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_2mv), which reflects the matching between muscle O₂ delivery and utilization, following the onset of contractions in old rats. Spinotrapezius muscle blood flow (radiolabeled microspheres), Po_2mv (phosphorescence quenching), O₂ utilization (V̇o₂; 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 V̇o₂ by ∼23% (P < 0.05), which elevated basal Po_2mv by ∼18% (P < 0.05). During contractions, steady-state muscle blood flow, V̇o₂, Po_2mv, and force production were not altered after SMTC (P > 0.05 for all). The overall Po_2mv 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.

nitric oxide (NO) is a ubiquitous signaling messenger synthesized primarily through the conversion of l-arginine to l-citrulline by the enzyme NO synthase (NOS). Within skeletal muscle, NO plays a critical role in the modulation of several physiological processes including vascular relaxation, oxidative metabolism, and excitation-contraction coupling (56). All three major NOS isoforms are expressed in mammalian skeletal muscle, namely, neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS) (56). Several lines of evidence indicate that NO derived from nNOS participates significantly in the matching of muscle O₂ delivery and utilization (Q̇o₂/V̇o₂) at rest and during contractions as well as submaximal force production in healthy young individuals (12; see also Refs. 13, 15, 19, 28, 33, 36, 39, 45, 52, 53, 60).

Advancing age is associated with impairments in the O₂ transport pathway and exercise capacity (47). Derangements in NO-mediated function likely represent one of the main mechanisms underlying temporal Q̇o₂/V̇o₂ mismatch during transitions in metabolic demand in aged skeletal muscle (5, 16, 23, 25). Impairments in the ability to regulate Q̇o₂ relative to V̇o₂ diminish muscle microvascular O₂ pressures (Po_2mv) and thus the driving force for blood-myocyte O₂ flux as dictated by Fick's law of diffusion. These alterations are of functional significance given that reductions in Po_2mv impact negatively on mitochondrial control and could explain, at least in part, poor exercise capacity with aging (29, 57). In view of the typical deterioration of endothelial function in the elderly (54, 65), age-related decrements in NO bioavailability have been ascribed traditionally to eNOS dysfunction. Whether nNOS dysfunction is potentially involved in impaired muscle Q̇o₂/V̇o₂ control with aging remains unexplored.

Given that advanced aging might impact both nNOS and eNOS function possibly because of prominent oxidative stress that promotes NOS uncoupling and/or direct NO inactivation (21, 49, 54, 58), we examined whether nNOS-derived control of skeletal muscle microvascular and contractile function is altered in old rats. The hypothesis was tested that, contrary to what was observed previously in young rats (12), selective nNOS inhibition in old rats would result in attenuated or absent alterations in resting and contracting muscle blood flow, V̇o₂, Po_2mv, and submaximal force production, thus indicating impaired nNOS-mediated microvascular and contractile control with aging.

METHODS

A total of 21 old (27 to 28 mo; and body mass, 614 ± 11 g) male Fischer 344 × Brown Norway (F344×BN) rats were used in the present study for measurements of Po_2mv and muscle blood flow (phosphorescence quenching and radiolabeled microspheres, respectively; n = 11), force production (n = 6), and time-control experiments (n = 4). Rats were obtained from Charles Rivers Laboratories and maintained on a 12-h:12-h light-dark cycle with food and water provided ad libitum. The selected age represents senescent rats according to the life span of the F344×BN rodent strain (38). The F344×BN rat has the distinct advantage of not acquiring many of the age-related pathologies that proliferate in their highly inbred counterparts (40). Upon completion of the study, rats were euthanized with intra-arterial pentobarbital sodium overdose (∼50 mg/kg). All procedures described herein were conducted under the guidelines established by the National Institutes of Health and approved by the Institutional Animal Care and Use Committee of Kansas State University.

Experimental design consideration.

Comparison with young rats is facilitated using data from Copp et al. (12). The rationale for this procedure is based on the Institutional Animal Care and Use Committee stipulation that additional animals not be euthanized for replication of data. In addition, direct comparison between old and young (12) animals is facilitated by the fact that both experimental groups underwent the exact same protocols and old and young animal experiments were temporally interdigitated.

Surgical preparation.

