Aging impacts microvascular oxygen pressures during recovery from contractions in rat skeletal muscle

Aging-induced alterations in peripheral circulatory control during contractions reduce the microvascular partial pressure of O2 (PO2mvPO2mv; which reflects the dynamic balance in the O2 delivery-to-O2 uptake ratio), resulting in exaggerated intramuscular metabolic disturbances and premature fatigue. However, the extent to which this altered PO2mvPO2mv during contractions is associated with prolongated muscle metabolic recovery is not known. We tested the hypothesis that the aging-induced speeding of the PO2mvPO2mv on-kinetics would presage slowed PO2mvPO2mv off-kinetics. The spinotrapezius muscle was exposed in six young (6–8 months) and seven old (26–28 months) male Fischer 344 × Brown Norway F1-hybrid rats. The PO2mvPO2mv kinetic profile was measured via phosphorescence quenching at rest, during electrically stimulated contractions (1 Hz, 7–9 V, 2 ms pulse duration, 180 s), and throughout recovery (180 s). Aged rats which evidenced faster PO2mvPO2mv on-kinetics (reduced mean response time (MRTon), young: 27.3 ± 3.6 s, old: 19.2 ± 1.6 s; P < 0.05) exhibited markedly slowed PO2mvPO2mv off-kinetics (increased MRToff, young: 46.5 ± 5.9 s, old: 84.8 ± 7.9 s; P < 0.05). Accordingly, a greater degree of PO2mvPO2mv on–off asymmetry (MRToff–MRTon) in the aged muscle was observed (young: 19.1 ± 4.5 s, old: 65.6 ± 8.6 s; P < 0.01). We conclude that aging-induced speeding of the PO2mvPO2mv on-kinetics does indeed presage a slowed PO2mvPO2mv off-kinetics, which likely compromises muscle metabolic recovery and may reduce subsequent contractile performance. Moreover, the greater degree of PO2mvPO2mv on–off asymmetry in the aged muscle suggests a mechanistic link between impaired microvascular oxygenation and altered muscle metabolic responses during exercise transitions.