Effects of aging on microvascular oxygen pressures in rat skeletal muscle

Aging alters skeletal muscle vascular geometry and control such that the dynamics of muscular blood flow (0) and 02 delivery ((Q) over dot O-2) may be impaired across the rest-exercise transition. If, at the onset of muscle contractions, 0 dynamics are slowed disproportionately to those of muscle O-2 uptake ((V) over dot O-2), microvascular PO2 (PO(2)m) would be reduced and blood-tissue 0, transfer compromised. This investigation determined the effects of aging on PO(2)m (a direct reflection of the (Q) over dot O-2-to-(V) over dot O-2 ratio), at rest and across the rest-contractions transition in the spinotrapezius of young (similar to 6 months, n = 9) and old (> 24 months, n = 10) male Fisher 344/Brown Norway hybrid rats. Phosphorescence quenching techniques were used to quantify PO(2)m, and test the hypothesis that, across the rest-contractions (twitch, 1 Hz; 4-6 V, 240 s) transition, aging would transiently reduce the (Q) over dot O-2-to-(V) over dot O-2 ratio causing a biphasic profile in which PO(2)m fell below steady-state contracting values. Old rats had a lower pre-contraction baseline PO(2)m than young (27.1 +/- 1.9 versus 33.8 +/- 1.6 mmHg, P < 0.05, respectively). In addition, in old rats PO(2)m demonstrated a pronounced difference between the absolute nadir and end-contracting values (2.5 +/- 0.9 mmHg), which was absent in young rats. In conclusion, unlike their young counterparts, old rats exhibited a transiently reduced PO(2)m across the rest-contractions transition that may impair blood-tissue O-2 exchange and elevate the O-2 deficit, thereby contributing to premature fatigue. (c) 2004 Elsevier B.V. All rights reserved.