Muscle microvascular oxygenation in chronic heart failure: role of nitric oxide availability

Effects of acute phosphodiesterase type 5 inhibition on skeletal muscle interstitial PO2 during contractions and recovery

The oxygen partial pressure within the interstitial space (PO2is; mmHg) provides the driving force for oxygen diffusion into the myocyte thereby supporting oxidative phosphorylation. We tested the hypothesis that potentiation of the nitric oxide pathway with sildenafil (phosphodiesterase type 5 inhibitor) would enhance PO2is during muscle metabolic transitions, thereby slowing PO2is on- and accelerating PO2is off-kinetics. The rat spinotrapezius muscle (n = 17) was exposed for PO2is measurements via phosphorescence quenching under control (CON), low-dose sildenafil (1 mg/kg i.a., SIL1) and high-dose sildenafil (7 mg/kg i.a., SIL7). Data were collected at rest and during submaximal twitch contractions (1 Hz, 4-6 V, 3 min) and recovery (3 min). Mean arterial blood pressure (MAP; mmHg) was reduced with both SIL1 (pre:132 ± 5; post:99 ± 5) and SIL7 (pre:111 ± 6; post:99 ± 4) (p < 0.05). SIL7 elevated resting PO2is (18.4 ± 1.1) relative to both CON (15.7 ± 0.7) and SIL1 (15.2 ± 0.7) (p < 0.05). In addition, SIL7 increased end-recovery PO2is (17.7 ± 1.6) compared to CON (12.8 ± 0.9) and SIL1 (13.4 ± 0.8) (p < 0.05). The overall PO2is response during recovery (i.e., area under the PO2is curve) was greater in SIL7 (4107 ± 444) compared to CON (3493 ± 222) and SIL1 (3114 ± 205 mmHg s) (p < 0.05). Contrary to our hypothesis, there was no impact of acute SIL (1 or 7 mg/kg) on the speed of the PO2is response during contractions or recovery (p > 0.05). However, sildenafil lowered MAP and improved skeletal muscle interstitial oxygenation in healthy rats. Specifically, SIL7 enhanced PO2is at rest and during recovery from submaximal muscle contractions. Potentiation of the nitric oxide pathway with sildenafil enhances microvascular blood-myocyte O2 transport and is expected to improve repeated bouts of contractile activity.

Capillary hemodynamics and contracting skeletal muscle oxygen pressures in male rats with heart failure: Impact of soluble guanylyl cyclase activator

In heart failure with reduced ejection fraction (HFrEF), nitric oxide-soluble guanylyl cyclase (sGC) pathway dysfunction impairs skeletal muscle arteriolar vasodilation and thus capillary hemodynamics, contributing to impaired oxygen uptake (V̇O2) kinetics. Targeting this pathway with sGC activators offers a new treatment approach to HFrEF. We tested the hypotheses that sGC activator administration would increase the O2 delivery (Q̇O2)-to-V̇O2 ratio in the skeletal muscle interstitial space (PO2is) of HFrEF rats during twitch contractions due, in part, to increases in red blood cell (RBC) flux (fRBC), velocity (VRBC), and capillary hematocrit (Hctcap). HFrEF was induced in male Sprague-Dawley rats via myocardial infarction. After 3 weeks, rats were treated with 0.3 mg/kg of the sGC activator BAY 60-2770 (HFrEF + BAY; n = 11) or solvent (HFrEF; n = 9) via gavage b.i.d for 5 days prior to phosphorescence quenching (PO2is, in contracting muscle) and intravital microscopy (resting) measurements in the spinotrapezius muscle. Intravital microscopy revealed higher fRBC (70 ± 9 vs 25 ± 8 RBC/s), VRBC (490 ± 43 vs 226 ± 35 μm/s), Hctcap (16 ± 1 vs 10 ± 1%) and a greater number of capillaries supporting flow (91 ± 3 vs 82 ± 3%) in HFrEF + BAY vs HFrEF (all P < 0.05). Additionally, PO2is was especially higher during 12-34s of contractions in HFrEF + BAY vs HFrEF (P < 0.05). Our findings suggest that sGC activators improved resting Q̇O2 via increased fRBC, VRBC, and Hctcap allowing for better Q̇O2-to-V̇O2 matching during the rest-contraction transient, supporting sGC activators as a potential therapeutic to target skeletal muscle vasomotor dysfunction in HFrEF.

Respiratory muscle work influences locomotor convective and diffusive oxygen transport in human heart failure during exercise

Introduction

It remains unclear if naturally occurring respiratory muscle (RM) work influences leg diffusive O₂ transport during exercise in heart failure patients with reduced ejection fraction (HFrEF). In this retrospective study, we hypothesized that RM unloading during submaximal exercise will lead to increases in locomotor muscle O₂ diffusion capacity (D_MO₂) contributing to the greater leg VO₂.

Methods

Ten HFrEF patients and 10 healthy control matched participants performed two submaximal exercise bouts (i.e., with and without RM unloading). During exercise, leg blood flow was measured via constant infusion thermodilution. Intrathoracic pressure was measured via esophageal balloon. Radial arterial and femoral venous blood gases were measured and used to calculate leg arterial and venous content (CaO₂ and CvO₂, respectively), VO₂, O₂ delivery, and D_MO₂.

Results

From CTL to RM unloading, leg VO₂, O₂ delivery, and D_MO₂ were not different in healthy participants during submaximal exercise (all, p > .15). In HFrEF, leg VO₂ (CTL: 0.7 ± 0.3 vs. RM unloading: 1.0 ± 0.4 L/min, p < .01), leg O₂ delivery (CTL: 0.9 ± 0.4 vs. RM unloading: 1.4 ± 0.5 L/min, p < .01), and leg D_MO₂ (CTL: 31.5 ± 11.4 vs. RM unloading: 49.7 ± 18.6 ml min⁻¹ mmHg⁻¹) increased from CTL to RM unloading during submaximal exercise (all, p < .01), whereas CaO₂‐CvO₂ was not different (p = .51). The degree of RM unloading (i.e., % decrease in esophageal pressure‐time integral during inspiration) was related to the % increase in leg D_MO₂ with RM unloading (r = −.76, p = .01).

Conclusion

Our data suggest RM unloading leads to increased leg VO₂ due to greater convective and diffusive O₂ transport during submaximal exercise in HFrEF patients.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

RNA-sequencing reveals transcriptional signature of pathological remodeling in the diaphragm of rats after myocardial infarction

The diaphragm is the main inspiratory muscle, and the chronic phase post-myocardial infarction (MI) is characterized by diaphragm morphological, contractile, and metabolic abnormalities. However, the mechanisms of diaphragm weakness are not fully understood. In the current study, we aimed to identify the transcriptome changes associated with diaphragm abnormalities in the chronic stage MI. We ligated the left coronary artery to cause MI in rats and performed RNA-sequencing (RNA-Seq) in diaphragm samples 16 weeks post-surgery. The sham group underwent thoracotomy and pericardiotomy but no artery ligation. We identified 112 differentially expressed genes (DEGs) out of a total of 9664 genes. Myocardial infarction upregulated and downregulated 42 and 70 genes, respectively. Analysis of DEGs in the framework of skeletal muscle-specific biological networks suggest remodeling in the neuromuscular junction, extracellular matrix, sarcomere, cytoskeleton, and changes in metabolism and iron homeostasis. Overall, the data are consistent with pathological remodeling of the diaphragm and reveal potential biological targets to prevent diaphragm weakness in the chronic stage MI.

Transcapillary PO2 gradients in contracting muscles across the fibre type and oxidative continuum

KEY POINTS

Within skeletal muscle the greatest resistance to oxygen transport is thought to reside across the short distance at the red blood cell-myocyte interface. These structures generate a significant transmural oxygen pressure (PO₂ ) gradient in mixed fibre-type muscle. Increasing O₂ flux across the capillary wall during exercise depends on: (i) the transmural O₂ pressure gradient, which is maintained in mixed-fibre muscle, and/or (ii) elevating diffusing properties between microvascular and interstitial compartments resulting, in part, from microvascular haemodynamics and red blood cell distribution. We evaluated the PO₂ within the microvascular and interstitial spaces of muscles spanning the slow- to fast-twitch fibre and high- to low-oxidative capacity spectrums, at rest and during contractions, to assess the magnitude of transcapillary PO₂ gradients in rats. Our findings demonstrate that, across the metabolic rest-contraction transition, the transcapillary pressure gradient for O₂ flux is: (i) maintained in all muscle types, and (ii) the lowest in contracting highly oxidative fast-twitch muscle.

ABSTRACT

In mixed fibre-type skeletal muscle transcapillary PO₂ gradients (PO₂ mv-PO₂ is; microvascular and interstitial, respectively) drive O₂ flux across the blood-myocyte interface where the greatest resistance to that O₂ flux resides. We assessed a broad spectrum of fibre-type and oxidative-capacity rat muscles across the rest-to-contraction (1 Hz, 120 s) transient to test the novel hypotheses that: (i) slow-twitch PO₂ is would be greater than fast-twitch, (ii) muscles with greater oxidative capacity have greater PO₂ is than glycolytic counterparts, and (iii) whether PO₂ mv-PO₂ is at rest is maintained during contractions across all muscle types. PO₂ mv and PO₂ is were determined via phosphorescence quenching in soleus (SOL; 91% type I+IIa fibres and CSa: ∼21 μmol min^-1 g^-1 ), peroneal (PER; 33% and ∼20 μmol min^-1 g^-1 ), mixed (MG; 9% and ∼26 μmol min^-1 g^-1 ) and white gastrocnemius (WG; 0% and ∼8 μmol min^-1 g^-1 ) across the rest-contraction transient. PO₂ mv was higher than PO₂ is in each muscle (∼6-13 mmHg; P < 0.05). SOL PO₂ is_area was greater than in the fast-twitch muscles during contractions (P < 0.05). Oxidative muscles had greater PO₂ is_nadir (9.4 ± 0.8, 7.4 ± 0.9 and 6.4 ± 0.4; SOL, PER and MG, respectively) than WG (3.0 ± 0.3 mmHg, P < 0.05). The magnitude of PO₂ mv-PO₂ is at rest decreased during contractions in MG only (∼11 to 7 mmHg; time × (PO₂ mv-PO₂ is) interaction, P < 0.05). These data support the hypothesis that, since transcapillary PO₂ gradients during contractions are maintained in all muscle types, increased O₂ flux must occur via enhanced intracapillary diffusing conductance, which is most extreme in highly oxidative fast-twitch muscle.

