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Effects of chronic nitric oxide synthase inhibition on responses to acute exercise in swine

¹Departments of Biomedical Sciences and ²Medicine & Surgery, College of Veterinary Medicine, University of Missouri, Columbia, Missouri

Submitted 6 July 2007 ; accepted in final form 31 October 2007

Nitric oxide (NO) is potentially involved in several responses to acute exercise. We tested the hypotheses that inhibition of NO formation reduces maximal O₂ delivery to muscle, but does not affect O₂ utilization by muscle, therefore lowering maximal O₂ consumption. To test these hypotheses, swine (30 kg) drank either tap water (Con, n = 25) or water with N^G-nitro-L-arginine methyl ester (8.0 ± 0.4 mg·kg^–1·day^–1 for 4 wk; LN, n = 24). Treatment efficacy was reflected by higher mean arterial pressure and lower plasma NO metabolite concentration in LN than Con (both P < 0.05). Swine completed two graded treadmill running tests to maximum. In the first test, O₂ consumption was determined at rest through maximal exercise intensity. O₂ consumption did not differ between groups at rest or at most exercise intensities, including maximum (Con, 40.8 ± 1.8 ml·min^–1·kg^–1; LN, 40.4 ± 2.9; not significant). In the second test, tissue-specific blood flows were determined using the radiolabeled-microsphere technique. At rest, blood flows were lower (P < 0.05) in LN compared with Con for a number of tissues, including kidney, adrenal, lung, and several skeletal muscles. During both submaximal and maximal exercise, however, blood flows were similar between Con and LN for all 16 muscles examined; only blood flows to kidney (Con, 99 ± 16 ml·min^–1·100 g; LN, 55 ± 15; P < 0.05) and pancreas (Con, 25 ± 7; LN, 6 ± 2; P < 0.05) were lower in LN at maximum. Endothelium-dependent, but not -independent, relaxation of renal arterial segments was reduced (P < 0.05) in vitro. These data indicate that exercise-induced increases in muscle blood flows are maintained with chronic inhibition of NO formation and that maximal O₂ consumption is therefore preserved. Redundant vasodilatory pathways and/or upregulation of these pathways may underlie these findings.

exercise; blood flow; oxygen consumption; muscle; kidney