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

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V̇O2max ), critical power (CP) and speed of the V̇O2 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 V̇O2 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 V̇O2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q̇O2 ) dependency that slows V̇O2 kinetics, decreasing CP and V̇O2max , 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 Q̇O2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V̇O2 values. Recent discoveries, especially in the muscle microcirculation and Q̇O2 -to- V̇O2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V̇O2 kinetics and determine CP and V̇O2max 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.

Sexual dimorphism in vascular ATP-sensitive K+ channel function supporting interstitial PO2 via convective and/or diffusive O2 transport.

KEY POINTS: Inhibition of pancreatic ATP-sensitive K+ (KATP ) channels is the intended effect of oral sulphonylureas to increase insulin release in diabetes. However, pertinent to off-target effects of sulphonylurea medication, sex differences in cardiac KATP channel function exist, whereas potential sex differences in vascular KATP channel function remain unknown. In the present study, we assessed vascular KATP channel function (topical glibenclamide superfused onto fast-twitch oxidative skeletal muscle) supporting blood flow and interstitial O2 delivery-utilization matching ( PO2 is) during twitch contractions in male, female during pro-oestrus and ovariectomized female (F+OVX) rats. Glibenclamide decreased blood flow (convective O2 transport) and interstitial PO2 in male and female, but not F+OVX, rats. Compared to males, females also demonstrated impaired diffusive O2 transport and a faster fall in interstitial PO2 . Our demonstration, in rats, that sex differences in vascular KATP channel function exist support the tentative hypothesis that oral sulphonylureas may exacerbate exercise intolerance and morbidity, especially in premenopausal females. ABSTRACT: Vascular ATP-sensitive K+ (KATP ) channels support skeletal muscle blood flow ( Q̇m ), interstitial O2 delivery ( Q̇O2 )-utilization ( V̇O2 ) matching (i.e. interstitial-myocyte O2 flux driving pressure; PO2 is) and exercise tolerance. Potential sex differences in skeletal muscle vascular KATP channel function remain largely unexplored. We hypothesized that local skeletal muscle KATP channel inhibition via glibenclamide superfusion (5 mg kg-1 GLI; sulphonylurea diabetes medication) in anaesthetized female Sprague-Dawley rats, compared to males, would demonstrate greater reductions in contracting (1 Hz, 7 V, 180 s) fast-twitch oxidative mixed gastrocnemius (97% type IIA+IID/X+IIB) Q̇m (15 µm microspheres) and PO2 is (phosphorescence quenching), resulting from more compromised convective ( Q̇O2 ) and diffusive ( DO2  ) O2 conductances. Furthermore, these GLI-induced reductions in ovary-intact females measured during pro-oestrus would be diminished following ovariectomy (F+OVX). GLI similarly impaired mixed gastrocnemius V̇O2 in both males (↓28%) and females (↓33%, both P < 0.032) via reduced Q̇m (male: ↓31%, female: ↓35%, both P < 0.020), Q̇O2 (male: 5.6 ± 0.5 vs. 4.0 ± 0.5, female: 6.4 ± 1.1 vs. 4.2 ± 0.6 mL O2  min-1 100 g tissue-1 , P < 0.022) and the resulting PO2 is, with females also demonstrating a reduced DO2  (0.40 ± 0.07 vs. 0.30 ± 0.04 mL O2  min-1 100 g tissue-1 , P < 0.042) and a greater GLI-induced speeding of PO2 is fall (mean response time: Sex × Drug interaction, P = 0.026). Conversely, GLI did not impair the mixed gastrocnemius of F+OVX rats. Therefore, in patients taking sulphonylureas, these results support the potential for impaired vascular KATP channel function to compromise muscle Q̇m and therefore exercise tolerance. Such an effect, if present, would likely contribute to adverse cardiovascular events in premenopausal females more than males.

Aerobic exercise training reduces cardiac function and coronary flow-induced vasodilation in mice lacking adiponectin.