Central and peripheral factors mechanistically linked to exercise intolerance in heart failure with reduced ejection fraction

Exercise intolerance is a primary symptom of heart failure (HF); however, the specific contribution of central and peripheral factors to this intolerance is not well described. The hyperbolic relationship between exercise intensity and time to exhaustion (speed-duration relationship) defines exercise tolerance but is underused in HF. We tested the hypotheses that critical speed (CS) would be reduced in HF, resting central functional measurements would correlate with CS, and the greatest HF-induced peripheral dysfunction would occur in more oxidative muscle. Multiple treadmill-constant speed runs to exhaustion were used to quantify CS and D' (distance coverable above CS) in healthy control (Con) and HF rats. Central function was determined via left ventricular (LV) Doppler echocardiography [fractional shortening (FS)] and a micromanometer-tipped catheter [LV end-diastolic pressure (LVEDP)]. Peripheral O₂ delivery-to-utilization matching was determined via phosphorescence quenching (interstitial Po₂, Po_2 is) in the soleus and white gastrocnemius during electrically induced twitch contractions (1 Hz, 8V). CS was lower in HF compared with Con (37 ± 1 vs. 44 ± 1 m/min, P < 0.001), but D' was not different (77 ± 8 vs. 69 ± 13 m, P = 0.6). HF reduced FS (23 ± 2 vs. 47 ± 2%, P < 0.001) and increased LVEDP (15 ± 1 vs. 7 ± 1 mmHg, P < 0.001). CS was related to FS (r = 0.72, P = 0.045) and LVEDP (r = -0.75, P = 0.02) only in HF. HF reduced soleus Po_2 is at rest and during contractions (both P < 0.01) but had no effect on white gastrocnemius Po_2 is (P > 0.05). We show in HF rats that decrements in central cardiac function relate directly with impaired exercise tolerance (i.e., CS) and that this compromised exercise tolerance is likely due to reduced perfusive and diffusive O₂ delivery to oxidative muscles.NEW & NOTEWORTHY We show that critical speed (CS), which defines the upper boundary of sustainable activity, can be resolved in heart failure (HF) animals and is diminished compared with controls. Central cardiac function is strongly related with CS in the HF animals, but not controls. Skeletal muscle O₂ delivery-to-utilization dysfunction is evident in the more oxidative, but not glycolytic, muscles of HF rats and is explained, in part, by reduced nitric oxide bioavailability.

Edward F. Adolph Distinguished Lecture. Contemporary model of muscle microcirculation: gateway to function and dysfunction

This review strikes at the very heart of how the microcirculation functions to facilitate blood-tissue oxygen, substrate, and metabolite fluxes in skeletal muscle. Contemporary evidence, marshalled from animals and humans using the latest techniques, challenges iconic perspectives that have changed little over the past century. Those perspectives include the following: the presence of contractile or collapsible capillaries in muscle, unitary control by precapillary sphincters, capillary recruitment at the onset of contractions, and the notion of capillary-to-mitochondrial diffusion distances as limiting O₂ delivery. Today a wealth of physiological, morphological, and intravital microscopy evidence presents a completely different picture of microcirculatory control. Specifically, capillary red blood cell (RBC) and plasma flux is controlled primarily at the arteriolar level with most capillaries, in healthy muscle, supporting at least some flow at rest. In healthy skeletal muscle, this permits substrate access (whether carried in RBCs or plasma) to a prodigious total capillary surface area. Pathologies such as heart failure or diabetes decrease access to that exchange surface by reducing the proportion of flowing capillaries at rest and during exercise. Capillary morphology and function vary disparately among tissues. The contemporary model of capillary function explains how, following the onset of exercise, muscle O₂ uptake kinetics can be extremely fast in health but slowed in heart failure and diabetes impairing contractile function and exercise tolerance. It is argued that adoption of this model is fundamental for understanding microvascular function and dysfunction and, as such, to the design and evaluation of effective therapeutic strategies to improve exercise tolerance and decrease morbidity and mortality in disease.

Sexual dimorphism in the control of skeletal muscle interstitial Po2 of heart failure rats: effects of dietary nitrate supplementation

Sex differences in the mechanisms underlying cardiovascular pathophysiology of O₂ transport in heart failure (HF) remain to be explored. In HF, nitric oxide (NO) bioavailability is reduced and contributes to deficits in O₂ delivery-to-utilization matching. Females may rely more on NO for cardiovascular control and as such experience greater decrements in HF. We tested the hypotheses that moderate HF induced by myocardial infarction would attenuate the skeletal muscle interstitial Po₂ response to contractions (Po_2is; determined by O₂ delivery-to-utilization matching) compared with healthy controls and females would express greater dysfunction than male counterparts. Furthermore, we hypothesized that 5 days of dietary nitrate supplementation (Nitrate; 1 mmol·kg^-1·day^-1) would raise Po_2is in HF rats. Forty-two Sprague-Dawley rats were randomly assigned to healthy, HF, or HF + Nitrate groups (each n = 14; 7 female/7 male). Spinotrapezius Po_2is was measured via phosphorescence quenching during electrically induced twitch contractions (180 s; 1 Hz). HF reduced resting Po_2is for both sexes compared with healthy controls (P < 0.01), and females were lower than males (14 ± 1 vs. 17 ± 2 mmHg) (P < 0.05). In HF both sexes expressed reduced Po_2is amplitudes following the onset of muscle contractions compared with healthy controls (female: -41 ± 7%, male: -26 ± 12%) (P < 0.01). In HF rats, Nitrate elevated resting Po_2is to values not different from healthy rats and removed the sex difference. Female HF + Nitrate rats expressed greater resting Po_2is and amplitudes compared with female HF (P < 0.05). In this model of moderate HF, O₂ delivery-to-utilization matching in the interstitial space is diminished in a sex-specific manner and dietary nitrate supplementation may serve to offset this reduction in HF rats with greater effects in females. NEW & NOTEWORTHY Interstitial Po₂ (Po_2is; indicative of O₂ delivery-to-utilization matching) determines, in part, O₂ flux into skeletal muscle. We show that heart failure (HF) reduces Po_2is at rest and during skeletal muscle contractions in rats and this negative effect is amplified for females. However, elevating NO bioavailability with dietary nitrate supplementation increases resting Po_2is and alters the dynamic response with greater efficacy in female HF rats, particularly at rest and following the onset of muscle contractions.

Small-hairpin RNA and pharmacological targeting of neutral sphingomyelinase prevent diaphragm weakness in rats with heart failure and reduced ejection fraction

Heart failure with reduced ejection fraction (HFREF) increases neutral sphingomyelinase (NSMase) activity and mitochondrial reactive oxygen species (ROS) emission and causes diaphragm weakness. We tested whether a systemic pharmacological NSMase inhibitor or short-hairpin RNA (shRNA) targeting NSMase isoform 3 (NSMase3) would prevent diaphragm abnormalities induced by HFREF caused by myocardial infarction. In the pharmacological intervention, we used intraperitoneal injection of GW4869 or vehicle. In the genetic intervention, we injected adeno-associated virus serotype 9 (AAV9) containing shRNA targeting NSMase3 or a scrambled sequence directly into the diaphragm. We also studied acid sphingomyelinase-knockout mice. GW4869 prevented the increase in diaphragm ceramide content, weakness, and tachypnea caused by HFREF. For example, maximal specific forces (in N/cm²) were vehicle [sham 31 ± 2 and HFREF 26 ± 2 ( P < 0.05)] and GW4869 (sham 31 ± 2 and HFREF 31 ± 1). Respiratory rates were (in breaths/min) vehicle [sham 61 ± 3 and HFREF 84 ± 11 ( P < 0.05)] and GW4869 (sham 66 ± 2 and HFREF 72 ± 2). AAV9-NSMase3 shRNA prevented heightening of diaphragm mitochondrial ROS and weakness [in N/cm², AAV9-scrambled shRNA: sham 31 ± 2 and HFREF 27 ± 2 ( P < 0.05); AAV9-NSMase3 shRNA: sham 30 ± 1 and HFREF 30 ± 1] but displayed tachypnea. Both wild-type and ASMase-knockout mice with HFREF displayed diaphragm weakness. Our study suggests that activation of NSMase3 causes diaphragm weakness in HFREF, presumably through accumulation of ceramide and elevation in mitochondrial ROS. Our data also reveal a novel inhibitory effect of GW4869 on tachypnea in HFREF likely mediated by changes in neural control of breathing.