Rats were anesthetized initially with 5% isoflurane-O₂ mixture and maintained on 2 to 3% isoflurane-O₂. The caudal (tail) artery was isolated surgically and cannulated (PE-50; Intra-Medic Tubing, Clay Adams Brand) for continuous monitoring of heart rate and mean arterial pressure (HR and MAP, respectively; Digi-Med BPA model 200) and infusion of the phosphorescent probe palladium meso-tetra (4-carboxyphenyl) porphyrin dendrimer (R2; 15 mg/kg; Oxygen Enterprises). Blood from the tail artery catheter was sampled at the end of each experimental condition (control and selective nNOS inhibition with S-methyl-l-thiocitrulline; SMTC) for the determination of arterial blood gases, pH, systemic hematocrit, and plasma lactate (n = 11). For blood flow measurements, an additional catheter (PE-10 connected to PE-50) was placed in the ascending aorta via the right carotid artery to allow injection of differently radiolabeled microspheres into the aortic arch. Anesthetized rats were maintained on a heating pad to maintain core temperature at ∼37 to 38°C as measured via rectal probe.

Isoflurane-O₂ mixture inhalation was progressively discontinued after catheter placement procedures, and rats were then kept under anesthesia with pentobarbital sodium (administered intra-arterially to effect). The level of anesthesia was monitored frequently via the toe pinch and blink reflexes and supplemented as necessary. Overlying skin and fascia from the middorsal region of the rat were reflected surgically to expose the right spinotrapezius muscle. The muscle was moistened constantly throughout the surgery and experimental protocol via superfusion of Krebs-Henseleit bicarbonate-buffered solution, consisting of (in mM) 4.7 KCl, 2.0 CaCl₂, 2.4 MgSO₄, 131 NaCl, and 22 NaHCO₃, equilibrated with 5% CO₂-95% N₂ (pH 7.4, warmed to 37–38°C), and the surrounding tissue was covered with Saran wrap (Dow Brands). Stainless steel electrodes were sutured to the rostral (cathode) and caudal (anode) regions of the spinotrapezius muscle for electrically induced contractions. Our laboratory has demonstrated previously that these surgical procedures do not alter the microvascular integrity and responsiveness of the spinotrapezius muscle (3).

Experimental protocol.

Two separate contraction bouts were performed under control (1.2 ml heparinized saline) and selective nNOS inhibition (2.1 μmol/kg SMTC dissolved in 1.2 ml heparinized saline) conditions. This dose of SMTC was selected based on previous studies designed to inhibit selectively nNOS in both humans and rodents (18, 30, 53, 64) and our analysis of the highest possible SMTC dose that could be administered without affecting the hypotensive response to ACh, indicative of nonspecific eNOS inhibition (12, 13). Each solution was infused at a rate of 0.2 ml/min into the tail artery catheter for a total time of 6 min, after which a ∼2-min period was allowed for resting muscle Po_2mv to stabilize. Subsequently, 1-Hz twitch contractions (∼7 V, 2-ms pulse duration) were evoked via a stimulator (model s48; Grass Technologies) for 3 min. The muscle was then allowed to recover for a minimum of 25 min before the next condition was initiated (stimulation parameters held constant). Due to its relatively long half-life (∼40 min; refs. 18, 64), SMTC was always the last condition to prevent residual effects on vascular and skeletal muscle function. Importantly, there is no ordering (priming) effect on the Po_2mv response to muscle contractions when a minimum of 20 min of recovery is allowed between stimulations (7, 16).

Effects of SMTC and N^G-nitro-l-arginine methyl ester on the hypotensive responses to ACh.

In a subset of animals (n = 8 of 21 total rats), rapid ACh infusions (5 μg/kg in 0.2 ml of heparinized saline) were performed under control and SMTC conditions as well as after nonselective NOS inhibition with N^G-nitro-l-arginine methyl ester (l-NAME; 10 mg/kg) administered into the caudal artery. The hypotensive responses to these infusions were recorded via the carotid artery catheter to confirm the efficacy of selective nNOS inhibition with SMTC (12, 13).

Measurement of Po_2mv.

Po_2mv was measured by phosphorescence quenching using a frequency domain phosphorometer (PMOD 5000; Oxygen Enterprises). As described in detail previously (6), this method applies the Stern-Volmer relationship (51) that describes quantitatively the O₂ dependence of the phosphorescent probe (i.e., R2) via the following equation:

$${\text{Po}}_{2\text{mv}}=\left[\left(\tau °/\tau \right)-1\right]/\left(k_{\text{Q}}\times \tau °\right)$$

where k_Q is the quenching constant and τ and τ° are the phosphorescence lifetimes in the absence of O₂ and at the ambient O₂ pressure, respectively. The τ of phosphorescence decay was determined using 10 scans (100 ms) in the single-frequency mode. The phosphorescent probe R2 (τ° = 601 μs and k_Q = 409 mmHg⁻¹·s⁻¹ at pH 7.4 and temperature 38°C) (51, 63) was infused ∼15 min before initiation of muscle contractions. R2 is bound to albumin and is distributed uniformly in the plasma, thus providing a signal corresponding to the volume-weighted O₂ pressure within the microvascular compartment (i.e., mainly the Po₂ within the capillaries, which volumetrically constitutes the major intramuscular space) (48). The negative charge of the R2 probe also facilitates its restriction to the intravascular space (46).