We tested the hypothesis that adiponectin deficiency attenuates cardiac and coronary microvascular function and prevents exercise training-induced adaptations of the myocardium and the coronary microvasculature in adult mice. Adult wild-type (WT) or adiponectin knockout (adiponectin KO) mice underwent treadmill exercise training or remained sedentary for 8-10 wk. Systolic and diastolic functions were assessed before and after exercise training or cage confinement. Vasoreactivity of coronary resistance arteries was assessed at the end of exercise training or cage confinement. Before exercise training, ejection fraction and fractional shortening were similar in adiponectin KO and WT mice, but isovolumic contraction time was significantly lengthened in adiponectin KO mice. Exercise training increased ejection fraction (12%) and fractional shortening (20%) with no change in isovolumic contraction time in WT mice. In adiponectin KO mice, both ejection fraction (-9%) and fractional shortening (-12%) were reduced after exercise training and these decreases were coupled to a further increase in isovolumic contraction time (20%). In sedentary mice, endothelium-dependent dilation to flow was higher in arterioles from adiponectin KO mice as compared with WT mice. Exercise training enhanced dilation to flow in WT mice but decreased flow-induced dilation in adiponectin KO mice. These data suggest that compensatory mechanisms contribute to the maintenance of cardiac and coronary microvascular function in sedentary mice lacking adiponectin; however, in the absence of adiponectin, cardiac and coronary microvascular adaptations to exercise training are compromised.NEW & NOTEWORTHY We report that compensatory mechanisms contribute to the maintenance of cardiac and coronary microvascular function in sedentary mice in which adiponectin has been deleted; however, when mice lacking adiponectin are subjected to the physiological stress of exercise training, beneficial coronary microvascular and cardiac adaptations are compromised or absent.

Vascular ATP-sensitive K+ channels support maximal aerobic capacity and critical speed via convective and diffusive O2 transport.

KEY POINTS: Oral sulphonylureas, widely prescribed for diabetes, inhibit pancreatic ATP-sensitive K+ (KATP ) channels to increase insulin release. However, KATP channels are also located within vascular (endothelium and smooth muscle) and muscle (cardiac and skeletal) tissue. We evaluated left ventricular function at rest, maximal aerobic capacity ( V̇ O2 max) and submaximal exercise tolerance (i.e. speed-duration relationship) during treadmill running in rats, before and after systemic KATP channel inhibition via glibenclamide. Glibenclamide impaired critical speed proportionally more than V̇ O2 max but did not alter resting cardiac output. Vascular KATP channel function (topical glibenclamide superfused onto hindlimb skeletal muscle) resolved a decreased blood flow and interstitial PO2 during twitch contractions reflecting impaired O2 delivery-to-utilization matching. Our findings demonstrate that systemic KATP channel inhibition reduces V̇ O2 max and critical speed during treadmill running in rats due, in part, to impaired convective and diffusive O2 delivery, and thus V̇ O2 , especially within fast-twitch oxidative skeletal muscle. ABSTRACT: Vascular ATP-sensitive K+ (KATP ) channels support skeletal muscle blood flow and microvascular oxygen delivery-to-utilization matching during exercise. However, oral sulphonylurea treatment for diabetes inhibits pancreatic KATP channels to enhance insulin release. Herein we tested the hypotheses that: i) systemic KATP channel inhibition via glibenclamide (GLI; 10 mg kg-1 i.p.) would decrease cardiac output at rest (echocardiography), maximal aerobic capacity ( V̇ O2 max) and the speed-duration relationship (i.e. lower critical speed (CS)) during treadmill running; and ii) local KATP channel inhibition (5 mg kg-1 GLI superfusion) would decrease blood flow (15 µm microspheres), interstitial space oxygen pressures (PO2 is; phosphorescence quenching) and convective and diffusive O2 transport ( Q̇ O2 and DO2 , respectively; Fick Principle and Law of Diffusion) in contracting fast-twitch oxidative mixed gastrocnemius muscle (MG: 9% type I+IIa fibres). At rest, GLI slowed left ventricular relaxation (2.11 ± 0.59 vs. 1.70 ± 0.23 cm s-1 ) and decreased heart rate (321 ± 23 vs. 304 ± 22 bpm, both P < 0.05) while cardiac output remained unaltered (219 ± 64 vs. 197 ± 39 ml min-1 , P > 0.05). During exercise, GLI reduced V̇ O2 max (71.5 ± 3.1 vs. 67.9 ± 4.8 ml kg-1 min-1 ) and CS (35.9 ± 2.4 vs. 31.9 ± 3.1 m min-1 , both P < 0.05). Local KATP channel inhibition decreased MG blood flow (52 ± 25 vs. 34 ± 13 ml min-1 100 g tissue-1 ) and PO2 isnadir (5.9 ± 0.9 vs. 4.7 ± 1.1 mmHg) during twitch contractions. Furthermore, MG V̇ O2 was reduced via impaired Q̇ O2 and DO2 (P < 0.05 for each). Collectively, these data support that vascular KATP channels help sustain submaximal exercise tolerance in healthy rats. For patients taking sulfonylureas, KATP channel inhibition may exacerbate exercise intolerance.