Impact of cell-free hemoglobin on contracting skeletal muscle microvascular oxygen pressure dynamics

Free hemoglobin (Hb) associated with hemolysis extravasates into vascular tissue and depletes nitric oxide (NO), which leads to impaired vascular function and could impair skeletal muscle metabolic control during exercise. We tested the hypothesis that: 1) free Hb would extravasate into skeletal muscle tissue, reducing the contracting skeletal muscle O₂ delivery/O₂ utilization ratio (microvascular PO₂, PO₂mv) to a similar extent as that observed following NO synthase (NOS) blockade, and 2) that the Hb scavenging protein haptoglobin (Hp) would prevent Hb extravasation and inhibit these skeletal muscle tissue effects. PO₂mv was measured in eight rats (phosphorescence quenching) at rest and during 180 s of electrically induced (1-Hz) twitch spinotrapezius muscle contractions (experiment 1). A second group of seven rats was also used to investigate the effects of Hb + Hp (experiment 2). For both experiments, measurements were made: 1) during control conditions, 2) following a bolus infusion of either Hb (50 mg/kg) or Hb + Hp (50 mg/kg), and 3) following local superfusion of NG-nitro-l-arginine methyl ester (L-NAME; 10 mg/kg). Additional experiments were completed to visualize Hb extravasation into the muscular tissue using Click chemistry techniques. There were no significant differences in the PO₂mv observed at rest for any condition in either experiment (p > 0.05 for all). In experiment 1, both Hb and L-NAME reduced the PO₂mv significantly during the steady-state of muscle contractions when compared to control conditions with no differences between Hb and L-NAME (control: 24 ± 1, Hb: 21 ± 1, L-NAME: 20 ± 1 mmHg, p < 0.05). In experiment 2, only L-NAME resulted in a significantly lower PO₂mv during the steady-state of muscle contractions (control: 25 ± 1, Hb + Hp: 22 ± 2, L-NAME: 18 ± 1 mmHg, p < 0.05). Free Hb lowered the blood-myocyte O₂ driving force to a level not significantly different from L-NAME. However, infusing Hb bound to Hp resulted in no significant differences in steady-state PO₂mv during muscle contractions when compared to control. Surprisingly, we did not observe Hb accumulation in skeletal muscle tissue. Taken together these data suggests that free Hb impairs O₂ delivery/utilization via a NO scavenging effect. Furthermore, the unchanged PO₂mv steady-state observed following Hb + Hp further indicates that vascular compartmentalization of Hb by the scavenger protein haptoglobin may improve skeletal muscle metabolic control and potentially exercise tolerance in those afflicted with hemolytic diseases.

Sex and nitric oxide bioavailability interact to modulate interstitial Po2 in healthy rat skeletal muscle

Premenopausal women express reduced blood pressure and risk of cardiovascular disease relative to age-matched men. This purportedly relates to elevated estrogen levels increasing nitric oxide synthase (NOS) activity and NO-mediated vasorelaxation. We tested the hypotheses that female rat skeletal muscle would: 1) evince a higher O₂ delivery-to-utilization ratio (Q̇o₂/V̇o₂) during contractions; and 2) express greater modulation of Q̇o₂/V̇o₂ with changes to NO bioavailability compared with male rats. The spinotrapezius muscle of Sprague-Dawley rats (females = 8, males = 8) was surgically exposed and electrically-stimulated (180 s, 1 Hz, 6 V). OxyphorG4 was injected into the muscle and phosphorescence quenching employed to determine the temporal profile of interstitial Po₂ (Po_2is, determined by Q̇o₂/V̇o₂). This was performed under three conditions: control (CON), 300 µM sodium nitroprusside (SNP; NO donor), and 1.5 mM N^ω-nitro-l-arginine methyl ester (l-NAME; NOS blockade) superfusion. No sex differences were found for the Po_2is kinetics parameters in CON or l-NAME ( P > 0.05), but females elicited a lower baseline following SNP (males 42 ± 3 vs. females 36 ± 2 mmHg, P < 0.05). Females had a lower ΔPo_2is during contractions following SNP (males 22 ± 3 vs. females 17 ± 2 mmHg, P < 0.05), but there were no sex differences for the temporal response to contractions ( P > 0.05). The total NO effect (SNP minus l-NAME) on Po_2is was not different between sexes. However, the spread across both conditions was shifted to a lower absolute range for females (reduced SNP baseline and greater reduction following l-NAME). These data support that females have a greater reliance on basal NO bioavailability and males have a greater responsiveness to exogenous NO and less responsiveness to reduced endogenous NO. NEW & NOTEWORTHY Interstitial Po₂ (Po_2is; determined by O₂ delivery-to-utilization matching) plays an important role for O₂ flux into skeletal muscle. We show that both sexes regulate Po_2is at similar levels at rest and during skeletal muscle contractions. However, modulating NO bioavailability exposes sex differences in this regulation with females potentially having a greater reliance on basal NO bioavailability and males having a greater responsiveness to exogenous NO and less responsiveness to reduced endogenous NO.

Exercise limitations in heart failure with reduced and preserved ejection fraction

The hallmark symptom of chronic heart failure (HF) is severe exercise intolerance. Impaired perfusive and diffusive O₂ transport are two of the major determinants of reduced physical capacity and lowered maximal O₂ uptake in patients with HF. It has now become evident that this syndrome manifests at least two different phenotypic variations: heart failure with preserved or reduced ejection fraction (HFpEF and HFrEF, respectively). Unlike HFrEF, however, there is currently limited understanding of HFpEF pathophysiology, leading to a lack of effective pharmacological treatments for this subpopulation. This brief review focuses on the disturbances within the O₂ transport pathway resulting in limited exercise capacity in both HFpEF and HFrEF. Evidence from human and animal research reveals HF-induced impairments in both perfusive and diffusive O₂ conductances identifying potential targets for clinical intervention. Specifically, utilization of different experimental approaches in humans (e.g., small vs. large muscle mass exercise) and animals (e.g., intravital microscopy and phosphorescence quenching) has provided important clues to elucidating these pathophysiological mechanisms. Adaptations within the skeletal muscle O₂ delivery-utilization system following established and emerging therapies (e.g., exercise training and inorganic nitrate supplementation, respectively) are discussed. Resolution of the underlying mechanisms of skeletal muscle dysfunction and exercise intolerance is essential for the development and refinement of the most effective treatments for patients with HF.

Vascular KATP channels mitigate severe muscle O2 delivery-utilization mismatch during contractions in chronic heart failure rats

The vascular ATP-sensitive K⁺ (K_ATP) channel is a mediator of skeletal muscle microvascular oxygenation (PO₂mv) during contractions in health. We tested the hypothesis that K_ATP channel function is preserved in chronic heart failure (CHF) and therefore its inhibition would reduce PO₂mv and exacerbate the time taken to reach the PO₂mv steady-state during contractions of the spinotrapezius muscle. Moreover, we hypothesized that subsequent K_ATP channel activation would oppose the effects of this inhibition. Muscle PO₂mv (phosphorescence quenching) was measured during 180s of 1-Hz twitch contractions (∼6V) under control, glibenclamide (GLI, K_ATP channel antagonist; 5mg/kg) and pinacidil (PIN, K_ATP channel agonist; 5mg/kg) conditions in 16 male Sprague-Dawley rats with CHF induced via myocardial infarction (coronary artery ligation, left ventricular end-diastolic pressure: 18±1mmHg). GLI reduced baseline PO₂mv (control: 28.3±0.9, GLI: 24.8±1.0mmHg, p<0.05), lowered mean PO₂mv (average PO₂mv during the overall time taken to reach the steady-state; control: 20.6±0.6, GLI: 17.6±0.3mmHg, p<0.05), and slowed the attainment of steady-state PO₂mv (overall mean response time; control: 66.1±10.2, GLI: 93.6±7.8s, p<0.05). PIN opposed these effects on the baseline PO₂mv, mean PO₂mv and time to reach the steady-state PO₂mv (p<0.05 for all vs. GLI). Inhibition of K_ATP channels exacerbates the transient mismatch between muscle O₂ delivery and utilization in CHF rats and this effect is opposed by PIN. These data reveal that the K_ATP channel constitutes one of the select few well-preserved mechanisms of skeletal muscle microvascular oxygenation control in CHF.

Effect of sodium nitrite on local control of contracting skeletal muscle microvascular oxygen pressure in healthy rats

Exercise intolerance characteristic of diseases such as chronic heart failure (CHF) and diabetes is associated with reduced nitric oxide (NO) bioavailability from nitric oxide synthase (NOS), resulting in an impaired microvascular O₂ driving pressure (Po₂mv; O₂ delivery/O₂ utilization) and metabolic control. Infusions of the potent NO donor sodium nitroprusside augment NO bioavailability yet decrease mean arterial pressure (MAP) thereby reducing its potential efficacy for patient populations. To eliminate or reduce hypotensive sequelae, [Formula: see text] was superfused onto the spinotrapezius muscle. It was hypothesized that local [Formula: see text] administration would elevate resting Po₂mv and slow Po₂mv kinetics [increased time constant (τ) and mean response time (MRT)] following the onset of muscle contractions without decreasing MAP. In 12 anesthetized male Sprague-Dawley rats, Po₂mv of the circulation-intact spinotrapezius muscle was measured by phosphorescence quenching during 180 s of electrically induced twitch contractions (1 Hz) before and after superfusion of sodium nitrite (NaNO₂ 30 mM). [Formula: see text] superfusion elevated resting Po₂mv (control: 28.4 ± 1.1 vs. [Formula: see text]: 31.6 ± 1.2 mmHg; P ≤ 0.05), τ (control: 12.3 ± 1.2 vs. [Formula: see text]: 19.7 ± 2.2 s; P ≤ 0.05), and MRT (control: 19.3 ± 1.9 vs. [Formula: see text]: 25.6 ± 3.3 s; P ≤ 0.05). Importantly, these effects occurred in the absence of any reduction in MAP (103 ± 4 vs. 105 ± 4 mmHg, pre- and postsuperfusion respectively; P > 0.05). These results indicate that [Formula: see text] supplementation delivered to the muscle directly through [Formula: see text] superfusion enhances the blood-myocyte oxygen driving pressure without compromising MAP at rest and following the onset of muscle contraction. This strategy has substantial clinical utility for a range of ischemic conditions.

