .05). SMT decreased both left ventricular end-systolic volume (10.4 ± 2 to 7.7 ± 4 mL;p < .05) and left ventricular end-diastolic volume (18.5 ± 3 to 14.2 ± 5 mL;p < .05), indicating improved left ventricular function, whereas L-NAME did not affect left ventricular volumes. Both SMT and PE corrected LPS-induced gastric mucosal acidosis, but L-NAME did not. We did not detect changes in plasma NOx concentrations in any of LPS groups. In the normal groups, all agents increased MAP without changes in plasma NOx concentrations. L-NAME caused a larger decrease in cardiac output, but the increase in MPAP was higher with SMT. Both NOS inhibitors led to left ventricular dilation, but PE did not. Only L-NAME caused right ventricular dilation. There were no changes in gastric-arterial Pco2 gradient. In LPS animals, we failed to detect changes in plasma NOx concentrations. Furthermore, for similar increases in MAP, SMT improved gastric mucosal acidosis, had less adverse effects on right ventricular function and MPAP, and may have improved left ventricular function. However, apart from its beneficial effects on left ventricular function, SMT was not superior to PE. The results from normal animals indicate that both NOS inhibitors have adverse effects on cardiac function beyond those attributed to increased MAP.