α-Adrenergic regulation of skeletal muscle blood flow during exercise in patients with heart failure with preserved ejection fraction.

Heart failure with preserved ejection fraction (HFpEF) has been characterized by lower blood flow to exercising limbs and lower peak oxygen utilization ( V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ ), possibly associated with disease-related changes in sympathetic (α-adrenergic) signaling. Thus, in seven patients with HFpEF (70 ± 6 years, 3 female/4 male) and seven controls (CON) (66 ± 3 years, 3 female/4 male), we examined changes (%Δ) in leg blood flow (LBF, Doppler ultrasound) and leg V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ to intra-arterial infusion of phentolamine (PHEN, α-adrenergic antagonist) or phenylephrine (PE, α1-adrenergic agonist) at rest and during single-leg knee-extension exercise (0, 5 and 10 W). At rest, the PHEN-induced increase in LBF was not different between groups, but PE-induced reductions in LBF were lower in HFpEF (-16% ± 4% vs. -26% ± 5%, HFpEF vs. CON; P < 0.05). During exercise, the PHEN-induced increase in LBF was greater in HFpEF at 10 W (16% ± 8% vs. 8% ± 5%; P < 0.05). PHEN increased leg V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ in HFpEF (10% ± 3%, 11% ± 6%, 15% ± 7% at 0, 5 and 10 W; P < 0.05) but not in controls (-1% ± 9%, -4% ± 2%, -1% ± 5%; P = 0.24). The 'magnitude of sympatholysis' (PE-induced %Δ LBF at rest - PE-induced %Δ LBF during exercise) was lower in patients with HFpEF (-6% ± 4%, -6% ± 6%, -7% ± 5% vs. -13% ± 6%, -17% ± 5%, -20% ± 5% at 0, 5 and 10 W; P < 0.05) and was positively related to LBF, leg oxygen delivery, leg V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ , and the PHEN-induced increase in LBF (P < 0.05). Together, these data indicate that excessive α-adrenergic vasoconstriction restrains blood flow and limits V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ of the exercising leg in patients with HFpEF, and is related to impaired functional sympatholysis in this patient group. KEY POINTS: Sympathetic (α-adrenergic)-mediated vasoconstriction is exaggerated during exercise in patients with heart failure with preserved ejection fraction (HFpEF), which may contribute to limitations of blood flow, oxygen delivery and oxygen utilization in the exercising muscle. The ability to adequately attenuate α1-adrenergic vasoconstriction (i.e. functional sympatholysis) within the vasculature of the exercising muscle is impaired in patients with HFpEF. These observations extend our current understanding of HFpEF pathophysiology by implicating excessive α-adrenergic restraint and impaired functional sympatholysis as important contributors to disease-related impairments in exercising muscle blood flow and oxygen utilization in these patients.

Statin administration improves vascular function in heart failure with preserved ejection fraction.

Heart failure with preserved ejection fraction (HFpEF) is characterized by impaired vascular endothelial function that may be improved by hydroxy-methylglutaryl-CoA (HMG-CoA) reductase enzyme inhibition. Thus, using a parallel, double-blind, placebo-controlled design, this study evaluated the efficacy of 30-day atorvastatin administration (10 mg daily) on peripheral vascular function and biomarkers of inflammation and oxidative stress in 16 patients with HFpEF [Statin: n = 8, 74 ± 6 yr, ejection fraction (EF) 52-73%; Placebo: n = 8, 67 ± 9 yr, EF 56-72%]. Flow-mediated dilation (FMD) and sustained-stimulus FMD (SS-FMD) during handgrip (HG) exercise, reactive hyperemia (RH), and blood flow during HG exercise were evaluated to assess conduit vessel function, microvascular function, and exercising muscle blood flow, respectively. FMD improved following statin administration (pre, 3.33 ± 2.13%; post, 5.23 ± 1.35%; P < 0.01), but was unchanged in the placebo group. Likewise, SS-FMD, quantified using the slope of changes in brachial artery diameter in response to increases in shear rate, improved following statin administration (pre: 5.31e-5 ± 3.85e-5 mm/s-1; post: 8.54e-5 ± 4.98e-5 mm/s-1; P = 0.03), with no change in the placebo group. Reactive hyperemia and exercise hyperemia responses were unchanged in both statin and placebo groups. Statin administration decreased markers of lipid peroxidation (malondialdehyde, MDA) (pre, 0.652 ± 0.095; post, 0.501 ± 0.094; P = 0.04), whereas other inflammatory and oxidative stress biomarkers were unchanged. Together, these data provide new evidence for the efficacy of low-dose statin administration to improve brachial artery endothelium-dependent vasodilation, but not microvascular function or exercising limb blood flow, in patients with HFpEF, which may be due in part to reductions in oxidative stress.NEW & NOTEWORTHY This is the first study to investigate the impact of statin administration on vascular function and exercise hyperemia in patients with heart failure with preserved ejection fraction (HFpEF). In support of our hypothesis, both conventional flow-mediated dilation (FMD) testing and brachial artery vasodilation in response to sustained elevations in shear rate during handgrip exercise increased significantly in patients with HFpEF following statin administration, beneficial effects that were accompanied by a decrease in biomarkers of oxidative damage. However, contrary to our hypothesis, reactive hyperemia and exercise hyperemia were unchanged in patients with HFpEF following statin therapy. These data provide new evidence for the efficacy of low-dose statin administration to improve brachial artery endothelium-dependent vasodilation, but not microvascular reactivity or exercising muscle blood flow in patients with HFpEF, which may be due in part to reductions in oxidative stress.