NEW & NOTEWORTHY

Ischemic conditions as diverse as chronic heart failure (CHF) and frostbite inflict tissue damage via inadequate O₂ delivery. Herein we demonstrate that direct application of sodium nitrite enhances the O₂ supply-O₂ demand relationship, raising microvascular O₂ pressure in healthy skeletal muscle. This therapeutic action of nitrite-derived nitric oxide occurred without inducing systemic hypotension and has the potential to relieve focal ischemia and preserve tissue vitality by enhancing O₂ delivery.

Dietary nitrate supplementation: impact on skeletal muscle vascular control in exercising rats with chronic heart failure

Chronic heart failure (CHF) results in central and peripheral derangements that ultimately reduce skeletal muscle O2 delivery and impair exercise tolerance. Dietary nitrate (NO3 (-)) supplementation improves skeletal muscle vascular function and tolerance to exercise. We tested the hypothesis that NO3 (-) supplementation would elevate exercising skeletal muscle blood flow (BF) and vascular conductance (VC) in CHF rats. Myocardial infarction (MI) was induced (coronary artery ligation) in young adult male rats. After 21 days of recovery, rats randomly received 5 days of NO3 (-)-rich beetroot juice (CHF + BR, n = 10) or a placebo (CHF, n = 10). Mean arterial pressure (carotid artery catheter) and skeletal muscle BF (radiolabeled microspheres) were measured during treadmill exercise (20 m/min, 5% grade). CHF-induced dysfunction, as determined by myocardial infarction size (29 ± 3% and 33 ± 4% in CHF and CHF + BR, respectively) and left ventricular end-diastolic pressure (18 ± 2 and 18 ± 2 mmHg in CHF and CHF + BR, respectively), and exercising mean arterial pressure (131 ± 3 and 128 ± 4 mmHg in CHF and CHF + BR, respectively) were not different (P > 0.05) between groups. Total exercising hindlimb skeletal muscle BF (95 ± 5 and 116 ± 9 ml·min(-1)·100 g(-1) in CHF and CHF + BR, respectively) and VC (0.75 ± 0.05 and 0.90 ± 0.05 ml·min(-1)·100 g(-1)·mmHg(-1) in CHF and CHF + BR, respectively) were 22% and 20% greater in BR-supplemented rats, respectively (P < 0.05). During exercise, BF in 9 and VC in 10 hindlimb muscles and muscle portions were significantly greater in the CHF + BR group. These results provide strong evidence that dietary NO3 (-) supplementation improves skeletal muscle vascular function during exercise in rats with CHF and, thus, support the use of BR as a novel therapeutic modality for the treatment of CHF.

Pharmacological targeting of mitochondrial reactive oxygen species counteracts diaphragm weakness in chronic heart failure

Diaphragm muscle weakness in chronic heart failure (CHF) is caused by elevated oxidants and exacerbates breathing abnormalities, exercise intolerance, and dyspnea. However, the specific source of oxidants that cause diaphragm weakness is unknown. We examined whether mitochondrial reactive oxygen species (ROS) cause diaphragm weakness in CHF by testing the hypothesis that CHF animals treated with a mitochondria-targeted antioxidant have normal diaphragm function. Rats underwent CHF or sham surgery. Eight weeks after surgeries, we administered a mitochondrial-targeted antioxidant (MitoTEMPO; 1 mg·kg(-1)·day(-1)) or sterile saline (Vehicle). Left ventricular dysfunction (echocardiography) pre- and posttreatment and morphological abnormalities were consistent with the presence of CHF. CHF elicited a threefold (P < 0.05) increase in diaphragm mitochondrial H2O2 emission, decreased diaphragm glutathione content by 23%, and also depressed twitch and maximal tetanic force by ∼20% in Vehicle-treated animals compared with Sham (P < 0.05 for all comparisons). Diaphragm mitochondrial H2O2 emission, glutathione content, and twitch and maximal tetanic force were normal in CHF animals receiving MitoTEMPO. Neither CHF nor MitoTEMPO altered the diaphragm protein levels of antioxidant enzymes: superoxide dismutases (CuZn-SOD or MnSOD), glutathione peroxidase, and catalase. In both Vehicle and MitoTEMPO groups, CHF elicited a ∼30% increase in cytochrome c oxidase activity, whereas there were no changes in citrate synthase activity. Our data suggest that elevated mitochondrial H2O2 emission causes diaphragm weakness in CHF. Moreover, changes in protein levels of antioxidant enzymes or mitochondrial content do not seem to mediate the increase in mitochondria H2O2 emission in CHF and protective effects of MitoTEMPO.

The Spatial Distribution of Absolute Skeletal Muscle Deoxygenation During Ramp-Incremental Exercise Is Not Influenced by Hypoxia

Time-resolved near-infrared spectroscopy (TRS-NIRS) allows absolute quantitation of deoxygenated haemoglobin and myoglobin concentration ([HHb]) in skeletal muscle. We recently showed that the spatial distribution of peak [HHb] within the quadriceps during moderate-intensity cycling is reduced with progressive hypoxia and this is associated with impaired aerobic energy provision. We therefore aimed to determine whether reduced spatial distribution of skeletal muscle [HHb] was associated with impaired aerobic energy transfer during exhaustive ramp-incremental exercise in hypoxia. Seven healthy men performed ramp-incremental cycle exercise (20 W/min) to exhaustion at 3 fractional inspired O2 concentrations (FIO2): 0.21, 0.16, 0.12. Pulmonary O2 uptake ([Formula: see text]) was measured using a flow meter and gas analyser system. Lactate threshold (LT) was estimated non-invasively. Absolute muscle deoxygenation was quantified by multichannel TRS-NIRS from the rectus femoris and vastus lateralis (proximal and distal regions). [Formula: see text] and LT were progressively reduced (p<0.05) with hypoxia. There was a significant effect (p<0.05) of FIO2 on [HHb] at baseline, LT, and peak. However the spatial variance of [HHb] was not different between FIO2 conditions. Peak total Hb ([Hbtot]) was significantly reduced between FIO2 conditions (p<0.001). There was no association between reductions in the spatial distribution of skeletal muscle [HHb] and indices of aerobic energy transfer during ramp-incremental exercise in hypoxia. While regional [HHb] quantified by TRS-NIRS at exhaustion was greater in hypoxia, the spatial distribution of [HHb] was unaffected. Interestingly, peak [Hbtot] was reduced at the tolerable limit in hypoxia implying a vasodilatory reserve may exist in conditions with reduced FIO2.

Heterogeneity of Muscle Blood Flow and Metabolism: Influence of Exercise, Aging, and Disease States

The systematic increase in V˙O2 uptake and O2 extraction with increasing work rates conceals a substantial heterogeneity of O2 delivery (Q˙O2)-to- V˙O2 matching across and within muscles and other organs. We hypothesize that whether increased/decreased Q˙O2/V˙O2 heterogeneity can be judged as "good" or "bad," for example, after exercise training or in aged individuals or with disease (heart failure, diabetes) depends on the resultant effects on O2 transport and contractile performance.

Effects of nitrite infusion on skeletal muscle vascular control during exercise in rats with chronic heart failure

Chronic heart failure (CHF) reduces nitric oxide (NO) bioavailability and impairs skeletal muscle vascular control during exercise. Reduction of NO2 (-) to NO may impact exercise-induced hyperemia, particularly in muscles with pathologically reduced O2 delivery. We tested the hypothesis that NO2 (-) infusion would increase exercising skeletal muscle blood flow (BF) and vascular conductance (VC) in CHF rats with a preferential effect in muscles composed primarily of type IIb + IId/x fibers. CHF (coronary artery ligation) was induced in adult male Sprague-Dawley rats. After a >21-day recovery, mean arterial pressure (MAP; carotid artery catheter) and skeletal muscle BF (radiolabeled microspheres) were measured during treadmill exercise (20 m/min, 5% incline) with and without NO2 (-) infusion. The myocardial infarct size (35 ± 3%) indicated moderate CHF. NO2 (-) infusion increased total hindlimb skeletal muscle VC (CHF: 0.85 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1) and CHF + NO2 (-): 0.93 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1), P < 0.05) without changing MAP (CHF: 123 ± 4 mmHg and CHF + NO2 (-): 120 ± 4 mmHg, P = 0.17). Total hindlimb skeletal muscle BF was not significantly different (CHF: 102 ± 7 and CHF + NO2 (-): 109 ± 7 ml·min(-1)·100 g(-1) ml·min(-1)·100 g(-1), P > 0.05). BF increased in 6 (∼21%) and VC in 8 (∼29%) of the 28 individual muscles and muscle parts. Muscles and muscle portions exhibiting greater BF and VC after NO2 (-) infusion comprised ≥63% type IIb + IId/x muscle fibers. These data demonstrate that NO2 (-) infusion can augment skeletal muscle vascular control during exercise in CHF rats. Given the targeted effects shown herein, a NO2 (-)-based therapy may provide an attractive "needs-based" approach for treatment of the vascular dysfunction in CHF.

