Effects of neuronal nitric oxide synthase inhibition on microvascular and contractile function in skeletal muscle of aged rats

Abstract

Advanced age is associated with derangements in skeletal muscle microvascular function during the transition from rest to contractions. We tested the hypothesis that, contrary to what was reported previously in young rats, selective neuronal nitric oxide (NO) synthase (nNOS) inhibition would result in attenuated or absent alterations in skeletal muscle microvascular oxygenation (Po(2)(mv)), which reflects the matching between muscle O(2) delivery and utilization, following the onset of contractions in old rats. Spinotrapezius muscle blood flow (radiolabeled microspheres), Po(2)(mv) (phosphorescence quenching), O(2) utilization (Vo(2); Fick calculation), and submaximal force production were measured at rest and following the onset of contractions in anesthetized old male Fischer 344 × Brown Norway rats (27 to 28 mo) pre- and postselective nNOS inhibition (2.1 μmol/kg S-methyl-l-thiocitrulline; SMTC). At rest, SMTC had no effects on muscle blood flow (P > 0.05) but reduced Vo(2) by ∼23% (P < 0.05), which elevated basal Po(2)(mv) by ∼18% (P < 0.05). During contractions, steady-state muscle blood flow, Vo(2), Po(2)(mv), and force production were not altered after SMTC (P > 0.05 for all). The overall Po(2)(mv) dynamics following onset of contractions was also unaffected by SMTC (mean response time: pre, 19.7 ± 1.5; and post, 20.0 ± 2.0 s; P > 0.05). These results indicate that the locus of nNOS-derived NO control in skeletal muscle depends on age and metabolic rate (i.e., rest vs. contractions). Alterations in nNOS-mediated regulation of contracting skeletal muscle microvascular function with aging may contribute to poor exercise capacity in this population.

Fig. 1.

Effects of selective neuronal nitric oxide synthase (nNOS) inhibition (S-methyl-l-thiocitrulline; SMTC) and nonselective NOS inhibition (N^G-nitro-l-arginine methyl ester; l-NAME) on the hypotensive responses to acetylcholine. Top and bottom: relative drop in mean arterial pressure (MAP) and time to 50% recovery from the hypotensive response to acetylcholine, respectively. *P < 0.05 vs. control and SMTC.

Fig. 2.

Mean resting spinotrapezius muscle microvascular oxygenation (Po_2mv) during infusion of saline (control) or SMTC (selective nNOS inhibition). Time zero denotes start of infusion. *P < 0.05 vs. control for end-infusion Po_2mv (last 10-s average).

Fig. 3.

Mean spinotrapezius muscle Po_2mv at rest and following the onset of contractions under control and selective nNOS inhibition (SMTC) conditions. Time zero denotes the onset of muscle contractions. Average kinetics parameters are displayed in Table 2. See text for further details.

Fig. 4.

Mean spinotrapezius muscle blood flow (top) and O₂ utilization (V̇o₂; bottom) at rest and during contractions under control and selective nNOS inhibition (SMTC) conditions. Note different scales on vertical axes.*P < 0.05 vs. control.

Fig. 5.

Mean spinotrapezius muscle force production under control and selective nNOS inhibition (SMTC) conditions. Note that muscle force production was not significantly different throughout the contraction period between control and SMTC. Inset : force-time integral values were also not significantly different between conditions.

Fig. 6.

Effects of selective nNOS inhibition (SMTC) on contracting spinotrapezius muscle blood flow, V̇o₂, overall Po_2mv kinetics (MRT, mean response time), and submaximal force production in young Sprague-Dawley (data from Ref. ; n = 10) and old Fischer 344 × Brown Norway (present study; n = 11) rats. Note that SMTC evoked significant changes in these variables in young but not old rats. *P < 0.05 vs. zero. See text for discussion.