Impaired cardiopulmonary baroreflex function and altered cardiovascular responses to hypovolemia in patients with heart failure with preserved ejection fraction.

Heart failure with preserved ejection fraction (HFpEF) is associated with autonomic dysregulation, which may be related to baroreflex dysfunction. Thus, we tested the hypothesis that cardiac and peripheral vascular responses to baroreflex activation via lower-body negative pressure (LBNP; -10, -20, -30, -40 mmHg) would be diminished in patients with HFpEF (n = 10, 71 ± 7 yr) compared with healthy controls (CON, n = 9, 69 ± 5 yr). Changes in heart rate (HR), mean arterial pressure (MAP, Finapres), forearm blood flow (FBF, ultrasound Doppler), and thoracic impedance (Z) were determined. Mild levels of LBNP (-10 and -20 mmHg) were used to specifically assess the cardiopulmonary baroreflex, whereas responses across the greater levels of LBNP represented an integrated baroreflex response. LBNP significantly increased in HR in CON subjects at -30 and -40 mmHg (+3 ± 3 and +6 ± 5 beats/min, P < 0.01), but was unchanged in patients with HFpEF across all LBNP levels. LBNP provoked progressive peripheral vasoconstriction, as quantified by changes in forearm vascular conductance (FVC), in both groups. However, a marked (40%-60%) attenuation in FVC responses was observed in patients with HFpEF (-6 ± 8, -15 ± 6, -16 ± 5, and -19 ± 7 mL/min/mmHg at -10, -20, -30, and -40 mmHg, respectively) compared with controls (-15 ± 10, -22 ± 6, -25 ± 10, and -28 ± 10 mL/min/mmHg, P < 0.01). MAP was unchanged in both groups. Together, these data provide new evidence for impairments in cardiopulmonary baroreflex function and diminished cardiovascular responsiveness during hypovolemia in patients with HFpEF, which may be an important aspect of the disease-related changes in autonomic cardiovascular control in this patient group.NEW & NOTEWORTHY Data from the current study demonstrate diminished cardiovascular responsiveness during hypovolemia induced by incremental lower-body negative pressure in patients with heart failure with preserved ejection fraction (HFpEF). These diminished responses imply impaired cardiopulmonary baroreflex function and altered autonomic cardiovascular regulation which may represent an important aspect of HFpEF pathophysiology.

Evidence of impaired functional sympatholysis in patients with heart failure with preserved ejection fraction.