Exercise training in chronic heart failure: improving skeletal muscle O2 transport and utilization

Chronic heart failure (CHF) impairs critical structural and functional components of the O2 transport pathway resulting in exercise intolerance and, consequently, reduced quality of life. In contrast, exercise training is capable of combating many of the CHF-induced impairments and enhancing the matching between skeletal muscle O2 delivery and utilization (Q̇mO2 and V̇mO2 , respectively). The Q̇mO2 /V̇mO2 ratio determines the microvascular O2 partial pressure (PmvO2 ), which represents the ultimate force driving blood-myocyte O2 flux (see Fig. 1). Improvements in perfusive and diffusive O2 conductances are essential to support faster rates of oxidative phosphorylation (reflected as faster V̇mO2 kinetics during transitions in metabolic demand) and reduce the reliance on anaerobic glycolysis and utilization of finite energy sources (thus lowering the magnitude of the O2 deficit) in trained CHF muscle. These adaptations contribute to attenuated muscle metabolic perturbations (e.g., changes in [PCr], [Cr], [ADP], and pH) and improved physical capacity (i.e., elevated critical power and maximal V̇mO2 ). Preservation of such plasticity in response to exercise training is crucial considering the dominant role of skeletal muscle dysfunction in the pathophysiology and increased morbidity/mortality of the CHF patient. This brief review focuses on the mechanistic bases for improved Q̇mO2 /V̇mO2 matching (and enhanced PmvO2 ) with exercise training in CHF with both preserved and reduced ejection fraction (HFpEF and HFrEF, respectively). Specifically, O2 convection within the skeletal muscle microcirculation, O2 diffusion from the red blood cell to the mitochondria, and muscle metabolic control are particularly susceptive to exercise training adaptations in CHF. Alternatives to traditional whole body endurance exercise training programs such as small muscle mass and inspiratory muscle training, pharmacological treatment (e.g., sildenafil and pentoxifylline), and dietary nitrate supplementation are also presented in light of their therapeutic potential. Adaptations within the skeletal muscle O2 transport and utilization system underlie improvements in physical capacity and quality of life in CHF and thus take center stage in the therapeutic management of these patients.

Skeletal Muscle Vascular Control During Exercise: Impact of Nitrite Infusion During Nitric Oxide Synthase Inhibition in Healthy Rats

The nitric oxide synthase (NOS)-independent pathway of nitric oxide (NO) production in which nitrite (NO2 (-)) is reduced to NO may have therapeutic applications for those with cardiovascular diseases in which the NOS pathway is downregulated. We tested the hypothesis that NO2 (-) infusion would reduce mean arterial pressure (MAP) and increase skeletal muscle blood flow (BF) and vascular conductance (VC) during exercise in the face of NOS blockade via L-NAME. Following infusion of L-NAME (10 mg kg(-1), L-NAME), male Sprague-Dawley rats (3-6 months, n = 8) exercised without N(G)-nitro-L arginine methyl ester (L-NAME) and after infusion of sodium NO2 (-) (7 mg kg(-1); L-NAME + NO2 (-)). MAP and hindlimb skeletal muscle BF (radiolabeled microsphere infusions) were measured during submaximal treadmill running (20 m min(-1), 5% grade). Across group comparisons were made with a published control data set (n = 11). Relative to L-NAME, NO2 (-) infusion significantly reduced MAP (P < 0.03). The lower MAP in L-NAME+NO2 (-) was not different from healthy control animals (control: 137 ± 3 L-NAME: 157 ± 7, L-NAME + NO2 (-): 136 ± 5 mm Hg). Also, NO2 (-) infusion significantly increased VC when compared to L-NAME (P < 0.03), ultimately negating any significant differences from control animals (control: 0.78 ± 0.05, L-NAME: 0.57 ± 0.03, L-NAME + NO2 (-); 0.69 ± 0.04 mL min(-1) 100 g(-1) mm Hg(-1)) with no apparent fiber-type preferential effect. Overall, hindlimb BF was decreased significantly by L-NAME; however, in L-NAME + NO2 (-), BF improved to a level not significantly different from healthy controls (control: 108 ± 8, L-NAME: 88 ± 3, L-NAME + NO2 (-): 94 ± 6 mL min(-1) 100 g(-1), P = 0.38 L-NAME vs L-NAME + NO2 (-)). Individuals with diseases that impair NOS activity, and thus vascular function, may benefit from a NO2 (-)-based therapy in which NO bioavailability is elevated in an NOS-independent manner.

Skeletal muscle microvascular oxygenation dynamics in heart failure: exercise training and nitric oxide-mediated function

Chronic heart failure (CHF) impairs nitric oxide (NO)-mediated regulation of skeletal muscle O2 delivery-utilization matching such that microvascular oxygenation falls faster (i.e., speeds PO2mv kinetics) during increases in metabolic demand. Conversely, exercise training improves (slows) muscle PO2mv kinetics following contractions onset in healthy young individuals via NO-dependent mechanisms. We tested the hypothesis that exercise training would improve contracting muscle microvascular oxygenation in CHF rats partly via improved NO-mediated function. CHF rats (left ventricular end-diastolic pressure = 17 ± 2 mmHg) were assigned to sedentary (n = 11) or progressive treadmill exercise training (n = 11; 5 days/wk, 6-8 wk, final workload of 60 min/day at 35 m/min; -14% grade downhill running) groups. PO2mv was measured via phosphorescence quenching in the spinotrapezius muscle at rest and during 1-Hz twitch contractions under control (Krebs-Henseleit solution), sodium nitroprusside (SNP; NO donor; 300 μM), and N(G)-nitro-l-arginine methyl ester (L-NAME, nonspecific NO synthase blockade; 1.5 mM) superfusion conditions. Exercise-trained CHF rats had greater peak oxygen uptake and spinotrapezius muscle citrate synthase activity than their sedentary counterparts (p < 0.05 for both). The overall speed of the PO2mv fall during contractions (mean response time; MRT) was slowed markedly in trained compared with sedentary CHF rats (sedentary: 20.8 ± 1.4, trained: 32.3 ± 3.0 s; p < 0.05), and the effect was not abolished by L-NAME (sedentary: 16.8 ± 1.5, trained: 31.0 ± 3.4 s; p > 0.05). Relative to control, SNP increased MRT in both groups such that trained CHF rats had slower kinetics (sedentary: 43.0 ± 6.8, trained: 55.5 ± 7.8 s; p < 0.05). Improved NO-mediated function is not obligatory for training-induced improvements in skeletal muscle microvascular oxygenation (slowed PO2mv kinetics) following contractions onset in rats with CHF.

The effects of dietary fish oil on exercising skeletal muscle vascular and metabolic control in chronic heart failure rats

Impaired vasomotor control in chronic heart failure (CHF) is due partly to decrements in nitric oxide synthase (NOS) mediated vasodilation. Exercising muscle blood flow (BF) is augmented with polyunsaturated fatty acid (PUFA) supplementation via fish oil (FO) in healthy rats. We hypothesized that FO would augment exercising muscle BF in CHF rats via increased NO-bioavailability. Myocardial infarction (coronary artery ligation) induced CHF in Sprague-Dawley rats which were subsequently randomized to dietary FO (20% docosahexaenoic acid, 30% eicosapentaenoic acid, n = 15) or safflower oil (SO, 5%, n = 10) for 6-8 weeks. Mean arterial pressure (MAP), blood [lactate], and hindlimb muscles BF (radiolabeled microspheres) were determined at rest, during treadmill exercise (20 m·min(-1), 5% incline) and exercise + N(G)-nitro-l-arginine-methyl-ester (l-NAME) (a nonspecific NOS inhibitor). FO did not change left ventricular end-diastolic pressure (SO: 14 ± 2; FO: 11 ± 1 mm Hg, p > 0.05). During exercise, MAP (SO: 128 ± 3; FO: 132 ± 3 mm Hg) and blood [lactate] (SO: 3.8 ± 0.4; FO: 4.6 ± 0.5 mmol·L(-1)) were not different (p > 0.05). Exercising hindlimb muscle BF was lower in FO than SO (SO: 120 ± 11; FO: 93 ± 4 mL·min(-1)·100 g(-1), p < 0.05) but was not differentially affected by l-NAME. Specifically, 17 of 28 individual muscle BF's were lower (p < 0.05) in FO demonstrating that PUFA supplementation with FO in CHF rats does not augment muscle BF during exercise but may lower metabolic cost.

The NO donor sodium nitroprusside: evaluation of skeletal muscle vascular and metabolic dysfunction

The nitric oxide (NO) donor sodium nitroprusside (SNP) may promote cyanide-induced toxicity and systemic and/or local responses approaching maximal vasodilation. The hypotheses were tested that SNP superfusion of the rat spinotrapezius muscle exerts 1) residual impairments in resting and contracting blood flow, oxygen utilization (VO(2)) and microvascular O(2) pressure (PO(2)mv); and 2) marked hypotension and elevation in resting PO(2)mv. Two superfusion protocols were performed: 1) Krebs-Henseleit (control 1), SNP (300 μM; a dose used commonly in superfusion studies) and Krebs-Henseleit (control 2), in this order; 2) 300 and 1200 μM SNP in random order. Spinotrapezius muscle blood flow (radiolabeled microspheres), VO(2) (Fick calculation) and PO(2)mv (phosphorescence quenching) were determined at rest and during electrically-induced (1 Hz) contractions. There were no differences in spinotrapezius blood flow, VO(2) or PO(2)mv at rest and during contractions pre- and post-SNP condition (control 1 and control 2; p>0.05 for all). With regard to dosing, SNP produced a graded elevation in resting PO(2)mv (p<0.05) with a reduction in mean arterial pressure only at the higher concentration (p<0.05). Contrary to our hypotheses, skeletal muscle superfusion with the NO donor SNP (300 μM) improved microvascular oxygenation during the transition from rest to contractions (PO(2)mv kinetics) without precipitating residual impairment of muscle hemodynamic or metabolic control or compromising systemic hemodynamics. These data suggest that SNP superfusion (300 μM) constitutes a valid and important tool for assessing the functional roles of NO in resting and contracting skeletal muscle function without incurring residual alterations consistent with cyanide accumulation and poisoning.