The common end of the light guide was placed ∼2–4 mm superficial to the dorsal surface of the exposed right spinotrapezius muscle. The phosphorometer modulates sinusoidal excitation frequencies between 100 Hz and 20 kHz and allows phosphorescence lifetime measurements from 10 μs to ∼2.5 ms. The excitation light (524 nm) was focused on a randomly selected area of ∼2 mm diameter within the central region of the exposed muscle and has a penetration depth of ∼500 μm. Po_2mv was measured continuously and recorded at 2-s intervals throughout the duration of the experimental protocols.

Analysis of Po_2mv kinetics.

The kinetics of Po_2mv were described by nonlinear regression analysis using the Marquardt-Levenberg algorithm (SigmaPlot 11.2; Systat software) from the onset of contractions. Transient Po_2mv responses were fit with either a one- or two-component model:

One component:

$${\text{Po}}_{2\text{mv}\left(t\right)}={\text{Po}}_{2\text{mv}\left(\text{BL}\right)}-\Delta {\text{Po}}_{2\text{mv}}\left(1-e^{-\left(t-\text{TD}\right)/\tau }\right)$$

Two-component:

$$\begin{gathered} {\text{Po}}_{2\text{mv}\left(t\right)}={\text{Po}}_{2\text{mv}\left(\text{BL}\right)}-{\Delta }_1{\text{Po}}_{2\text{mv}}\left(1-e^{-\left(t-{\text{TD}}_{\text{1}}\right)/{\tau }_1}\right) \\ +{\Delta }_2{\text{Po}}_{2\text{mv}}\left(1-e^{-\left(t-{\text{TD}}_{\text{2}}\right)/{\tau }_2}\right) \end{gathered}$$

where Po_2mv(t) is the Po_2mv at a given time t; Po_2mv(BL) corresponds to the precontracting resting Po_2mv; Δ₁ and Δ₂ are the amplitudes for the first and second components, respectively; TD₁ and TD₂ are the independent time delays for each component; and τ₁ and τ₂ are the time constants (i.e., time to reach 63% of the response) for each component. Goodness of fit was determined using three criteria: 1) the coefficient of determination, 2) the sum of squared residuals, and 3) visual inspection.

The amplitude of the first component was normalized to its time constant (Δ₁Po_2mv/τ₁) to provide an index of the relative rate of Po_2mv fall. The overall time necessary to attain 63% of the final amplitude of the Po_2mv response following contractions onset was determined independent of modeling procedures (T₆₃) (34) to ensure appropriateness of the model fits.

The mean response time (MRT) (43) was employed to describe the overall dynamics of the Po_2mv fall

$$\text{MRT}={\text{TD}}_1+{\tau }_1$$

where TD₁ and τ₁ are defined above. The MRT analysis was constrained to the first phase of the Po_2mv response since inclusion of the emergent second phase underestimates the actual rate of Po_2mv fall following initiation of contractions (25).

The area under the Po_2mv curve plotted as function of time (Po_2Area) (23) was calculated during the 3-min contraction protocol to provide an index of the overall muscle microvascular oxygenation (i.e., incorporating from time zero to contracting steady-state Po_2mv, time delays, amplitudes, and time constants of the response to yield a value, expressed in mmHg·s).

Measurement of blood flow.

