The role of nitric oxide (NO) in the regulation of 02 consumption was studied in chronically instrumented conscious dogs. A specific NO synthesis inhibitor, nitro-L-arginine (NLA, 30 mg/kg IV), significantly increased mean arterial pressure from 100±4 to 134±5 mm Hg (mean±SEM) and total peripheral resistance by 157±16% and reduced cardiac output by 47±3% and heart rate by 34±6% after 120 minutes. Changes in arterial blood gases were not observed. There were significant changes in Po2 (-14±2 mm Hg), 02 saturation (-21±2%), the percentage of hemoglobin as oxyhemoglobin (-21±2%), and 02 content (-3.0±0.9 vol%) and a significant increase in percent reduced hemoglobin (21±1%) in mixed venous blood, associated with an increase in 02 extraction (5.1±0.2 vol%) (all P<.01). 02 consumption was increased from 124± 6 to 155 ±9 mL/min (P<.05). Methoxamine, titrated to have hemodynamic effects similar to those of NLA (eg, itric oxide (NO) formed during the metabolism N of L-arginine1-3 has been identified as an important endothelium-derived relaxing factor (EDRF), which is released from the endothelium and results in vascular smooth muscle relaxation through a cGMP-dependent mechanism.3-5 NO is also found in the central nervous system, where it may mediate some forms of interneuronal communication.6 In addition, NO has been found in activated macrophages, where it is responsible for cytotoxicity and its production is induced after cytokine stimulation.7-9 The mechanism for cytotoxicity of activated macrophages is dependent on NO production, which has multiple actions including inhibition of mitochondrial respiration in target cells. Three mitochondrial enzymes are affected by NO: (1) aconitase in the tricarboxylic acid cycle and (2) NADH+-ubiquinone oxidoreductase and (3) succinateubiquinone oxidoreductase, which are part of complex I and complex II of the mitochondrial electron transport chain.9-11It was recently reported that in cultured rat hepatocytes and rat aortic smooth muscle cells, NO production induced by cytokines results in significant inhibition of aerobic energy metabolism through the inhibition of the mitochondrial enzymes.12-14 Because all observations of the effects of NO on respiratory function previous to these findings were examined in vitro and involved the induction of NO synthase, which is associated with the production of relatively large local concentrations of Received January 28, 1994; accepted August 24, 1994 (Circ Res. 1994;75:1086-1095 Key Words * constitutive nitric oxide synthase * cardiac output * nitro-L-arginine * mitochondrial function . oxygen extraction * barbiturates NO and with pathophysiological conditions, we wondered whether the constitutive pathways of NO production could regulate tissue metabolism under normal physiological conditions. Therefore, the present study addressed the question whether tissue 02 consumption is modulated physiologically in vivo by NO produced by the constitutive NO synthase.In the present study, nitro-L-arginine (NLA), a specific NO synthesis i...

Inhibition of NO synthesis has recently been shown to increase oxygen extraction in vivo, and NO has been proposed to play a significant role in the regulation of oxygen consumption by both skeletal and cardiac muscle in vivo and in vitro. It was our aim to determine whether NO also has such a role in the kidney, a tissue with a relatively low basal oxygen extraction. In chronically instrumented conscious dogs, administration of an inhibitor of NO synthase, nitro-L-arginine (NLA, 30 mg/kg i.v.), caused a maintained increase in mean arterial pressure and renal vascular resistance and a decrease in heart rate (all P<0.05). At 60 minutes, urine flow rate and glomerular flow rate decreased by 44+/-12% and 45+/-7%, respectively; moreover, the amount of sodium reabsorbed fell from 16+/-1.7 to 8.5+/-1.1 mmol/min (all P<0.05). At this time, oxygen uptake and extraction increased markedly by 115+/-37% and 102+/-34%, respectively (P<0.05). Oxygen consumption also significantly increased from 4.5+/-0.6 to 7.1+/-0.9 mL O2/min. Most important, the ratio of oxygen consumption to sodium reabsorbed increased dramatically from 0.33+/-0.07 to 0.75+/-0.11 mL O2/mmol Na+ (P<0.05), suggesting a reduction in renal efficiency for transporting sodium. In vitro, both a NO-donating agent and the NO synthase-stimulating agonist bradykinin significantly decreased both cortical and medullary renal oxygen consumption. In conclusion, NO plays a role in maintaining a balance between oxygen consumption and sodium reabsorption, the major ATP-consuming process in the kidney, in conscious dogs, and NO can inhibit mitochondrial oxygen consumption in canine renal slices in vitro.