Exercising muscle blood flow is reduced in patients with heart failure with a preserved ejection fraction (HFpEF), which may be related to disease-related changes in the ability to overcome sympathetic nervous system (SNS)-mediated vasoconstriction during exercise, (i.e., "functional sympatholysis"). Thus, in 12 patients with HFpEF (69 ± 7 yr) and 11 healthy controls (Con, 69 ± 4 yr), we examined forearm blood flow (FBF), mean arterial pressure (MAP), and forearm vascular conductance (FVC) during rhythmic handgrip exercise (HG) at 30% of maximum voluntary contraction with or without lower-body negative pressure (LBNP, -20 mmHg) to increase SNS activity and elicit peripheral vasoconstriction. SNS-mediated vasoconstrictor responses were determined as LBNP-induced changes (%Δ) in FVC, and the "magnitude of sympatholysis" was calculated as the difference between responses at rest and during exercise. At rest, the LBNP-induced change in FVC was significantly lesser in HFpEF compared with Con (HFpEF: -9.5 ± 5.5 vs. Con: -21.0 ± 8.0%; P < 0.01). During exercise, LBNP-induced %ΔFVC was significantly attenuated in Con compared with rest (HG: -5.8 ± 6.0%; P < 0.05) but not in HFpEF (HG: -9.9 ± 2.5%; P = 0.88). Thus, the magnitude of sympatholysis was lesser in HFpEF compared with Con (HFpEF: 0.4 ± 4.7 vs. Con: -15.2 ± 11.8%; P < 0.01). These data demonstrate a diminished ability to attenuate SNS-mediated vasoconstriction in HFpEF and provide new evidence suggesting impaired functional sympatholysis in this patient group.NEW & NOTEWORTHY Data from the current study suggest that functional sympatholysis, or the ability to adequately attenuate sympathetic nervous system (SNS)-mediated vasoconstriction during exercise, is impaired in patients with heart failure with preserved ejection fraction (HFpEF). These observations extend the current understanding of HFpEF pathophysiology by implicating inadequate functional sympatholysis as an important contributor to reduced exercising muscle blood flow in this patient group.

Preserved endothelium-independent vascular function with aging in men and women: evidence from the peripheral and cerebral vasculature.

In the peripheral and cerebral vasculature, the impact of aging and sex on the endothelial-independent functional capacity of vascular smooth muscle cells (VSMCs) is not well understood, nor is it known whether such VSMC functions in these vascular beds reflect one another. Therefore, endothelium-independent dilation, at both the conduit (Δ diameter) and microvascular (Δ vascular conductance, VC) level, elicited by sublingual nitroglycerin (NTG, 0.8 mg of Nitrostat), compared with sham-delivery (control), was assessed using Doppler ultrasound in the popliteal (PA) and middle cerebral (MCA) artery of 20 young [23 ± 4 yr, 10 males (YM)/10 females (YF)] and 21 old [69 ± 5 yr, 11 males (OM)/10 females (OF)] relatively healthy adults. In the PA, compared with zero, NTG significantly increased diameter in all groups (YM: 0.29 ± 0.13, YF: 0.35 ± 0.26, OM: 0.30 ± 0.18, OF: 0.31 ± 0.14 mm), while control did not. The increase in VC only achieved significance in the OF (0.22 ± 0.31 mL/min/mmHg). In the MCA, compared with zero, NTG significantly increased diameter and VC in all groups (YM: 0.89 ± 0.30, 1.06 ± 1.28; YF: 0.97 ± 0.31, 1.84 ± 1.07; OM: 0.90 ± 0.42, 0.72 ± 0.99; OF: 0.74 ± 0.32, 1.19 ± 1.18, mm and mL/min/mmHg, respectively), while control did not. There were no age or sex differences or age-by-sex interactions for both the NTG-induced PA and MCA dilation and VC. In addition, PA and MCA dilation and VC responses to NTG were not related when grouped by age, sex, or as all subjects (r = 0.04-0.44, P > 0.05). Thus, peripheral and cerebral endothelial-independent VSMC function appears to be unaffected by age or sex, and variations in such VSMC function in one of these vascular beds are not reflected in the other.NEW & NOTEWORTHY To confidently explain peripheral and cerebral vascular dysfunction, it is essential to have a clear understanding of the endothelial-independent function of VSMCs across age and sex. By assessing endothelium-independent dilation using sublingual nitroglycerin, endothelial-independent VSMC function in the periphery (popliteal artery), and in the cerebral circulation (middle cerebral artery), was not different due to age or sex. In addition, endothelial-independent VSMC function in one of these vascular beds is not reflected in the other.

Pharmacological modulation of adrenergic tone alters the vasodilatory response to passive leg movement in young but not in old adults.