Spinotrapezius muscle blood flow was measured using the radiolabeled microsphere technique, as described in detail previously (44). In each condition (control and SMTC), the stimulated right and nonstimulated left spinotrapezius muscles represented the contracting and resting blood flow measurements, respectively. Briefly, the tail artery catheter was connected to a 1-ml syringe, and blood withdrawal at a constant rate of 0.25 ml/min was performed via a Harvard pump (model 907). Differently radiolabeled microspheres (⁴⁶Sc or ⁸⁵Sr; 15 μm diameter; Perkin Elmer Life and Analytical Sciences) were injected in random order into the aortic arch via the carotid artery catheter during the contracting steady state (i.e., ∼3 min after onset of stimulation). Upon completion of the experiment, the right and left spinotrapezius muscles, right and left kidneys, and organs of the splanchnic region (stomach, adrenals, spleen, pancreas, small intestine, large intestine, and liver) were carefully dissected, removed, and weighed immediately after euthanasia. The thorax was opened and placement of the carotid artery catheter into the aortic arch was confirmed by anatomic dissection. Tissue radioactivity was determined on a gamma scintillation counter (Auto Gamma Spectrometer, Cobra model 5003; Hewlett-Packard), and blood flow was determined by the reference sample method (31) and expressed as milliliters per minute per 100 g of tissue. Adequate mixing of the microspheres was verified for each injection by demonstrating a <15% difference in blood flow between the right and left kidneys. Blood flow data were also normalized to MAP and expressed as vascular conductance (VC; ml·min⁻¹·100 g⁻¹·mmHg⁻¹).

Calculation of muscle V̇o₂.

Muscle V̇o₂ was calculated from Po_2mv and blood flow measurements as described previously (12, 27). Briefly, arterial O₂ concentration (Ca_O2) was calculated directly from arterial blood samples, whereas venous O₂ concentration (Cv_O2) was estimated from either the baseline (rest) or the contracting steady-state (contractions) Po_2mv using the rat O₂ dissociation curve (Hill coefficient of 2.6), the measured Hb concentration, a P₅₀ of 38 mmHg, and an O₂ carrying capacity of 1.34 ml O₂/g Hb (1). The measures of the resting and contracting spinotrapezius blood flow (Q̇_m) were then used to determine V̇o₂ via the Fick equation, i.e., V̇o₂ = Q̇_m(Ca_O2 − Cv_O2).

Measurement of submaximal muscle force production.

The caudal end of the spinotrapezius muscle was exteriorized and sutured to a swivel apparatus and a nondistensible light weight (0.4 g) cable, which linked the muscle to a force transducer (model FTO3; Grass Technologies). The preload tension was set at ∼0.04 N to elicit the optimal length of the muscle for twitch force production (12, 27). Muscle force production was measured throughout control and SMTC contraction bouts which were identical to the contraction protocols described above for the measurement of Po_2mv and blood flow. Force production was expressed as Newtons per gram of muscle.

Time-control experiments.

The stability and reproducibility of the spinotrapezius muscle preparation has been addressed previously (12, 27) and was reconfirmed in the current study via time-control experiments (i.e., 2 control contraction bouts performed as described above). The average coefficient of variation for Po_2mv kinetics and muscle force production was 9 ± 5% with no ordering effects (P > 0.05). These data provide confidence that the significant effects (or lack thereof) detected herein were the direct result of selective nNOS inhibition with SMTC.

Statistical analyses.

Data comparison was performed using paired Student's t-tests, one-way repeated-measures ANOVA or two-way repeated-measures ANOVA where appropriate. Post hoc analyses were performed with the Student-Newman-Keuls test when a significant F-ratio was detected. The z-statistic was calculated to determine differences from zero. The significance level was set at P < 0.05. Results are reported as means ± SE.

RESULTS

Blood sampling, hemodynamic variables, and ACh injections.

There were no differences in arterial blood O₂ saturation (control, 95.3 ± 0.2; and SMTC, 94.4 ± 0.7%), pH (control, 7.41 ± 0.01; and SMTC, 7.40 ± 0.01), lactate concentration (control, 1.0 ± 0.1; SMTC, 1.1 ± 0.1 mM) and systemic hematocrit (control, 35.6 ± 0.6; and SMTC, 34.8 ± 0.6%) between conditions (P > 0.05 for all).

The HR and MAP responses during control and SMTC conditions are displayed in Table 1. HR and MAP did not change during the control saline infusion, whereas MAP increased and HR decreased during the infusion of SMTC. Within each condition, there were no significant differences between rest (postinfusion) and contractions (steady state).

Table 1.

HR and MAP at rest (pre- and postinfusion) and during the steady state of muscle contractions before (control) and after selective nNOS inhibition with SMTC

	Control	SMTC
At rest (preinfusion)
HR, beats/min	297 ± 9	307 ± 4
MAP, mmHg	109 ± 3	112 ± 3
At rest (postinfusion)
HR, beats/min	305 ± 6	294 ± 4*†
MAP, mmHg	111 ± 4	126 ± 3*†
Contractions (steady state)
HR, beats/min	309 ± 7	292 ± 6*†
MAP, mmHg	107 ± 3	129 ± 3*†

Values are means ± SE. HR, heart rate; MAP, mean arterial pressure; nNOS, neuronal nitric oxide synthase; SMTC, S-methyl-l-thiocitrulline.