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 isarea was greater than in the fast-twitch muscles during contractions (P < 0.05). Oxidative muscles had greater PO2 isnadir (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.

Daily Passive Muscle Stretching Improves Flow-Mediated Dilation of Popliteal Artery and 6-minute Walk Test in Elderly Patients with Stable Symptomatic Peripheral Artery Disease.

BACKGROUND: Patients with peripheral arterial disease (PAD) often have walking impairment due to insufficient oxygen supply to skeletal muscle. In aged rats, we have shown that daily stretching of calf muscles improves endothelium-dependent dilation of arterioles from the soleus muscle and increases capillarity and muscle blood flow during exercise. Therefore, we hypothesized that daily muscle stretching of calf muscles would improve endothelium-dependent vasodilation of the popliteal artery and walking function in PAD patients. METHODS: We performed a randomized, non-blinded, crossover study whereby 13 patients with stable symptomatic PAD were randomized to undergo either 4 weeks of passive calf muscle stretching (ankle dorsiflexion applied 30 min/d, 5 days/wk) followed by 4 weeks of no muscle stretching and vice versa. Endothelium-dependent flow-mediated dilation (FMD) and endothelium-independent nitroglycerin-induced dilation of the popliteal artery and 6 minute walk test (6MWT) were evaluated at baseline and after each 4 week interval. RESULTS: After 4 weeks of muscle stretching, FMD and 6MWT improved significantly in the muscle stretching group vs. the control (FMD: 5.1 ±â€¯0.5% vs. 3.7 ±â€¯0.3%, P = 0.005; 6MWT continuous walking distance: 304 ±â€¯43 m vs. 182 ±â€¯34 m; P = 0.0006). No difference in nitroglycerin-induced dilation was found between groups (10.9 ±â€¯1.2 vs. 9.9 ±â€¯0.4%, P = 0.48). Post-stretching, 6MWT total walking distance was positively correlated with normalized FMD (R = 0.645, P = 0.02). CONCLUSIONS: Passive calf muscle stretching enhanced vascular endothelial function and improved walking function in elderly patients with stable symptomatic PAD. These findings merit further investigation in a prospective randomized trial.

Aging reduces skeletal blood flow, endothelium-dependent vasodilation, and NO bioavailability in rats.

UNLABELLED: We determined whether aging diminishes bone blood flow and impairs endothelium-dependent vasodilation. Femoral perfusion was lower in old animals, as well as endothelium-dependent vasodilation and NO bioavailability. These effects could contribute to old age-related bone loss and the increased risk of fracture. INTRODUCTION: Aging has been shown to diminish bone blood flow in rats and humans. The purpose of this study was to determine whether blood flow to regions of the femur perfused primarily through the principal nutrient artery (PNA) are diminished with aging and whether this putative reduction in flow is associated with impaired endothelium-dependent vasodilation. MATERIALS AND METHODS: Blood flow was measured in conscious young adult (4-6 mo old) and aged (24-26 mo old) male Fischer-344 rats using radiolabeled microspheres. Endothelium-dependent vasodilation of the PNA was assessed in vitro using acetylcholine (ACh), whereas the contribution of the NO synthase (NOS) and cyclooxygenase (COX) signaling pathways to endothelium-dependent vasodilation was determined using the NOS and COX inhibitors L-NAME and indomethacin, respectively. RESULTS: Femoral blood flow in the aged rats was 21% and 28% lower in the proximal and distal metaphyses, respectively, and 45% lower in the diaphyseal marrow. Endothelium-dependent vasodilation was reduced with old age (young: 83 +/- 6% maximal relaxation; aged: 62 +/- 5% maximal relaxation), whereas endothelium-independent vasodilation (sodium nitroprusside) was unaffected by age. The reduction in endothelium-dependent vasodilation was mediated through impairment of the NOS signaling pathway, which resulted in lower NO bioavailability (young: 168 +/- 56 nM; aged: 50 +/- 7 nM). CONCLUSIONS: These data show that reductions in metaphyseal bone and diaphyseal marrow perfusion with old age are associated with diminished endothelium-dependent vasodilation through an impairment of the NOS mechanism. Such age-related changes in bone perfusion and vascular NO signaling could impact clinical bone loss, increase risk of fracture, and impair fracture healing in the elderly.