(-)-Epicatechin administration and exercising skeletal muscle vascular control and microvascular oxygenation in healthy rats

Consumption of the dietary flavanol (-)-epicatechin (EPI) is associated with enhanced endothelial function and augmented skeletal muscle capillarity and mitochondrial volume density. The potential for EPI to improve peripheral vascular function and muscle oxygenation during exercise is unknown. We tested the hypothesis that EPI administration in healthy rats would improve treadmill exercise performance secondary to elevated skeletal muscle blood flow and vascular conductance [VC, blood flow/mean arterial pressure (MAP)] and improved skeletal muscle microvascular oxygenation. Rats received water (control, n = 12) or 4 mg/kg EPI (n = 12) via oral gavage daily for 24 days. Exercise endurance capacity and peak O(2) uptake (Vo(2) peak) were measured via treadmill runs to exhaustion. MAP (arterial catheter) and blood flow (radiolabeled microspheres) were measured and VC was calculated during submaximal treadmill exercise (25 m/min, 5% grade). Spinotrapezius muscle microvascular O(2) pressure (Po(2mv)) was measured (phosphorescence quenching) during electrically induced twitch (1 Hz) contractions. In conscious rats, EPI administration resulted in lower (↓~5%) resting (P = 0.03) and exercising (P = 0.04) MAP. There were no differences in exercise endurance capacity, Vo(2) peak, total exercising hindlimb blood flow (control, 154 ± 13; and EPI, 159 ± 8 ml·min(-1)·100 g(-1), P = 0.68), or VC (control, 1.13 ± 0.10; and EPI, 1.24 ± 0.08 ml·min(-1)·100 g(-1)·mmHg(-1), P = 0.21) between groups. Following anesthesia, EPI resulted in lower MAP (↓~16%) but did not impact resting Po(2mv) or any kinetics parameters (P > 0.05 for all) during muscle contractions compared with control. EPI administration (4 mg·kg(-1)·day(-1)) improved modestly cardiovascular function (i.e., ↓MAP) with no impact on exercise performance, total exercising skeletal muscle blood flow and VC, or contracting muscle microvascular oxygenation in healthy rats.

Impact of dietary nitrate supplementation via beetroot juice on exercising muscle vascular control in rats

Dietary nitrate (NO(3)(-)) supplementation, via its reduction to nitrite (NO(2)(-)) and subsequent conversion to nitric oxide (NO) and other reactive nitrogen intermediates, reduces blood pressure and the O(2) cost of submaximal exercise in humans. Despite these observations, the effects of dietary NO(3)(-) supplementation on skeletal muscle vascular control during locomotory exercise remain unknown. We tested the hypotheses that dietary NO(3)(-) supplementation via beetroot juice (BR) would reduce mean arterial pressure (MAP) and increase hindlimb muscle blood flow in the exercising rat. Male Sprague-Dawley rats (3-6 months) were administered either NO(3)(-) (via beetroot juice; 1 mmol kg(-1) day(-1), BR n = 8) or untreated (control, n = 11) tap water for 5 days. MAP and hindlimb skeletal muscle blood flow and vascular conductance (radiolabelled microsphere infusions) were measured during submaximal treadmill running (20 m min(-1), 5% grade). BR resulted in significantly lower exercising MAP (control: 137 ± 3, BR: 127 ± 4 mmHg, P < 0.05) and blood [lactate] (control: 2.6 ± 0.3, BR: 1.9 ± 0.2 mm, P < 0.05) compared to control. Total exercising hindlimb skeletal muscle blood flow (control: 108 ± 8, BR: 150 ± 11 ml min(-1) (100 g)(-1), P < 0.05) and vascular conductance (control: 0.78 ± 0.05, BR: 1.16 ± 0.10 ml min(-1) (100 g)(-1) mmHg(-1), P < 0.05) were greater in rats that received BR compared to control. The relative differences in blood flow and vascular conductance for the 28 individual hindlimb muscles and muscle parts correlated positively with their percentage type IIb + d/x muscle fibres (blood flow: r = 0.74, vascular conductance: r = 0.71, P < 0.01 for both). These data support the hypothesis that NO(3)(-) supplementation improves vascular control and elevates skeletal muscle O(2) delivery during exercise predominantly in fast-twitch type II muscles, and provide a potential mechanism by which NO(3)(-) supplementation improves metabolic control.

Sildenafil improves microvascular O2 delivery-to-utilization matching and accelerates exercise O2 uptake kinetics in chronic heart failure

Nitric oxide (NO) can temporally and spatially match microvascular oxygen (O(2)) delivery (Qo(2mv)) to O(2) uptake (Vo(2)) in the skeletal muscle, a crucial adjustment-to-exercise tolerance that is impaired in chronic heart failure (CHF). To investigate the effects of NO bioavailability induced by sildenafil intake on muscle Qo(2mv)-to-O(2) utilization matching and Vo(2) kinetics, 10 males with CHF (ejection fraction = 27 ± 6%) undertook constant work-rate exercise (70-80% peak). Breath-by-breath Vo(2), fractional O(2)extraction in the vastus lateralis {∼deoxygenated hemoglobin + myoglobin ([deoxy-Hb + Mb]) by near-infrared spectroscopy}, and cardiac output (CO) were evaluated after sildenafil (50 mg) or placebo. Sildenafil increased exercise tolerance compared with placebo by ∼20%, an effect that was related to faster on- and off-exercise Vo(2) kinetics (P < 0.05). Active treatment, however, failed to accelerate CO dynamics (P > 0.05). On-exercise [deoxy-Hb + Mb] kinetics were slowed by sildenafil (∼25%), and a subsequent response "overshoot" (n = 8) was significantly lessened or even abolished. In contrast, [deoxy-Hb + Mb] recovery was faster with sildenafil (∼15%). Improvements in muscle oxygenation with sildenafil were related to faster on-exercise Vo(2) kinetics, blunted oscillations in ventilation (n = 9), and greater exercise capacity (P < 0.05). Sildenafil intake enhanced intramuscular Qo(2mv)-to-Vo(2) matching with beneficial effects on Vo(2) kinetics and exercise tolerance in CHF. The lack of effect on CO suggests that improvement in blood flow to and within skeletal muscles underlies these effects.

Effects of chronic heart failure on neuronal nitric oxide synthase-mediated control of microvascular O2 pressure in contracting rat skeletal muscle

Chronic heart failure (CHF) impairs nitric oxide (NO)-mediated regulation of the skeletal muscle microvascular O(2) delivery/V(O(2)) ratio (which sets the microvascular O(2) pressure, PO(2)mv). Given the pervasiveness of endothelial dysfunction in CHF, this NO-mediated dysregulation is attributed generally to eNOS. It is unknown whether nNOS-mediated PO(2)mv regulation is altered in CHF. We tested the hypothesis that CHF impairs nNOS-mediated PO(2)mv control. In healthy and CHF (left ventricular end diastolic pressure (LVEDP): 6 ± 1 versus 14 ± 1 mmHg, respectively, P < 0.05) rats spinotrapezius muscle blood flow (radiolabelled microspheres), PO(2)mv (phosphorescence quenching), and V(O(2)) (Fick calculation) were measured before and after 0.56 mg kg(-1)i.a. of the selective nNOS inhibitor S-methyl-l-thiocitrulline (SMTC). In healthy rats, SMTC increased baseline PO(2)mv (

CONTROL

29.7 ± 1.4, SMTC: 34.4 ± 1.9 mmHg, P < 0.05) by reducing V(O(2)) (↓20%) without any effect on blood flow and speeded the mean response time (MRT, time to reach 63% of the overall kinetics response,

CONTROL

24.2 ± 2.0, SMTC: 18.5 ± 1.3 s, P < 0.05). In CHF rats, SMTC did not alter baseline PO(2)mv (

CONTROL

25.7 ± 1.6, SMTC: 28.6 ± 2.1 mmHg, P > 0.05), V(O(2)) at rest, or the MRT (CONTROL: 22.8 ± 2.6, SMTC: 21.3 ± 3.0 s, P > 0.05). During the contracting steady-state, SMTC reduced blood flow (↓15%) and V(O(2)) (↓15%) in healthy rats such that PO(2)mv was unaltered (

CONTROL

19.8 ± 1.7, SMTC: 20.7 ± 1.8 mmHg, P > 0.05). In marked contrast, in CHF rats SMTC did not change contracting steady-state blood flow, V(O(2)), or PO(2)mv (

CONTROL

17.0 ± 1.4, SMTC: 17.7 ± 1.8 mmHg, P > 0.05). nNOS-mediated control of skeletal muscle microvascular function is compromised in CHF versus healthy rats. Treatments designed to ameliorate microvascular dysfunction in CHF may benefit by targeting improvements in nNOS function.