^*P < 0.05 vs. control;

^†P < 0.05 vs. rest (preinfusion). Within each condition, there were no significant differences between rest (postinfusion) and contractions (steady state).

The hypotensive responses to ACh infusion during control, SMTC, and l-NAME conditions are depicted in Fig. 1. The relative change in MAP with ACh was not different between control and SMTC but decreased significantly with l-NAME. Similarly, the time to 50% recovery from the drop in MAP with ACh was not different between control and SMTC but speeded significantly with l-NAME. These data are consistent with the notion that SMTC did not impair eNOS function in the present study.

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.

Spinotrapezius Po_2mv.

Mean spinotrapezius muscle Po_2mv during control and SMTC infusions are shown in Fig. 2. The control infusion did not change Po_2mv (preinfusion, 36.1 ± 1.2; and postinfusion, 37.4 ± 1.3 mmHg; P > 0.05), whereas SMTC infusion increased Po_2mv from 35.3 ± 1.6 to 41.9 ± 2.2 mmHg (P < 0.05). Po_2mv was not different between control and SMTC at the start or during the first 5 min of infusion (P > 0.05) but was increased significantly by the end of SMTC infusion.

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).

Mean spinotrapezius muscle Po_2mv values following the onset of contractions during control and SMTC conditions are shown in Fig. 3 and average kinetics parameters displayed in Table 2. Although the time delay of the Po_2mv fall following the onset of contractions (TD₁; P < 0.05) was reduced, SMTC did not change the Po_2mv time constant (τ₁; P > 0.05). Furthermore, no differences in the overall dynamics of Po_2mv as represented by the MRT (model dependent), T₆₃ (model independent), and Δ₁Po_2mv/τ₁ (relative rate of Po_2mv fall) were observed between conditions (P > 0.05 for all). SMTC did, however, increase the amplitude of the Po_2mv fall during contractions (Δ₁Po_2mv; P < 0.05), such that the contracting steady-state Po_2mv [Po_2mv(SS); P > 0.05] was not different between conditions. Although SMTC elevated resting Po_2mv, the overall muscle microvascular oxygenation during contractions as represented by the Po_2Area was not significantly different between conditions.

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.

Table 2.

Muscle Po_2mv at rest and following the onset of contractions before (control) and after selective nNOS inhibition with SMTC

	Control	SMTC
Po2mv(BL), mmHg	37.7 ± 1.2	44.2 ± 2.1*
Δ1Po2mv, mmHg	12.4 ± 0.9	17.6 ± 2.1*
Δ2Po2mv, mmHg	2.3 ± 0.2	2.6 ± 0.5
ΔTotalPo2mv, mmHg	11.0 ± 0.7	17.2 ± 2.1*
Po2mv(SS), mmHg	26.9 ± 1.3	27.3 ± 2.4
TD1, s	10.1 ± 1.5	6.3 ± 0.7*
TD2, s	73.1 ± 11.7	52.8 ± 14.5
τ1, s	9.6 ± 0.9	13.7 ± 1.8
τ2, s	59.5 ± 15.1	55.7 ± 4.7
MRT, s	19.7 ± 1.5	20.0 ± 2.0
T63, s	20.9 ± 1.9	19.6 ± 1.4
Δ1Po2mv/τ1, mmHg/s	1.3 ± 0.1	1.4 ± 0.2
Po2Area, mmHg·s	4,920 ± 244	5,195 ± 402

Values are means ± SE.

Po_2mv, microvascular oxygenation; Po_2mv(BL), precontracting Po_2mv; Δ₁Po_2mv, amplitude of the first component; Δ₂Po_2mv, amplitude of the second component; Δ_TotalPo_2mv, overall amplitude regardless of one- or two-component model fit; Po_2mv(SS), contracting steady-state Po_2mv; TD₁, time delay for the first component; TD₂, time delay for the second component; τ₁, time constant for the first component; τ₂, time constant for the second component; MRT, mean response time; T₆₃, time to reach 63% of the overall amplitude as determined independent of modeling procedures; Δ₁Po_2mv/τ₁, relative rate of Po_2mv fall; Po_2Area, area under the Po_2mv curve over the 3-min contraction period.

The two-component exponential model was used to analyze the Po_2mv kinetics in the majority of instances (7 out of 11) in the control condition, whereas the one-component model was required to fit the SMTC response in most rats (9 out of 11).

^*P < 0.05 vs. control.