Effects of Type II diabetes on muscle microvascular oxygen pressures.

We tested the hypothesis that muscle microvascular O2 pressure (PmvO2; reflecting the O2 delivery (QO2) to O2 uptake (VO2) ratio) would be lowered in the spinotrapezius muscle of Goto-Kakizaki (GK) Type II diabetic rats (n=7) at rest and during twitch contractions when compared to control (CON; n=5) rats. At rest, PmvO2 was lower in GK versus CON rats (CON: 29+/-2; GK: 18+/-2Torr; P<0.05). At the onset of contractions, GK rats evidenced a faster change in PmvO2 than CON (i.e., time constant (tau); CON: 16+/-4; GK: 6+/-2s; P<0.05). In contrast to the monoexponential fall in PmvO2 to the steady-state level seen in CON, GK rats exhibited a biphasic PmvO2 response that included a blunted (or non-existent) PmvO2 decrease followed by recovery to a steady-state PmvO2 that was at, or slightly above, resting values. Compared with CON, this decreased PmvO2 across the transition to a higher metabolic rate in Type II diabetes would be expected to impair blood-muscle O2 exchange and contractile function.

Effects of arterial hypotension on microvascular oxygen exchange in contracting skeletal muscle.

In healthy animals under normotensive conditions (N), contracting skeletal muscle perfusion is regulated to maintain microvascular O2 pressures (PmvO2) at levels commensurate with O2 demands. Hypovolemic hypotension (H) impairs muscle contractile function; we tested whether this condition would alter the matching of O2 delivery (Qo2) to O2 utilization (Vo2), as determined by PmvO2 at the onset of muscle contractions. PmvO2 in the spinotrapezius muscles of seven female Sprague-Dawley rats (280+/-6 g) was measured every 2 s across the transition from rest to 1-Hz twitch contractions. Measurements were made under N (mean arterial pressure, 97+/-4 mmHg) and H (induced by arterial section; mean arterial pressure, 58+/-3 mmHg, P<0.05) conditions; PmvO2 profiles were modeled using a multicomponent exponential fitted with independent time delays. Hypotension reduced muscle blood flow at rest (24+/-8 vs. 6+/-1 ml-1.min-1.100 g-1 for N and H, respectively; P<0.05) and during contractions (74+/-20 vs. 22+/-4 ml-1.min-1.100 g-1 for N and H, respectively; P<0.05). H significantly decreased resting PmvO2 and steady-state contracting PmvO2(19.4+/-2.4 vs. 8.7+/-1.6 Torr for N and H, respectively, P<0.05). At the onset of contractions, H reduced the time delay (11.8+/-1.7 vs. 5.9+/-0.9 s for N and H, respectively, P<0.05) before the fall in PmvO2 and accelerated the rate of PmvO2 decrease (time constant, 12.6+/-1.4 vs. 7.3+/-0.9 s for N and H, respectively, P<0.05). Muscle Vo2 was reduced by 71% at rest and 64% with contractions in H vs. N, and O2 extraction during H averaged 78% at rest and 94% during contractions vs. 51 and 78% in N. These results demonstrate that H constrains the increase of skeletal muscle Qo2 relative to that of Vo2 at the onset of contractions, leading to a decreased PmvO2. According to Fick's law, this scenario will decrease blood-myocyte O2 flux, thereby slowing Vo2 kinetics and exacerbating the O2 deficit generated at exercise onset.

