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

Abstract

Aged rats exhibit a decreased muscle microvascular O2 partial pressure (PmvO2) at rest and during contractions compared with young rats. Age-related reductions in nitric oxide bioavailability due, in part, to elevated reactive O2 species, constrain muscle blood flow (Q̇m). Antioxidants may restore nitric oxide bioavailability, Q̇m, and ameliorate the reduced PmvO2. We tested the hypothesis that antioxidants would elevate Q̇m and, therefore, PmvO2 in aged rats. Spinotrapezius muscle PmvO2 and Q̇m were measured, and oxygen consumption (V̇mO2) was estimated in anesthetized male Fisher 344 × 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 PmvO2 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 PmvO2 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 Q̇m and V̇mO2 during contractions ( P < 0.05), while decreasing force production 16.5% ( P < 0.05) and elevating the force production-to-V̇mO2 ratio (0.92 ± 0.03 to 1.06 ± 0.6, P < 0.05). Thus antioxidants increased PmvO2 by altering the balance between muscle O2 delivery and V̇mO2 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 O2 delivery and V̇mO2.