Spinotrapezius blood flow and V̇o₂.

At rest, blood flow (Fig. 4, top left) and VC (control, 0.09 ± 0.02; and SMTC, 0.07 ± 0.01 ml·min⁻¹·100 g⁻¹·mmHg⁻¹; P > 0.05) were not different between control and SMTC. During contractions, blood flow (Fig. 4, top right) was not different, whereas VC (control, 1.01 ± 0.09; and SMTC, 0.82 ± 0.08 ml·min⁻¹·100 g⁻¹·mmHg⁻¹; P < 0.05) was reduced with SMTC compared with control. Accordingly, SMTC did not alter the change in blood flow from rest to contractions (Δblood flow: control, 98.0 ± 9.1; and SMTC, 97.1 ± 10.8 ml·min⁻¹·100 g⁻¹; P > 0.05) but attenuated the change in VC during the rest-contraction transient (ΔVC: control, 0.92 ± 0.09; and SMTC: 0.75 ± 0.07 ml·min⁻¹·100 g⁻¹·mmHg⁻¹; P < 0.05).

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.

Relative to the control condition, SMTC reduced significantly resting but not contracting spinotrapezius muscle V̇o₂ (Fig. 4, bottom). However, from rest to contractions, the change in muscle V̇o₂ was not different between conditions (ΔV̇o₂: control, 10.5 ± 1.0; and SMTC, 10.2 ± 1.5 ml·min⁻¹·100 g⁻¹; P > 0.05).

Spinotrapezius muscle force production.

As illustrated in Fig. 5, mean spinotrapezius muscle force production throughout the contraction protocol and the force-time integral were not significantly different between control and SMTC conditions. Similarly, the average steady-state force production-to-V̇o₂ ratio was also unaffected by SMTC (force/V̇o₂: control, 0.024 ± 0.001; and SMTC: 0.023 ± 0.001 N·ml O₂⁻¹·min⁻¹; P > 0.05).

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.

Abdominal organ blood flow and VC.

The effects of SMTC on resting blood flow and VC in the kidneys and organs of the splanchnic region are displayed in Table 3. Relative to control, SMTC decreased blood flow in the kidneys, stomach, adrenals, spleen, and large intestine (P < 0.05 for all). SMTC decreased VC in the kidneys, stomach, adrenals, spleen, pancreas, small intestine, and large intestine (P < 0.05 for all).

Table 3.

Resting blood flow and vascular conductance in the kidneys and organs of the splanchnic region before (control) and after selective nNOS inhibition with SMTC

	Blood Flow, ml·min−1·100 g−1		Vascular Conductance, ml·min−1·100 g−1·mmHg−1
	Control	SMTC	Control	SMTC
Kidneys	515 ± 52	319 ± 25*	4.88 ± 0.57	2.49 ± 0.19*
Stomach	55 ± 7	34 ± 2*	0.52 ± 0.08	0.27 ± 0.02*
Adrenals	798 ± 75	542 ± 55*	7.47 ± 0.72	4.20 ± 0.39*
Spleen	256 ± 28	217 ± 26*	2.43 ± 0.29	1.72 ± 0.21*
Pancreas	105 ± 19	101 ± 26	0.98 ± 0.16	0.76 ± 0.17*
Small intestine	342 ± 37	290 ± 18	3.24 ± 0.40	2.27 ± 0.17*
Large intestine	116 ± 8	95 ± 5*	1.09 ± 0.08	0.75 ± 0.05*
Liver†	23 ± 3	25 ± 5	0.21 ± 0.03	0.19 ± 0.04

Values are means ± SE.

^†Arterial, not portal, blood flow and vascular conductance.

^*P < 0.05 vs. control.

DISCUSSION

The present investigation determined the effects of selective nNOS inhibition on skeletal muscle function at rest and during contractions in old rats. The principal novel finding was that the changes in contracting muscle blood flow, V̇o₂, Po_2mv kinetics, and submaximal force production produced by nNOS inhibition in healthy young rats (12) were absent in old rats (see Fig. 6). Specifically, selective nNOS inhibition in old rats resulted in 1) alterations in resting muscle V̇o₂ (↓23%) and Po_2mv (↑18%) but not blood flow; 2) no changes in steady-state blood flow, V̇o₂, or Po_2mv during contractions; 3) no changes in the overall dynamics of Po_2mv following the onset of contractions; and 4) no changes in muscle force production. These data suggest that nNOS-mediated control of contracting skeletal muscle microvascular and contractile function is altered in old rats.

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. 12; 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.