The age-related increase in α-adrenergic tone may contribute to decreased leg vascular conductance (LVC) both at rest and during exercise in the old. However, the effect on passive leg movement (PLM)-induced LVC, a measure of vascular function, which is markedly attenuated in this population, is unknown. Thus, in eight young (25 ± 5 yr) and seven old (65 ± 7 yr) subjects, this investigation examined the impact of systemic ß-adrenergic blockade (propanalol, PROP) alone, and PROP combined with either α1-adrenergic stimulation (phenylephrine, PE) or α-adrenergic inhibition (phentolamine, PHEN), on PLM-induced vasodilation. LVC, calculated from femoral artery blood flow and pressure, was determined and PLM-induced Δ peak (LVCΔpeak) and total vasodilation (LVCAUC, area under curve) were documented. PROP decreased LVCΔpeak (PROP: 4.8 ± 1.8, Saline: 7.7 ± 2.7 mL·mmHg-1, P < 0.001) and LVCAUC (PROP: 1.1 ± 0.7, Saline: 2.4 ± 1.6 mL·mmHg-1, P = 0.002) in the young, but not in the old (LVCΔpeak, P = 0.931; LVCAUC, P = 0.999). PE reduced baseline LVC (PE: 1.6 ± 0.4, PROP: 2.3 ± 0.4 mL·min-1·mmHg-1, P < 0.01), LVCΔpeak (PE: 3.2 ± 1.3, PROP: 4.8 ± 1.8 mL·min-1·mmHg-1, P = 0.004), and LVCAUC (PE: 0.5 ± 0.4, PROP: 1.1 ± 0.7 mL·mmHg-1, P = 0.011) in the young, but not in the old (baseline LVC, P = 0.199; LVCΔpeak, P = 0.904; LVCAUC, P = 0.823). PHEN increased LVC at rest and throughout PLM in both groups (drug effect: P < 0.05), however LVCΔpeak was only improved in the young (PHEN: 6.4 ± 3.1, PROP: 4.4 ± 1.5 mL·min-1·mmHg-1, P = 0.004), and not in the old (P = 0.904). Furthermore, the magnitude of α-adrenergic modulation (PHEN - PE) of LVCΔpeak was greater in the young compared with the old (Young: 3.35 ± 2.32, Old: 0.40 ± 1.59 mL·min-1·mmHg-1, P = 0.019). Therefore, elevated α-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.NEW & NOTEWORTHY Stimulation of α1-adrenergic receptors eliminated age-related differences in passive leg movement (PLM) by decreasing PLM-induced vasodilation in the young. Systemic ß-blockade attenuated the central hemodynamic component of the PLM response in young individuals. Inhibition of α-adrenergic receptors did not improve the PLM response in older individuals, though withdrawal of α-adrenergic modulation augmented baseline and maximal vasodilation in both groups. Accordingly, α-adrenergic signaling plays a role in modulating the PLM vasodilatory response in young but not in old adults, and elevated α-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.

Impaired hemodynamic response to exercise in patients with peripheral artery disease: evidence of a link to inflammation and oxidative stress.

An exaggerated mean arterial blood pressure (MAP) response to exercise in patients with peripheral artery disease (PAD), likely driven by inflammation and oxidative stress and, perhaps, required to achieve an adequate blood flow response, is well described. However, the blood flow response to exercise in patients with PAD actually remains equivocal. Therefore, eight patients with PAD and eight healthy controls completed 3 min of plantar flexion exercise at both an absolute work rate (WR) (2.7 W, to evaluate blood flow) and a relative intensity (40%WRmax, to evaluate MAP). The exercise-induced change in popliteal artery blood flow (BF, Ultrasound Doppler), MAP (Finapress), and vascular conductance (VC) were quantified. In addition, resting markers of inflammation and oxidative stress were measured in plasma and muscle biopsies. Exercise-induced ΔBF, assessed at 2.7 W, was lower in PAD compared with controls (PAD: 251 ± 150 vs. Controls: 545 ± 187 mL/min, P < 0.001), whereas ΔMAP, assessed at 40%WRmax, was greater for PAD (PAD: 23 ± 14 vs. Controls: 11 ± 6 mmHg, P = 0.028). The exercise-induced ΔVC was lower for PAD during both the absolute WR (PAD: 1.9 ± 1.6 vs. Controls: 4.7 ± 1.9 mL/min/mmHg) and relative intensity exercise (PAD: 1.9 ± 1.8 vs. Controls: 5.0 ± 2.2 mL/min/mmHg) trials (both, P < 0.01). Inflammatory and oxidative stress markers, including plasma interleukin-6 and muscle protein carbonyls, were elevated in PAD (both, P < 0.05), and significantly correlated with the hemodynamic changes during exercise (r = -0.57 to -0.78, P < 0.05). Thus, despite an exaggerated ΔMAP response, patients with PAD exhibit an impaired exercise-induced ΔBF and ΔVC, and both inflammation and oxidative stress likely play a role in this attenuated hemodynamic response.

