Effects of neuronal nitric oxide synthase inhibition on resting and exercising hindlimb muscle blood flow in the rat

Nitric oxide (NO) derived from endothelial NO synthase (eNOS) is an integral mediator of vascular control during muscle contractions. However, it is not known whether neuronal NOS (nNOS)-derived NO regulates tissue hyperaemia in healthy subjects, particularly during exercise. We tested the hypothesis that selective nNOS inhibition would reduce blood flow and vascular conductance (VC) in rat hindlimb locomotor muscle(s), kidneys and splanchnic organs at rest and during dynamic treadmill exercise (20 m min-1, 10% grade). Nineteen male Sprague-Dawley rats (555 +/- 23 g) were assigned to either rest (n = 9) or exercise (n = 10) groups. Blood flow and VC were determined via radiolabelled microspheres before and after the intra-arterial administration of the selective nNOS inhibitor S-methyl-l-thiocitrulline (SMTC, 2.1 +/- 0.1 mu mol kg-1). Total hindlimb muscle blood flow (control: 20 +/- 2 ml min-1 100g-1, SMTC: 12 +/- 2 ml min-1 100g-1, P < 0.05) and VC (control: 0.16 +/- 0.02 ml min-1 100 g-1 mmHg-1, SMTC: 0.09 +/- 0.01 ml min-1 100 g-1 mmHg-1, P < 0.05) were reduced substantially at rest. Moreover, the magnitude of the absolute reduction in blood flow and VC correlated (P < 0.05) with the proportion of oxidative muscle fibres found in the individual muscles or muscle parts of the hindlimb. During exercise, total hindlimb blood flow (control: 108 +/- 7 ml min-1 100 g-1, SMTC: 105 +/- 8 ml min-1 100 g-1) and VC (control: 0.77 +/- 0.06 ml min-1 100g-1 mmHg-1; SMTC: 0.70 +/- 0.05 ml min-1 100g-1 mmHg-1) were not different (P > 0.05) between control and SMTC conditions. SMTC reduced (P < 0.05) blood flow and VC at rest and during exercise in the kidneys, adrenals and liver. These results enhance our understanding of the role of NO-mediated circulatory control by demonstrating that nNOS does not appear to subserve an obligatory role in the exercising muscle hyperaemic response in the rat.