Effects of selective nNOS inhibition on skeletal muscle function in old rats.

Skeletal muscle Po_2mv is dictated by the dynamic Q̇o₂/V̇o₂ matching within the microvascular space (6). We reported recently that selective nNOS inhibition with SMTC in healthy young rats increased resting spinotrapezius Po_2mv via reductions in V̇o₂ concomitant with no alterations in muscle blood flow (12). In the present study, similar effects were observed following SMTC infusion in old rats (Figs. 2 and 4, Table 2), thus suggesting that nNOS-mediated function is preserved within aged skeletal muscle at least at rest. The lack of an effect of SMTC on resting spinotrapezius muscle blood flow in both young (12) and old (Fig. 4, top left) anesthetized rats contrasts with that seen previously in the human forearm (53) and awake rat hindlimb (13) circulations. The reasons for this discrepancy are not entirely clear but could relate partially to the effects of anesthesia. However, the reductions in blood flow to the kidneys and organs of the abdominal region following SMTC infusion in old rats at rest (Table 3) are consistent with the role of nNOS-derived NO in regulating renal and splanchnic blood flow as reported previously in the awake young rat at rest (13, see also Refs. 30, 32). These findings also suggest preserved nNOS-mediated function in the renal and splanchnic circulations with aging at rest.

The SMTC-induced reductions in resting spinotrapezius V̇o₂ in both young (12) and old (Fig. 4, bottom left) rats are surprising considering the well-known inhibitory effects of NO on mitochondrial respiration (56) that may actually act to increase resting muscle V̇o₂ following nonselective NOS inhibition (e.g., dogs, Ref. 20; and humans, Ref. 22). Although our data could be interpreted as reflecting a stimulatory role of NO from nNOS on mitochondrial respiration (66; see also Refs. 4, 37) and/or relate to possible differences in NOS isoform compartmentalization across species, evidence from isolated cardiac muscle suggests rather that nNOS-derived NO does not control directly tissue V̇o₂ (35, 41). In the latter scenario, reduced resting V̇o₂ with SMTC in old rats could result from blockade of uncoupled nNOS which alleviates the inactivation of NO from eNOS and restores partially mitochondrial respiratory inhibition (cf. 35). These intriguing possibilities remain to be tested empirically in aged skeletal muscle.

Despite the apparent preserved contribution of nNOS-derived NO to resting skeletal muscle function with aging (as discussed above), the effects of SMTC on contracting muscle blood flow, V̇o₂, Po_2mv kinetics, and force production observed presently in old rats differ markedly from those reported previously in young rats (12). Specifically, selective nNOS inhibition in young rats 1) reduced contracting steady-state muscle blood flow and V̇o₂, 2) speeded the fall in Po_2mv following the onset of contractions (i.e., shorter MRT, T₆₃ and Δ₁Po_2mv /τ₁), and 3) increased submaximal force production (12). In old rats, however, SMTC had no significant effects on any of these variables during muscle contractions (Table 2, Figs. 4 and 5). Figure 6 summarizes these results and illustrates the contrasting effects of SMTC on skeletal muscle hemodynamic, metabolic, and contractile function in young (12) compared with old (present study) rats during contractions. These data thus suggest that nNOS-mediated regulation of skeletal muscle microvascular and contractile function during transitions in metabolic demand is altered with advanced aging.

Alterations in nNOS-mediated regulation of skeletal muscle during contractions, but not at rest, with aging could emanate from age-related disruptions in myocyte redox state. Enhanced reactive oxygen and nitrogen species accumulation during muscle contractions (2, 8) coupled with impaired antioxidant mechanisms in old individuals (65) may exacerbate the underlying oxidative stress characteristic of aging (17) and promote nNOS uncoupling and/or direct NO inactivation (21, 49, 58). This leads to reduced nNOS-derived NO bioavailability that likely impacts contracting skeletal muscle microvascular and contractile function via multiple mechanisms. Specifically, considerable evidence in young subjects suggests critical regulatory roles for NO derived from nNOS on contracting muscle blood flow (via cGMP formation and/or functional sympatholysis; Refs. 15, 39, 60), V̇o₂ (contribution to oxidative enzyme inertia at contractions onset; e.g., ref. 34), and, therefore, Po_2mv kinetics (12). In addition, nNOS-derived NO bioavailability likely modulates submaximal force production via inhibitory influences on myofibrillar contractile elements (36).

Clinical implications.