Regulation of capillary hemodynamics by KATP channels in resting skeletal muscle.

ATP-sensitive K+ channels (KATP ) have been implicated in the regulation of resting vascular smooth muscle membrane potential and tone. However, whether KATP channels modulate skeletal muscle microvascular hemodynamics at the capillary level (the primary site for blood-myocyte O2 exchange) remains unknown. We tested the hypothesis that KATP channel inhibition would reduce the proportion of capillaries supporting continuous red blood cell (RBC) flow and impair RBC hemodynamics and distribution in perfused capillaries within resting skeletal muscle. RBC flux (fRBC ), velocity (VRBC ), and capillary tube hematocrit (Hctcap ) were assessed via intravital microscopy of the rat spinotrapezius muscle (n = 6) under control (CON) and glibenclamide (GLI; KATP channel antagonist; 10 µM) superfusion conditions. There were no differences in mean arterial pressure (CON:120 ± 5, GLI:124 ± 5 mmHg; p > 0.05) or heart rate (CON:322 ± 32, GLI:337 ± 33 beats/min; p > 0.05) between conditions. The %RBC-flowing capillaries were not altered between conditions (CON:87 ± 2, GLI:85 ± 1%; p > 0.05). In RBC-perfused capillaries, GLI reduced fRBC (CON:20.1 ± 1.8, GLI:14.6 ± 1.3 cells/s; p < 0.05) and VRBC (CON:240 ± 17, GLI:182 ± 17 µm/s; p < 0.05) but not Hctcap (CON:0.26 ± 0.01, GLI:0.26 ± 0.01; p > 0.05). The absence of GLI effects on the %RBC-flowing capillaries and Hctcap indicates preserved muscle O2 diffusing capacity (DO2 m). In contrast, GLI lowered both fRBC and VRBC thus impairing perfusive microvascular O2 transport (Q̇m) and lengthening RBC capillary transit times, respectively. Given the interdependence between diffusive and perfusive O2 conductances (i.e., %O2 extraction∝DO2 m/Q̇m), such GLI alterations are expected to elevate muscle %O2 extraction to sustain a given metabolic rate. These results support that KATP channels regulate capillary hemodynamics and, therefore, microvascular gas exchange in resting skeletal muscle.

Impact of presymptomatic COVID-19 on vascular and skeletal muscle function: a case study.

The impact of COVID-19 has been largely described after symptom development. Although the SARS-CoV-2 virus elevates heart rate (HR) prior to symptom onset, whether this virus evokes other presymptomatic alterations is unknown. This case study details the presymptomatic impact of COVID-19 on vascular and skeletal muscle function in a young woman [24 yr, 173.5 cm, 89 kg, body mass index (BMI): 29.6 kg·m-2]. Vascular and skeletal muscle function were assessed as part of a separate study with the first and second visits separated by 2 wk. On the evening following the second visit, the participant developed a fever and a rapid antigen test confirmed a positive COVID-19 diagnosis. Compared with the first visit, the participant presented with a markedly elevated HR (∼30 beats/min) and a lower mean blood pressure (∼8 mmHg) at the second visit. Vascular function measured by brachial artery flow-mediated dilation, reactive hyperemia, and passive leg movement were all noticeably attenuated (25%-65%) as was leg blood flow during knee extension exercise. Muscle strength was diminished as was ADP-stimulated respiration (30%), assessed in vitro, whereas there was a 25% increase in the apparent Km. Lastly, an elevation in IL-10 was observed prior to symptom onset. Notably, 2.5 mo after diagnosis symptoms of fatigue and cough were still present. Together, these findings provide unique insight into the physiological responses immediately prior to onset of COVID-19 symptoms; they suggest that SARS-CoV-2 negatively impacts vascular and skeletal muscle function prior to the onset of common symptoms and may set the stage for the widespread sequelae observed following COVID-19 diagnosis.NEW & NOTEWORTHY This unique case study details the impact of SARS-CoV-2 infection on vascular and skeletal muscle function in a young predominantly presymptomatic woman. Prior to COVID-19 diagnosis, substantial reductions in vascular, skeletal muscle, and mitochondrial function were observed along with an elevation in IL-10. This integrative case study indicates that the presymptomatic impact of COVID-19 is widespread and may help elucidate the acute and long-term sequelae of this disease.

