Effects of antioxidants on contracting spinotrapezius muscle microvascular oxygenation and blood flow in aged rats

Aged rats exhibit a decreased muscle microvascular O-2 partial pressure (PmvO(2)) at rest and during contractions compared with young rats. Age-related reductions in nitric oxide bioavailability due, in part, to elevated reactive O-2 species, constrain muscle blood flow (Qm). Antioxidants may restore nitric oxide bioavailability, Qm, and ameliorate the reduced PmvO(2). We tested the hypothesis that antioxidants would elevate Qm and, therefore, PmvO(2) in aged rats. Spinotrapezius muscle PmvO(2) and Qm were measured, and oxygen consumption (VmO(2)) was estimated in anesthetized male Fisher 344 X Brown Norway hybrid rats at rest and during 1-Hz contractions, before and after antioxidant intravenous infusion (76 mg/kg vitamin C and 52 mg/kg tempol). Before infusion, contractions evoked a biphasic PmvO(2) that fell from 30.6 +/- 0.9 Torr to a nadir of 16.8 +/- 1.2 Torr with an undershoot of 2.8 +/- 0.7 Torr below the subsequent steady-state (19.7 +/- 1.2 Torr). The principal effect of antioxidants was to elevate baseline PmvO(2) from 30.6 +/- 0.9 to 35.7 +/- 0.8 Torr (P < 0.05) and reduce or abolish the undershoot (P < 0.05). Antioxidants reduced Qm and VmO(2) during contractions (P < 0.05), while decreasing force production 16.5% (P < 0.05) and elevating the force production-to-VmO(2) ratio (0.92 +/- 0.03 to 1.06 +/- 0.6, P < 0.05). Thus antioxidants increased PmvO(2) by altering the balance between muscle O-2 delivery and VmO(2) at rest and during contractions. It is likely that this effect arose from antioxidants reducing myocyte redox below the level optimal for contractile performance and directly (decreased tension) or indirectly (altered balance of vasoactive mediators) influencing O-2 delivery and VmO(2).