Understanding how aging impacts the functional role of nNOS-derived NO on skeletal muscle represents the initial step toward the development of potential therapeutic strategies targeting this isoform. The use of isoform-selective NOS inhibitors is therefore crucial in such investigations. As discussed in detail elsewhere (12, 13) and demonstrated by previous pharmacological studies (18, 64) and the current hypotensive responses to ACh (Fig. 1), SMTC represents a viable tool for this purpose based on its selectivity for nNOS over both eNOS and iNOS inhibition. Acute selective pharmacological inhibition as used herein also minimizes the potential for chronic compensation of genetically modified nNOS models by other isoforms (33).

It is noteworthy that alterations in skeletal muscle nNOS expression and/or activity with aging (9, 11, 50, 55) may not predict nNOS-mediated function particularly in conditions associated with significant oxidative stress (e.g., aging, chronic heart failure, and diabetes), which promotes nNOS uncoupling and/or direct NO inactivation (21, 49, 58). Our current results therefore suggest that nNOS-mediated regulation of skeletal muscle microvascular and contractile function is altered in old individuals. Important clinical implications arise from these findings when considering that endurance exercise training could improve nNOS-mediated function via upregulation of nNOS expression and/or activity (55) in conjunction with augmented antioxidant capacity (62).

Besides its effects on nNOS-mediated function, it is important to acknowledge that advanced age results in downregulation of eNOS alongside upregulation of iNOS, which is consistent with a greater role for NO in inflammatory processes and a reduced participation in contractile function in aged skeletal muscle (55). This shift in the muscle NOS isoform profile with aging is hallmarked by oxidative stress and endothelial dysfunction (14, 24, 26, 54, 59, 61, 65), which likely contribute to exercise intolerance in this population (47).

Experimental considerations.

In view of the greater relative expression and activity of nNOS in fast-twitch compared with slow-twitch muscles (36, 39), potential alterations in skeletal muscle fiber-type composition with aging could partly underlie the responses seen herein with SMTC. However, it must be noted that potential age-related shifts in muscle phenotype may not only favor an increased abundance of slow-twitch fibers as traditionally considered but also promote alterations in the opposite direction (i.e., increased abundance of fast-twitch fibers) in both humans and animals (see Ref. 10 for discussion). Despite these possibilities, F344×BN rats do not appear to experience significant changes in fiber-type composition within representative skeletal muscles across the age range used herein (42).

In accordance with the latter and the fact that F344×BN rats do not develop many of the age-related pathologies seen in their highly inbred counterparts (which is essential to discern healthy aging from pathological decay; ref. 40), the National Institutes of Health support currently the use of the F344×BN rat as a model of aging. In the absence of evidence to the contrary, we therefore consider that these key facets outweigh any likely interspecies differences in nNOS-mediated control and thus facilitate comparison with young Sprague-Dawley rats (12) and suggest that differences in strain are unlikely to play a major role in the responses seen herein following selective nNOS inhibition with SMTC (Fig. 6).

Conclusions.

Pharmacological isoform-specific NOS inhibition revealed that nNOS-mediated control of contracting skeletal muscle function is altered with advanced aging. In marked contrast to the responses seen previously in young rats (12), nNOS inhibition with SMTC evoked no alterations in contracting muscle blood flow, V̇o₂, Po_2mv kinetics, and submaximal force production in old rats (as illustrated in Fig. 6). These novel findings suggest that, in addition to the documented deterioration in eNOS function (e.g., refs. 54, 65), alterations in nNOS-mediated regulation of contracting skeletal muscle microvascular function with aging may contribute to reduced exercise capacity in this population.

GRANTS

This research was supported in part by a Fellowship from the Brazilian Ministry of Education/CAPES-Fulbright and a Doctoral Student Research grant from the American College of Sports Medicine Foundation (to D. M. Hirai); American Heart Association Heartland Affiliate Grant 0750090Z (to T. I. Musch); Kansas State University SMILE award, American Heart Association Midwest Affiliate Grant 10GRNT4350011, and National Institutes of Health Grant HL-108328 (to D. C. Poole).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

D.M.H., S.W.C., T.I.M., and D.C.P. conception and design of research; D.M.H., S.W.C., C.T.H., S.K.F., and T.I.M. performed experiments; D.M.H. analyzed data; D.M.H. interpreted results of experiments; D.M.H. prepared figures; D.M.H. drafted manuscript; D.M.H., S.W.C., C.T.H., S.K.F., T.I.M., and D.C.P. edited and revised manuscript; D.M.H., S.W.C., C.T.H., S.K.F., T.I.M., and D.C.P. approved final version of manuscript.