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

Abstract

Nitric oxide (NO) is potentially involved in several responses to acute exercise. We tested the hypotheses that inhibition of NO formation reduces maximal O(2) delivery to muscle, but does not affect O(2) utilization by muscle, therefore lowering maximal O(2) consumption. To test these hypotheses, swine (approximately 30 kg) drank either tap water (Con, n = 25) or water with N(G)-nitro-l-arginine methyl ester (8.0 +/- 0.4 mg x kg(-1) x day(-1) for >or=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, O(2) consumption was determined at rest through maximal exercise intensity. O(2) consumption did not differ between groups at rest or at most exercise intensities, including maximum (Con, 40.8 +/- 1.8 ml x min(-1) x 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 x min(-1) x 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 O(2) consumption is therefore preserved. Redundant vasodilatory pathways and/or upregulation of these pathways may underlie these findings.