Nitric oxide bioavailability modulates the dynamics of microvascular oxygen exchange during recovery from contractions

Aim: Lowered microvascular PO(2) (PO(2)mv) during the exercise off-transient likely impairs muscle metabolic recovery and limits the capacity to perform repetitive tasks. The current investigation explored the impact of altered nitric oxide (NO) bioavailability on PO(2)mv during recovery from contractions in healthy skeletal muscle. We hypothesized that increased NO bioavailability (sodium nitroprusside: SNP) would enhance PO(2)mv and speed its recovery kinetics while decreased NO bioavailability (l-nitro arginine methyl ester: l-NAME) would reduce PO(2)mv and slow its recovery kinetics. Methods: PO(2)mv was measured by phosphorescence quenching during transitions (rest-1 Hz twitch-contractions for 3 min-recovery) in the spinotrapezius muscle of Sprague-Dawley rats under SNP (300 mu m), Krebs-Henseleit (Control) and l-NAME (1.5 mm) superfusion conditions. Results: Relative to recovery in Control, SNP resulted in greater overall microvascular oxygenation as assessed by the area under the PO(2)mv curve (PO(2 AREA); Control: 3471 +/- 292 mmHg s; SNP: 4307 +/- 282 mmHg s; P < 0.05) and faster off-kinetics as evidenced by the mean response time (MRToff; Control: 60.2 +/- 6.9 s; SNP: 34.8 +/- 5.7 s; P < 0.05), whereas l-NAME produced lower PO(2 AREA) (2339 +/- 444 mmHg s; P < 0.05) and slower MRToff (86.6 +/- 14.5 s; P < 0.05). Conclusion: NO bioavailability plays a key role in determining the matching of O(2) delivery-to-O(2) uptake and thus the upstream O(2) pressure driving capillary-myocyte O(2) flux (i.e. PO(2)mv) following cessation of contractions in healthy skeletal muscle. Additionally, these data support a mechanistic link between reduced NO bioavailability and prolonged muscle metabolic recovery commonly observed in ageing and diseased populations.