Blood flow and O2 extraction as a function of O2 uptake in muscles composed of different fiber types.

We examined how the greater vasodilatory capacity of slow--(ST) versus fast-twitch (FT) muscles impacts the relationship between blood flow (Q ) and O2 uptake (VO2) and, consequently, the O2 extraction (a-vO2 diff.)-to-VO2 relationship. Q was measured with radiolabelled microspheres, while VO2 was calculated by the Fick principle using measurements of microvascular O2 pressure (phosphorescence quenching) at rest, low--(2.5 V) and high-intensity contractions (4.5 V) for soleus (Sol; ST, n=5), mixed-gastrocnemius (MG; FT, n=7) and white-gastrocnemius (WG; FT, n=7). The slope of the Q-to-VO2 relationship (delta Q/delta VO2] ) was not different among muscles (Sol = 5.5 +/- 0.2, MG = 6.0 +/- 0.11 and WG = 5.8 +/- 0.06; P > 0.05). In contrast, the intercept was greater (P < 0.05) for Sol (16.3 +/- 2.7 ml min(-1) 100 g(-1)) versus MG and WG (in ml min(-1) 100 g(-1): 1.39 +/- 0.26 and 1.45 +/- 0.23, respectively; MG and WG, P > 0.05). In addition, the a-vO2 diff.-to-VO2] relationship for Sol was shifted rightward compared to MG and WG. These data suggest that the increase in Q for a given change in VO2 is similar for slow- and fast-twitch muscles, at least for the range of metabolic rates and muscles studied herein and that a-vO2 diff. differences result from the lower resting Q in FT muscles.

Effects of hindlimb unloading and ionizing radiation on skeletal muscle resistance artery vasodilation and its relation to cancellous bone in mice.

Spaceflight has profound effects on vascular function as a result of weightlessness that may be further compounded by radiation exposure. The purpose of the present study was to assess the individual and combined effects of hindlimb unloading (HU) and radiation (Rad) on vasodilator responses in the skeletal muscle vasculature. Adult male C57BL/6J mice were randomized to one of four groups: control (Con), HU (tail suspension for 15 days), Rad (200 cGy of (137)Cs), and HU-Rad (15-day tail suspension and 200 cGy of (137)Cs). Endothelium-dependent vasodilation of gastrocnemius feed arteries was assessed in vitro using acetylcholine (ACh, 10(-9)-10(-4) M) and inhibitors of nitric oxide synthase (NOS) and cyclooxygenase (COX). Endothelium-independent vasodilation was assessed using Dea-NONOate (10(-9)-10(-4) M). Endothelium-dependent and -independent vasodilator responses were impaired relative to Con responses in all treatment groups; however, there was no further impairment from the combination of treatments (HU-Rad) relative to that in the HU and Rad groups. The NOS-mediated contribution to endothelium-dependent vasodilation was depressed with HU and Rad. This impairment in NOS signaling may have been partially compensated for by an enhancement of PGI2-mediated dilation. Changes in endothelium-dependent vasodilation were also associated with decrements in trabecular bone volume in the proximal tibia metaphysis. These data demonstrate that the simulated space environment (i.e., radiation exposure and unloading of muscle and bone) significantly impairs skeletal muscle artery vasodilation, mediated through endothelium-dependent reductions in NOS signaling and decrements in vascular smooth muscle cell responsiveness to NO.

Effects of High-LET Radiation Exposure and Hindlimb Unloading on Skeletal Muscle Resistance Artery Vasomotor Properties and Cancellous Bone Microarchitecture in Mice.