Exercise training and muscle microvascular oxygenation: functional role of nitric oxide

Exercise training induces multiple adaptations within skeletal muscle that may improve local O(2) delivery-utilization matching (i.e., Po(2)mv). We tested the hypothesis that increased nitric oxide (NO) function is intrinsic to improved muscle Po(2)mv kinetics from rest to contractions after exercise training. Healthy young Sprague-Dawley rats were assigned to sedentary (n = 18) or progressive treadmill exercise training (n = 10; 5 days/wk, 6-8 wk, final workload of 60 min/day at 35 m/min, -14% grade) groups. Po(2)mv was measured via phosphorescence quenching in the spinotrapezius muscle at rest and during 1-Hz twitch contractions under control (Krebs-Henseleit solution), sodium nitroprusside (SNP, NO donor; 300 μM), and N(G)-nitro-L-arginine methyl ester (l-NAME, nonspecific NO synthase blockade; 1.5 mM) superfusion conditions. Exercise-trained rats had greater peak oxygen uptake (Vo(2 peak)) than their sedentary counterparts (81 ± 1 vs. 72 ± 2 ml · kg(-1) · min(-1), respectively; P < 0.05). Exercise-trained rats had significantly slower Po(2)mv fall throughout contractions (τ(1); time constant for the first component) during control (sedentary: 8.1 ± 0.6; trained: 15.2 ± 2.8 s). Compared with control, SNP slowed τ(1) to a greater extent in sedentary rats (sedentary: 38.7 ± 5.6; trained: 26.8 ± 4.1 s; P > 0.05) whereas l-NAME abolished the differences in τ(1) between sedentary and trained rats (sedentary: 12.0 ± 1.7; trained: 11.2 ± 1.4 s; P < 0.05). Our results indicate that endurance exercise training leads to greater muscle microvascular oxygenation across the metabolic transient following the onset of contractions (i.e., slower Po(2)mv kinetics) partly via increased NO-mediated function, which likely constitutes an important mechanism for training-induced metabolic adaptations.

Plasticity of microvascular oxygenation control in rat fast-twitch muscle: effects of experimental creatine depletion

Aging, heart failure and diabetes each compromise the matching of O2 delivery (Q˙O2)-to-metabolic requirements (O2 uptake, V˙O2) in skeletal muscle such that the O2 pressure driving blood-myocyte O2 flux (microvascular PO2, PmvO2) is reduced and contractile function impaired. In contrast, β-guanidinopropionic acid (β-GPA) treatment improves muscle contractile function, primarily in fast-twitch muscle (Moerland and Kushmerick, 1994). We tested the hypothesis that β-GPA (2% wt/BW in rat chow, 8 weeks; n=14) would improve Q˙O2-to-V˙O2 matching (elevated PmvO2) during contractions (4.5V @ 1Hz) in mixed (MG) and white (WG) portions of the gastrocnemius, both predominantly fast-twitch). Compared with control (CON), during contractions PmvO2 fell less following β-GPA (MG -54%, WG -26%, P<0.05), elevating steady-state PmvO2 (CON, MG: 10±2, WG: 9±1; β-GPA, MG 16±2, WG 18±2 mmHg, P<0.05). This reflected an increased Q˙O2/V˙O2 ratio due primarily to a reduced V˙O2 in β-GPA muscles. It is likely that this adaptation helps facilitate the β-GPA-induced enhancement of contractile function in fast-twitch muscles.

Muscle oxygen transport and utilization in heart failure: implications for exercise (in)tolerance

The defining characteristic of chronic heart failure (CHF) is an exercise intolerance that is inextricably linked to structural and functional aberrations in the O(2) transport pathway. CHF reduces muscle O(2) supply while simultaneously increasing O(2) demands. CHF severity varies from moderate to severe and is assessed commonly in terms of the maximum O(2) uptake, which relates closely to patient morbidity and mortality in CHF and forms the basis for Weber and colleagues' (167) classifications of heart failure, speed of the O(2) uptake kinetics following exercise onset and during recovery, and the capacity to perform submaximal exercise. As the heart fails, cardiovascular regulation shifts from controlling cardiac output as a means for supplying the oxidative energetic needs of exercising skeletal muscle and other organs to preventing catastrophic swings in blood pressure. This shift is mediated by a complex array of events that include altered reflex and humoral control of the circulation, required to prevent the skeletal muscle "sleeping giant" from outstripping the pathologically limited cardiac output and secondarily impacts lung (and respiratory muscle), vascular, and locomotory muscle function. Recently, interest has also focused on the dysregulation of inflammatory mediators including tumor necrosis factor-α and interleukin-1β as well as reactive oxygen species as mediators of systemic and muscle dysfunction. This brief review focuses on skeletal muscle to address the mechanistic bases for the reduced maximum O(2) uptake, slowed O(2) uptake kinetics, and exercise intolerance in CHF. Experimental evidence in humans and animal models of CHF unveils the microvascular cause(s) and consequences of the O(2) supply (decreased)/O(2) demand (increased) imbalance emblematic of CHF. Therapeutic strategies to improve muscle microvascular and oxidative function (e.g., exercise training and anti-inflammatory, antioxidant strategies, in particular) and hence patient exercise tolerance and quality of life are presented within their appropriate context of the O(2) transport pathway.

Oxygen uptake kinetics in chronic heart failure: clinical and physiological aspects

One of the hallmark symptoms of patients with chronic heart failure (CHF) is exercise intolerance. Therefore, exercise testing has become an important tool for the evaluation and monitoring of heart failure. Whereas the maximal aerobic capacity (peak VO(2)) is a reliable indicator of the severity and prognosis of heart failure, submaximal exercise parameters may be more closely related to the ability to perform daily activities. As such, oxygen (O(2)) uptake kinetics, describing the rate change of O(2) uptake during onset or recovery of submaximal constant-load exercise (O(2) onset and recovery kinetics, respectively), have been shown to be useful parameters for objectively evaluating the functional capacity of CHF patients. However, their evaluation in this population is not a routine part of daily clinical practice. Possible reasons for this include a lack of standardisation of the assessment methodology and a limited number of studies evaluating the clinical use of O(2) uptake kinetics in CHF patients. In addition, the pathophysiological mechanisms underlying the delay in O(2) uptake kinetics in these patients are not completely understood. This review discusses the current literature on the clinical potency and physiological determinants of O(2) uptake kinetics in CHF patients and provides directions for future research. (Neth Heart J 2009;17:238-44.Neth Heart J 2009;17:238-44.).

Dynamics of muscle microcirculatory and blood-myocyte O(2) flux during contractions

The O(2) requirements of contracting skeletal muscle may increase 100-fold above rest. In 1919, August Krogh's brilliant insights recognized the capillary as the principal site for this increased blood-myocyte O(2) flux. Based on the premise that most capillaries did not sustain RBC flux at rest, Krogh proposed that capillary recruitment [i.e. initiation of red blood cell (RBC) flux in previously non-flowing capillaries] increased the capillary surface area available for O(2) flux and reduced mean capillary-to-mitochondrial diffusion distances. More modern experimental approaches reveal that most muscle capillaries may support RBC flux at rest. Thus, rather than contraction-induced capillary recruitment per se, increased RBC flux and haematocrit within already-flowing capillaries probably elevate perfusive and diffusive O(2) conductances and hence blood-myocyte O(2) flux. Additional surface area for O(2) exchange is recruited but, crucially, this may occur along the length of already-flowing capillaries (i.e. longitudinal recruitment). Today, the capillary is still considered the principal site for O(2) and substrate delivery to contracting skeletal muscle. Indeed, the presence of very low intramyocyte O(2) partial pressures (PO(2)s) and the absence of intramyocyte PO(2) gradients, whilst refuting the relevance of diffusion distances, place an even greater importance on capillary hemodynamics. This emergent picture calls for a paradigm-shift in our understanding of the function of capillaries by de-emphasizing de novo'capillary recruitment'. Diseases such as heart failure impair blood-myocyte O(2) flux, in part, by decreasing the proportion of RBC-flowing capillaries. Knowledge of capillary function in healthy muscle is requisite for identification of pathology and efficient design of therapeutic treatments.

Skeletal muscle arteriolar function following myocardial infarction: Analysis of branch-order effects

Diminished bioavailability of nitric oxide (NO) may impair skeletal muscle arteriolar function after myocardial infarction (MI). We tested the hypotheses that chronic MI induced would diminish 1) endothelial function in large (resting diameter ~75μm) feed arterioles, and 2) functional dilation in feed arterioles, but not smaller arcade (~25μm) or transverse (~15μm) arterioles, in the spinotrapezius muscle of female Sprague-Dawley rats. Additionally, we hypothesized that blockade of NO production with N(G)-nitro-l-arginine methyl ester (l-NAME; 30mg/kg i.v.) would have a greater blunting effect on control rats than MI rats. Endothelial function of the feed arterioles was assessed with an infusion of acetylcholine (1.5μg i.v.) after pretreatment with indomethacin (5mg/kgi.p.). MI blunted the response to acetylcholine in feed arterioles (p=0.037), but did not affect resting or post-contraction diameter at any branching order. l-NAME had similar effects on MI and SHAM rats; the response to acetylcholine was blunted in feed arterioles (p=0.003), resting diameter was diminished in arcade arterioles (p=0.003), and post-contraction diameter was diminished in both arcade arterioles (p=0.03) and transverse arterioles (p=0.05). In conclusion, despite endothelial dysfunction in feed arterioles, functional dilation was not affected by MI in any branching order studied. l-NAME had similar effects on MI and SHAM rats that were branch order-dependent. These branch-order effects should be considered in future studies of the control of blood flow.

Nitric oxide bioavailability modulates the dynamics of microvascular oxygen exchange during recovery from contractions

AIM

lowered microvascular PO(2) (PO(2) mv) during the exercise off-transient likely impairs muscle metabolic recovery and limits the capacity to perform repetitive tasks. The current investigation explored the impact of altered nitric oxide (NO) bioavailability on PO(2) mv during recovery from contractions in healthy skeletal muscle. We hypothesized that increased NO bioavailability (sodium nitroprusside: SNP) would enhance PO(2) mv and speed its recovery kinetics while decreased NO bioavailability (l-nitro arginine methyl ester: l-NAME) would reduce PO(2) mv and slow its recovery kinetics.