The dynamic adjustment of mean arterial pressure during exercise: a potential tool for discerning cardiovascular health status.

The regulation of mean arterial pressure (MAP) during exercise has important physiological and clinical implications. Kinetics analysis on numerous physiological variables following the transition from unloaded-to-loaded exercise has revealed important information regarding their control. Surprisingly, the dynamic response of MAP during this transition remains to be quantified. Therefore, ten healthy participants (5/5 M/F, 24 ± 3 yr) completed repeated transitions from unloaded to moderate- and heavy-intensity dynamic single-leg knee-extensor exercise to investigate the on-kinetics of MAP. Following the transition to loaded exercise, MAP increased in a first-order dynamic manner, subsequent to a time delay (moderate: 23 ± 10; heavy: 19 ± 9 s, P > 0.05) at a speed (τ, moderate: 59 ± 30; heavy: 66 ± 19 s, P > 0.05), which did not differ between intensities, but the MAP amplitude was doubled during heavy-intensity exercise (moderate: 12 ± 5; heavy: 24 ± 8 mmHg, P < 0.001). The reproducibility [coefficient of variation (CV)] during heavy intensity for unloaded baseline, amplitude, and mean response time, when assessed as individual transitions, was 7 ± 1%, 18 ± 2%, and 25 ± 4%, respectively. Averaging two transitions improved the CVs to 4 ± 1%, 8 ± 2%, and 13 ± 3%, respectively. Preliminary findings supporting the clinical relevance of evaluating MAP kinetics in middle-aged hypertensive (n = 5) and, age-matched, normotensive (n = 5) participants revealed an exaggerated MAP response in both older groups (P < 0.05), but the MAP response was slowed only for the patients with hypertension (P < 0.05). It is concluded that kinetics modeling of MAP is practical for heavy-intensity knee-extensor exercise and may provide insight into cardiovascular health and the effect of aging.NEW & NOTEWORTHY Kinetics analysis of physiological variables following workload transitions provides important information, but this has not been performed on mean arterial pressure (MAP), despite the clear clinical importance of this variable. This investigation reveals that kinetic modeling of MAP following unloaded-to-loaded knee-extensor exercise is practical and repeatable. Additional preliminary findings in hypertensive and, age-matched, normotensive subjects suggest that MAP kinetics may provide insight into cardiovascular health and the effect of aging.

The role of the endothelium in the hyperemic response to passive leg movement: looking beyond nitric oxide.