Weightlessness during spaceflight leads to functional changes in resistance arteries and loss of cancellous bone, which may be potentiated by radiation exposure. The purpose of this study was to assess the effects of hindlimb unloading (HU) and total-body irradiation (TBI) on the vasomotor responses of skeletal muscle arteries. Male C57BL/6 mice were assigned to control, HU (13-16 days), TBI (1 Gy (56)Fe, 600 MeV, 10 cGy/min) and HU-TBI groups. Gastrocnemius muscle feed arteries were isolated for in vitro study. Endothelium-dependent (acetylcholine) and -independent (Dea-NONOate) vasodilator and vasoconstrictor (KCl, phenylephrine and myogenic) responses were evaluated. Arterial endothelial nitric oxide synthase (eNOS), superoxide dismutase-1 (SOD-1) and xanthine oxidase (XO) protein content and tibial cancellous bone microarchitecture were quantified. Endothelium-dependent and -independent vasodilator responses were impaired in all groups relative to control, and acetylcholine-induced vasodilation was lower in the HU-TBI group relative to that in the HU and TBI groups. Reductions in endothelium-dependent vasodilation correlated with a lower cancellous bone volume fraction. Nitric oxide synthase inhibition abolished all group differences in endothelium-dependent vasodilation. HU and HU-TBI resulted in decreases in eNOS protein levels, while TBI and HU-TBI produced lower SOD-1 and higher XO protein content. Vasoconstrictor responses were not altered. Reductions in NO bioavailability (eNOS), lower anti-oxidant capacity (SOD-1) and higher pro-oxidant capacity (XO) may contribute to the deficits in NOS signaling in skeletal muscle resistance arteries. These findings suggest that the combination of insults experienced in spaceflight leads to impairment of vasodilator function in resistance arteries that is mediated through deficits in NOS signaling.

Effects of eccentric exercise on microcirculation and microvascular oxygen pressures in rat spinotrapezius muscle.

A single bout of eccentric exercise results in muscle damage, but it is not known whether this is correlated with microcirculatory dysfunction. We tested the following hypotheses in the spinotrapezius muscle of rats either 1 (DH-1; n = 6) or 3 (DH-3; n = 6) days after a downhill run to exhaustion (90-120 min; -14 degrees grade): 1) in resting muscle, capillary hemodynamics would be impaired, and 2) at the onset of subsequent acute concentric contractions, the decrease of microvascular O(2) pressure (Pmv(o(2))), which reflects the dynamic balance between O(2) delivery and O(2) utilization, would be accelerated compared with control (Con, n = 6) rats. In contrast to Con muscles, intravital microscopy observations revealed the presence of sarcomere disruptions in DH-1 and DH-3 and increased capillary diameter in DH-3 (Con: 5.2 +/- 0.1; DH-1: 5.1 +/- 0.1; DH-3: 5.6 +/- 0.1 mum; both P < 0.05 vs. DH-3). At rest, there was a significant reduction in the percentage of capillaries that sustained continuous red blood cell (RBC) flux in both DH running groups (Con: 90.0 +/- 2.1; DH-1: 66.4 +/- 5.2; DH-3: 72.9 +/- 4.1%, both P < 0.05 vs. Con). Capillary tube hematocrit was elevated in DH-1 but reduced in DH-3 (Con: 22 +/- 2; DH-1: 28 +/- 1; DH-3: 16 +/- 1%; all P < 0.05). Although capillary RBC flux did not differ between groups (P > 0.05), RBC velocity was lower in DH-1 compared with Con (Con: 324 +/- 43; DH-1: 212 +/- 30; DH-3: 266 +/- 45 mum/s; P < 0.05 DH-1 vs. Con). Baseline Pmv(O(2)) before contractions was not different between groups (P > 0.05), but the time constant of the exponential fall to contracting Pmv(O(2)) values was accelerated in the DH running groups (Con: 14.7 +/- 1.4; DH-1: 8.9 +/- 1.4; DH-3: 8.7 +/- 1.4 s, both P < 0.05 vs. Con). These findings are consistent with the presence of substantial microvascular dysfunction after downhill eccentric running, which slows the exercise hyperemic response at the onset of contractions and reduces the Pmv(O(2)) available to drive blood-muscle O(2) delivery.

Effects of aging on microvascular oxygen pressures in rat skeletal muscle.