METHODS

  PO(2) mv was measured by phosphorescence quenching during transitions (rest-1 Hz twitch-contractions for 3 min-recovery) in the spinotrapezius muscle of Sprague-Dawley rats under SNP (300 microm), Krebs-Henseleit (CONTROL) and l-NAME (1.5 mm) superfusion conditions.

RESULTS

relative to recovery in CONTROL, SNP resulted in greater overall microvascular oxygenation as assessed by the area under the PO(2) mv curve (PO(2 AREA) ;

CONTROL

3471 ± 292 mmHg s; SNP: 4307 ± 282 mmHg s; P < 0.05) and faster off-kinetics as evidenced by the mean response time (MRToff;

CONTROL

60.2 ± 6.9 s; SNP: 34.8 ± 5.7 s; P < 0.05), whereas l-NAME produced lower PO(2 AREA) (2339 ± 444 mmHg s; P < 0.05) and slower MRToff (86.6 ± 14.5s; P < 0.05).

CONCLUSION

no bioavailability plays a key role in determining the matching of O(2) delivery-to-O(2) uptake and thus the upstream O(2) pressure driving capillary-myocyte O(2) flux (i.e. PO(2) mv) following cessation of contractions in healthy skeletal muscle. Additionally, these data support a mechanistic link between reduced NO bioavailability and prolonged muscle metabolic recovery commonly observed in ageing and diseased populations.

Effects of combined aging and heart failure on visceral sympathetic nerve and cardiovascular responses to progressive hyperthermia in F344 rats

Sympathetic nerve discharge (SND) responses to hyperthermia are attenuated in aged rats without heart failure (HF) and in young HF (Y(HF)) rats, demonstrating that individually aging and HF alter SND regulation. However, the combined effects of aging and HF on SND regulation to heat stress are unknown, despite the high prevalence of HF in aged individuals. We hypothesized that SND responses to heating would be additive when aging and HF are combined, demonstrated by marked reductions in SND and mean arterial pressure (MAP) responses to heating in aged HF (A(HF)) compared with aged sham HF (A(SHAM)) rats, and in A(HF) compared with Y(HF) rats. Renal and splenic SND responses to hyperthermia (colonic temperature increased to 41.5°C) were determined in anesthetized Y(HF), young sham (Y(SHAM)), A(HF), and A(SHAM) Fischer rats. HF was induced by myocardial infarction and documented using echocardiographic, invasive, and postmortem measures. The severity of HF was similar in Y(HF) and A(HF) rats. SND responses to heating were attenuated in Y(HF) compared with Y(SHAM) rats, demonstrating an effect of HF on SND regulation in young rats. In contrast, A(HF) and A(SHAM) rats demonstrated similar SND responses to heating, suggesting a prominent influence of age on SND regulation in A(HF) rats. Splenic SND and MAP responses to heating were similar in Y(HF), A(HF), and A(SHAM) rats, indicating that the imposition of HF in young rats changes the regulatory status of these variables to one consistent with aged rats. These data suggest that the effect of HF on SND regulation to hyperthermia is age dependent.

Progressive chronic heart failure slows the recovery of microvascular O2 pressures after contractions in the rat spinotrapezius muscle

Chronic heart failure (CHF) induces muscle fiber-type specific alterations in skeletal muscle O(2) delivery and utilization during metabolic transitions. As a result, the recovery of microvascular Po(2) (Pmv(O(2))) is prolonged in slow-twitch skeletal muscle but not fast-twitch skeletal muscle in rats with CHF. We tested the hypothesis that CHF slows Pmv(O(2)) recovery in rat skeletal muscle of a mixed fiber-type analogous to human locomotory muscles and that the degree of slowing correlates with central indexes of heart failure. Healthy control [n = 6, left ventricular end-diastolic pressure (LVEDP): 10 ± 1 mmHg], moderate CHF (n = 6, LVEDP: 18 ± 2 mmHg), and severe CHF (n = 4, LVEDP: 34 ± 2 mmHg) female Sprague-Dawley rats had their right spinotrapezius muscles (41% type I, 7% type IIa, and 52% type IIb and d/x) exposed, and Pmv(O(2)) was measured via phosphorescence quenching during 180 s of recovery from 180 s of electrically induced twitch contractions (1 Hz, 4-6 V). CHF progressively slowed the mean response time (MRT; the time to reach 63% of the overall dynamic response) of Pmv(O(2)) recovery (MRT(off); control: 60.2 ± 6.9, moderate CHF: 72.8 ± 6.6, and severe CHF: 109.8 ± 6.6 s, P < 0.05 for all). MRT(off) correlated positively with central hemodynamic (LVEDP: r = 0.76, P < 0.01) and morphological (right ventricle-to-body weight ratio: r = 0.74, P < 0.01; and lung weight-to-body weight ratio: r = 0.79, P < 0.01) indexes of heart failure. The present investigation suggests that slowed Pmv(O(2)) kinetics during recovery in CHF constitutes a mechanistic link between impaired circulatory and metabolic recovery after contractions in CHF.

Recovery dynamics of skeletal muscle oxygen uptake during the exercise off-transient

The time course of muscle .V(O2) recovery from contractions (i.e., muscle .V(O2) off-kinetics), measured directly at the site of O(2) exchange, i.e., in the microcirculation, is unknown. Whereas biochemical models based upon creatine kinase flux rates predict slower .V(O2) off- than on-transients [Kushmerick, M.J., 1998. Comp. Biochem. Physiol. B: Biochem. Mol. Biol.], whole muscle .V(O2) data [Krustrup, et al. J. Physiol.] suggest on-off symmetry.

PURPOSE

We tested the hypothesis that the slowed recovery blood flow (Qm) kinetics profile in the spinotrapezius muscle [Ferreira et al., 2006. J. Physiol.] was associated with a slowed muscle .V(O2) recovery compared with that seen at the onset of contractions (time constant, tau approximately 23s, Behnke et al., 2002. Resp. Physiol.), i.e., on-off asymmetry.

METHODS

Measurements of capillary red blood cell flux and microvascular pressure of O(2) (P(O2) mv) were combined to resolve the temporal profile of muscle .V(O2) across the moderate intensity contractions-to-rest transition.

RESULTS

Muscle .V(O2) decreased from an end-contracting value of 7.7+/-0.2 ml/100 g/min to 1.7+/-0.1 ml/100g/min at the end of the 3 min recovery period, which was not different from pre-stimulation .V(O2). Contrary to our hypothesis, muscle .V(O2) in recovery began to decrease immediately (i.e., time delay <2s) and demonstrated rapid first-order kinetics (tau, 25.5+/-2.6s) not different (i.e., symmetrical to) to those during the on-transient. This resulted in a systematic increase in microvascular P(O2) during the recovery from contractions.

CONCLUSIONS

The slowed Qm kinetics in recovery serves to elevate the Qm/.V(O2) ratio and thus microvascular P(O2) . Whether this Qm response is obligatory to the rapid muscle .V(O2) kinetics and hence speeds the repletion of high-energy phosphates by maximizing conductive and diffusive O(2) flux is an important question that awaits resolution.

The final frontier: oxygen flux into muscle at exercise onset

In humans at exercise onset, intramuscular phosphocreatine decreases immediately, whereas muscle oxygen (O2) uptake seems to rise after a delay of up to 15 s which is inconsistent with models of metabolic control. Novel microcirculatory investigations reveal that elevated capillary-to-myocyte O2 flux in rat muscle is, in fact, initiated simultaneously with contractions.

Aging potentiates the effect of congestive heart failure on muscle microvascular oxygenation

Congestive heart failure (CHF) is most prevalent in aged individuals and elicits a spectrum of cardiovascular and muscular perturbations that impairs the ability to deliver (Qo(2)) and utilize (Vo(2)) oxygen in skeletal muscle. Whether aging potentiates the CHF-induced alterations in the Qo(2)-to-Vo(2) relationship [which determines microvascular Po(2) (Pmv(O(2)))] in resting and contracting skeletal muscle is unclear. We tested the hypothesis that old rats with CHF would demonstrate a greater impairment of skeletal muscle Pmv(O(2)) than observed in young rats with CHF. Phosphorescence quenching was utilized to measure spinotrapezius Pmv(O(2)) at rest and across the rest-to-contractions (1-Hz, 4-6 V) transition in young (Y) and old (O) male Fischer 344 Brown-Norway rats with CHF induced by myocardial infarction (mean left ventricular end-diastolic pressure >20 mmHg for Y(CHF) and O(CHF)). In CHF muscle, aging significantly reduced resting Pmv(O(2)) (32.3 +/- 3.4 Torr for Y(CHF) and 21.3 +/- 3.3 Torr for O(CHF); P < 0.05) and in both Y(CHF) and O(CHF) compared with their aged-matched counterparts, CHF reduced the rate of the Pmv(O(2)) fall at the onset of contractions. Moreover, across the on-transient and in the subsequent steady state, Pmv(O(2)) values in O(CHF) vs. Y(CHF) were substantially lower (for steady-state, 20.4 +/- 1.7 Torr for Y(CHF) and 16.4 +/- 2.0 Torr for O(CHF); P < 0.05). At rest and during contractions in CHF, the pressure driving blood-muscle O(2) diffusion (Pmv(O(2))) is substantially decreased in old animals. This finding suggests that muscle dysfunction and exercise intolerance in aged CHF patients might be due, in part, to the failure to maintain a sufficiently high Pmv(O(2)) to facilitate blood-muscle O(2) exchange and support mitochondrial ATP production.