Our previous studies uncovered an inhibitory effect of nitric oxide (NO) on leg skeletal muscle respiration in dogs at rest. The role of NO in the modulation of O2 consumption and O2 extraction in hindlimb muscle during elevated metabolic states was investigated in chronically instrumented dogs while walking and at three exercise intensities which markedly increased hindlimb blood flow. Walking resulted in increased O2 consumption by 17 +/- 4 mL min-1 and O2 extraction from 24 +/- 1 to 37 +/- 8%, with no alteration in hindlimb blood flow (BFLeg) and vascular resistance (VRLeg). Running at the highest speed (9.1 mph) resulted in an increase in BFLeg from 0.67 +/- 0.05 to 2.2 +/- 0.1 L min-1, a reduction of VRLeg and elevation of hindlimb O2 consumption from 33 +/- 3 to 226 +/- 21 mL min-1 and O2 extraction from 29 +/- 2 to 61 +/- 5%, with a decrease in leg venous PO2 from 38 +/- 1 to 25 +/- 1 mmHg. After nitro-L-arginine (NLA) (35 mg kg-1, i.v.) to inhibit endogenous NO synthesis, walking caused greater increases in hindlimb O2 consumption (29 +/- 5 mL min-1) and O2 extraction (43 +/- 1 to 60 +/- 3%) (both P < 0.05), with no significant change in BFLeg. During running at the highest speed, BFLeg was 1.9 +/- 0.1 L min-1 (P < 0. 05) and VRLeg was higher, accompanied by increases in hindlimb O2 consumption from 49 +/- 7 to 318 +/- 24 mL min-1 and O2 extraction from 41 +/- 2 to 79 +/- 4% (both P < 0.05), with a greater decrease in leg venous PO2 from 33 +/- 1 to 20 +/- 1 mmHg (P < 0.05). Similar results were found for intermediate levels of exercise. Our results indicate that NO modulates hindlimb skeletal muscle O2 extraction and O2 usage whether blood flow increased or not during exercise.

The contribution of endothelium-derived relaxing factor (EDRF) to the regulation of regional vascular resistance and tissue blood flow at rest and during acute moderate exercise was studied in chronically instrumented conscious dogs. Radioactive microspheres were injected before and during exercise to measure regional blood flow. An infusion of nitro-L-arginine (L-NA), an analogue of L-arginine, was used to inhibit the synthesis of EDRF and resulted in a significant increase in mean arterial pressure, associated with significantly elevated vascular resistance in heart, skeletal muscle, renal and splanchnic circulations and with decreases in tissue blood flow in those regions at rest. Acute exercise caused a typical redistribution of blood flow, in which there was vasodilation in heart and working skeletal muscles, accompanied by vasoconstriction in kidney and splanchnic circulations. L-NA resulted in significantly elevated vascular resistance during vasodilation in heart and working skeletal muscles and also significantly increased vasoconstriction in renal cortex, stomach, pancreas, liver, and colon during exercise. Blood flows during exercise were largely unaffected by L-NA treatment. Our results suggest that whereas EDRF functions to regulate basal vascular tone and vascular resistance during exercise, EDRF has a minor role in determining the pattern of the redistribution of tissue blood flow during exercise.

The acute inhibition of NO synthase by NLA causes a switch from fatty acids to lactate and glucose utilization by the heart which can be reversed by a NO donor, suggesting an important regulatory action of NO on cardiac metabolism.