Aging alters skeletal muscle vascular geometry and control such that the dynamics of muscular blood flow (Q) and O2 delivery (Q(O2)) may be impaired across the rest-exercise transition. If, at the onset of muscle contractions, Q dynamics are slowed disproportionately to those of muscle O2 uptake (V(O2), microvascular PO2 (PO2m) would be reduced and blood-tissue O2 transfer compromised. This investigation determined the effects of aging on PO2m (a direct reflection of the Q(O2)-to-V(O2) ratio), at rest and across the rest-contractions transition in the spinotrapezius of young (approximately 6 months, n = 9) and old (>24 months, n = 10) male Fisher 344/Brown Norway hybrid rats. Phosphorescence quenching techniques were used to quantify PO2m, and test the hypothesis that, across the rest-contractions (twitch, 1 Hz; 4-6 V, 240 s) transition, aging would transiently reduce the Q(O2)-to-V(O2) ratio causing a biphasic profile in which PO2m fell below steady-state contracting values. Old rats had a lower pre-contraction baseline PO2m than young (27.1+/-1.9 versus 33.8+/-1.6 mmHg, P<0.05, respectively). In addition, in old rats PO2m demonstrated a pronounced difference between the absolute nadir and end-contracting values (2.5+/-0.9 mmHg), which was absent in young rats. In conclusion, unlike their young counterparts, old rats exhibited a transiently reduced PO2m across the rest-contractions transition that may impair blood-tissue O2 exchange and elevate the O2 deficit, thereby contributing to premature fatigue.

Dynamics of muscle microcirculatory oxygen exchange.

PURPOSE: Beyond the initial cardiodynamic "Phase I," pulmonary oxygen uptake (VO(2)) kinetics are dictated largely by, and resemble closely, the VO(2) of the exercising muscles (VO(2)m). Within those muscles, the microcirculation is responsible for affecting almost all blood-myocyte O(2) transfer, and thus, observations at this site may provide key insights into muscle oxidative function in health and dysfunction in disease. METHODS: Recently, a novel combination of microscopy and phosphorescence quenching techniques has been utilized to understand the dynamics of microvascular O(2) delivery (VO(2)m) and muscle O(2) utilization (VO(2)m) at the onset of muscle contractions. RESULTS: These experiments have addressed longstanding questions regarding the site of control of VO(2)m kinetics and provide a first look at capillary hemodynamics at exercise onset in healthy muscle and their derangements resulting from chronic diseases such as heart failure and diabetes. CONCLUSION: This paper will review these novel findings within our current understanding of microcirculatory control and blood-myocyte O(2) transfer.

Dynamics of microvascular oxygen pressure in the rat diaphragm.

The relative amplitudes and rates of increase of muscle blood flow (and O(2) delivery) and O(2) uptake responses determine the O(2) pressure within the muscle microvasculature (Pm(O(2))) across the rest-to-contraction transition. Skeletal muscle function is a primary determinant of pulmonary O(2) uptake kinetics; however, it has never been determined whether the dynamics of muscle Pm(O(2)) are faster in a highly oxidative muscle [e.g., diaphragm (Dia), citrate synthase activity of 39 micromol. min(-1). g(-1)] compared with less oxidative muscles [e.g., spinotrapezius (Spino), citrate synthase activity of 14 micromol. min(-1). g(-1), male Sprague-Dawley rats; Delp MD and Duan C, J Appl Physiol 80: 261-270, 1996]. Phosphorescence quenching techniques (porphyrin dendrimer, R2) were used to determine Pm(O(2)) across the transition to electrically stimulated contractions (1 Hz) within the rat Dia. After a delay of 10.4 +/- 1.3 (SE) s at the beginning of Dia contractions, Pm(O(2)) decreased close to monoexponentially from 42 +/- 2 to 27 +/- 3 Torr (P < 0.05) with an extremely fast time constant of 7.1 +/- 1.1 s. Thus Dia Pm(O(2)) decreased with significantly (P < 0.05) faster kinetics than reported previously for the Spino muscle (delay, 19.2 +/- 2.8 s; time constant Pm(O(2)), 21.7 +/- 2.1 s; Behnke BJ, Kindig CA, Musch TI, Koga S, and Poole DC, Respir Physiol 126: 53-63, 2001). With the use of two specialized muscles with similar fiber-type composition but widely disparate oxidative capacities (Delp MD and Duan C, J Appl Physiol 80: 261-270, 1996), these data demonstrate that Pm(O(2)) kinetics are significantly faster in the highly oxidative Dia compared with the low-oxidative Spino muscle and that this effect is not dependent on muscle fiber-type composition.