Passive leg movement (PLM) evokes a robust and predominantly nitric oxide (NO)-mediated increase in blood flow that declines with age and disease. Consequently, PLM is becoming increasingly accepted as a sensitive assessment of endothelium-mediated vascular function. However, a substantial PLM-induced hyperemic response is still evoked despite nitric oxide synthase (NOS) inhibition. Therefore, in nine young healthy men (25 ± 4 yr), this investigation aimed to determine whether the combination of two potent endothelium-dependent vasodilators, specifically prostaglandin (PG) and endothelium-derived hyperpolarizing factor (EDHF), account for the remaining hyperemic response to the two variants of PLM, PLM (60 movements) and single PLM (sPLM, 1 movement), when NOS is inhibited. The leg blood flow (LBF, Doppler ultrasound) response to PLM and sPLM following the intra-arterial infusion of NG-monomethyl-l-arginine (l-NMMA), to inhibit NOS, was compared to the combined inhibition of NOS, cyclooxygenase (COX), and cytochrome P-450 (CYP450) by l-NMMA, ketorolac tromethamine (KET), and fluconazole (FLUC), respectively. NOS inhibition attenuated the overall LBF [area under the curve (LBFAUC)] response to both PLM (control: 456 ± 194, l-NMMA: 168 ± 127 mL, P < 0.01) and sPLM (control: 185 ± 171, l-NMMA: 62 ± 31 mL, P = 0.03). The combined inhibition of NOS, COX, and CYP450 (i.e., l-NMMA+KET+FLUC) did not further attenuate the hyperemic responses to PLM (LBFAUC: 271 ± 97 mL, P > 0.05) or sPLM (LBFAUC: 72 ± 45 mL, P > 0.05). Therefore, PG and EDHF do not collectively contribute to the non-NOS-derived NO-mediated, endothelium-dependent hyperemic response to either PLM or sPLM in healthy young men. These findings add to the mounting evidence and understanding of the vasodilatory pathways assessed by the PLM and sPLM vascular function tests.NEW & NOTEWORTHY Passive leg movement (PLM) evokes a highly nitric oxide (NO)-mediated hyperemic response and may provide a novel evaluation of vascular function. The contributions of endothelium-dependent vasodilatory pathways, beyond NO and including prostaglandins and endothelium-derived hyperpolarizing factor, to the PLM-induced hyperemic response to PLM have not been evaluated. With intra-arterial drug infusion, the combined inhibition of nitric oxide synthase (NOS), cyclooxygenase, and cytochrome P-450 (CYP450) pathways did not further diminish the hyperemic response to PLM compared with NOS inhibition alone.

Systemic NOS inhibition reduces contracting muscle oxygenation more in intact female than male rats.

Females respond to baroreceptor stimulation with enhanced modulation of heart rate (HR) to regulate blood pressure and also express greater reliance on nitric oxide (NO) for vascular control compared to males. Sex differences in muscle oxygenation consequent to central hemodynamic challenge induced by systemic NO synthase (NOS) inhibition are unknown. We tested the hypotheses that systemic NOS inhibition would induce lower contracting skeletal muscle oxygenation in females compared to males. The spinotrapezius of Sprague-Dawley rats (females (♀) = 9, males (♂) = 9) was surgically exposed and contracted by electrical stimulation (180s, 1 Hz, ~6 V) under pentobarbital sodium anesthesia. Oxyphor G4 was injected into the muscle and phosphorescence quenching was used to measure the interstitial PO2 (PO2is, determined by O2 delivery-to-utilization matching) under control (Krebs-Henseleit solution) and after intra-arterial infusion of nitro-l-arginine methyl ester (l-NAME; NOS blockade; 10 mg kg-1). At rest, females showed a greater PO2is increase (ΔPO2is/ΔMAP) and HR (ΔHR/ΔMAP) reduction than males in response to the elevated MAP induced by systemic NOS inhibition (both p < 0.05). Following l-NAME, during the contracting steady-state, females exhibited lower PO2is than males (♂: 17.1 ± 1.4 vs ♀: 10.8 ± 1.4 mmHg, p < 0.05). The rate pressure product was lower in females than males (♂: 482 ± 14 vs ♀: 392 ± 29, p < 0.05) and correlated with the steady-state PO2is (r = 0.66, p < 0.05). These results support that females express greater reductions in HR than males in response to l-NAME-induced elevation of MAP via the baroreceptor reflex and provide new insights on how central hemodynamics affect skeletal muscle oxygenation in a sex-specific manner.

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.

ATP-sensitive K+ channel inhibition in rats decreases kidney and skeletal muscle blood flow without increasing sympathetic nerve discharge.

ATP-sensitive K+ (KATP) channels contribute to exercise-induced hyperemia in skeletal muscle either locally by vascular hyperpolarization or by sympathoinhibition and decreased sympathetic vasoconstriction. However, mean arterial pressure (MAP) regulation via baroreceptors and subsequent efferent activity may confound assessment of vascular versus neural KATP channel function. We hypothesized that systemic KATP channel inhibition via glibenclamide (GLI) would increase MAP without increasing sympathetic nerve discharge (SND). Lumbar and renal nerve SND were measured in anesthetized male rats with intact baroreceptors (n = 12) and sinoaortic denervated (SAD; n = 4) counterparts and blood flow (BF) and vascular conductance (VC) assessed in conscious rats (n = 6). GLI increased MAP (p < 0.05) and transiently decreased HR in intact (p < 0.05), but not SAD rats. Renal (-30 %) and lumbar (-40 %) ΔSND decreased in intact but increased in SAD rats (∼40 % and 20 %; p < 0.05). BF and VC decreased in kidneys and total hindlimb skeletal muscle (p < 0.05). Thus, because KATP inhibition decreases SND, GLI-induced reductions in blood flow cannot result from enhanced sympathetic activity.