Recovery of microvascular PO2 during the exercise off-transient in muscles of different fiber type.

The speed with which muscle energetic status recovers after exercise is dependent on oxidative capacity and vascular O(2) pressures. Because vascular control differs between muscles composed of fast- vs. slow-twitch fibers, we explored the possibility that microvascular O(2) pressure (Pmv(O(2)); proportional to the O(2) delivery-to-O(2) uptake ratio) would differ during recovery in fast-twitch peroneal (Per: 86% type II) compared with slow-twitch soleus (Sol: 84% type I). Specifically, we hypothesized that, in Per, Pmv(O(2)) would be reduced immediately after contractions and would recover more slowly during the off-transient from contractions compared with Sol. The Per and Sol muscles of six female Sprague-Dawley rats (weight = approximately 220 g) were studied after the cessation of electrical stimulation (120 s; 1 Hz) to compare the recovery profiles of Pmv(O(2)). As hypothesized, Pmv(O(2)) was lower throughout recovery in Per compared with Sol (end contraction: 13.4 +/- 2.2 vs. 20.2 +/- 0.9 Torr; end recovery: 24.0 +/- 2.4 vs. 27.4 +/- 1.2 Torr, Per vs. Sol; P

Arterial blood gas control in the upright versus recumbent Asian elephant.

In the elephant, there is concern that lateral recumbency (LR) impairs respiratory muscle and lung function resulting in clinically significant arterial hypoxemia. Using healthy adult female Asian elephants (Elephas maximus, n=6), the hypothesis was tested that, given the O(2) binding characteristics of elephant blood, substantial reductions in arterial O(2) pressure (Pa(O(2))) in LR could be tolerated without lowering arterial O(2) content appreciably. Fifteen minutes of LR decreased Pa(O(2)) from 103+/-2 (upright, U) to 77+/-4 mmHg (P<0.05) and hemoglobin O(2) saturation (U, 97.8+/-0.1, LR, 95.3+/-0.5%, P<0.05). However, due to a recumbency-induced hemoconcentration, arterial O(2) content was unchanged (U, 18.2+/-2.4, LR, 18.3+/-2.1 ml O(2) per 100 ml). In addition, there was a mild hyperventilation in LR that reduced arterial CO(2) pressure (P(CO(2))) from 39.4+/-0.3 to 37.1+/-1.0 mmHg (P<0.05). These data indicate that the Asian elephant can endure at least short periods of LR without lowering arterial O(2) content.

Dynamics of oxygen uptake following exercise onset in rat skeletal muscle.

Technical limitations have precluded measurement of the V(O(2)) profile within contracting muscle (mV(O(2))) and hence it is not known to what extent V(O(2)) dynamics measured across limbs in humans or muscles in the dog are influenced by transit delays between the muscle microvasculature and venous effluent. Measurements of capillary red blood cell flux and microvascular P(O(2)) (P(O(2)m)) were combined to resolve the time course of mV(O(2)) across the rest-stimulation transient (1 Hz, twitch contractions). mV(O(2)) began to rise at the onset of contractions in a close to monoexponential fashion (time constant, J = 23.2 +/- 1.0 sec) and reached it's steady-state value at 4.5-fold above baseline. Using computer simulation in healthy and disease conditions (diabetes and chronic heart failure), our findings suggest that: (1) mV(O(2)) increases essentially immediately (< 2 sec) following exercise onset; (2) within healthy muscle the J blood flow (thus O(2) delivery, J Q(O(2)m)) is faster than JmV(O(2)) such that oxygen delivery is not limiting, and 3) a faster P(O(2)m) fall to a P(O(2)m) value below steady-state values within muscle from diseased animals is consistent with a relatively sluggish Q(O(2)m) response compared to that of mV(O(2)).