Guidelines for animal exercise and training protocols for cardiovascular studies.

Whole body exercise tolerance is the consummate example of integrative physiological function among the metabolic, neuromuscular, cardiovascular, and respiratory systems. Depending on the animal selected, the energetic demands and flux through the oxygen transport system can increase two orders of magnitude from rest to maximal exercise. Thus, animal models in health and disease present the scientist with flexible, powerful, and, in some instances, purpose-built tools to explore the mechanistic bases for physiological function and help unveil the causes for pathological or age-related exercise intolerance. Elegant experimental designs and analyses of kinetic parameters and steady-state responses permit acute and chronic exercise paradigms to identify therapeutic targets for drug development in disease and also present the opportunity to test the efficacy of pharmacological and behavioral countermeasures during aging, for example. However, for this promise to be fully realized, the correct or optimal animal model must be selected in conjunction with reproducible tests of physiological function (e.g., exercise capacity and maximal oxygen uptake) that can be compared equitably across laboratories, clinics, and other proving grounds. Rigorously controlled animal exercise and training studies constitute the foundation of translational research. This review presents the most commonly selected animal models with guidelines for their use and obtaining reproducible results and, crucially, translates state-of-the-art techniques and procedures developed on humans to those animal models.

The role of endothelin A receptors in peripheral vascular control at rest and during exercise in patients with hypertension.

KEY POINTS: Exercise in patients with hypertension can be accompanied by an abnormal cardiovascular response that includes attenuated blood flow and an augmented pressor response. Endothelin-1, a very potent vasoconstrictor, is a key modulator of blood flow and pressure during in health and has been implicated as a potential cause of the dysfunction in hypertension. We assessed the role of endothelin-1, acting through endothelin A (ETA ) receptors, in modulating the central and peripheral cardiovascular responses to exercise in patients with hypertension via local antagonism of these receptors during exercise. ETA receptor antagonism markedly increased leg blood flow, vascular conductance, oxygen delivery, and oxygen consumption during exercise; interestingly, these changes occurred in the presence of reduced leg perfusion pressure, indicating that these augmentations were driven by changes in vascular resistance. These data indicate that ETA receptor antagonism could be a viable therapeutic approach to improve blood flow during exercise in hypertension. ABSTRACT: Patients with hypertension can exhibit impaired muscle blood flow and exaggerated increases in blood pressure during exercise. While endothelin (ET)-1 plays a role in regulating blood flow and pressure during exercise in health, little is known about the role of ET-1 in the cardiovascular response to exercise in hypertension. Therefore, eight volunteers diagnosed with hypertension were studied during exercise with either saline or BQ-123 (ETA receptor antagonist) infusion following a 2-week withdrawal of anti-hypertensive medications. The common femoral artery and vein were catheterized for drug infusion, blood collection and blood pressure measurements, and leg blood flow was measured by Doppler ultrasound. Patients exercised at both absolute (0, 5, 10, 15 W) and relative (40, 60, 80% peak power) intensities. BQ-123 increased blood flow at rest (79 ± 87 ml/min; P = 0.03) and augmented the exercise-induced hyperaemia at most intensities (80% saline: Δ3818±1222 vs. BQ-123: Δ4812±1469 ml/min; P = 0.001). BQ-123 reduced leg MAP at rest (-8 ± 4 mmHg; P < 0.001) and lower intensities (0-10 W; P < 0.05). Systemic diastolic blood pressure was reduced (0 W, 40%; P < 0.05), but systemic MAP was defended by an increased cardiac output. The exercise pressor response (ΔMAP) did not differ between conditions (80% saline: 25 ± 10, BQ-123: 30 ± 7 mmHg; P = 0.17). Thus, ET-1, acting through the ETA receptors, contributes to the control of blood pressure at rest and lower intensity exercise in these patients. Furthermore, the finding that ET-1 constrains the blood flow response to exercise suggests that ETA receptor antagonism could be a therapeutic approach to improve blood flow during exercise in hypertension.

Skeletal muscle interstitial Po2 kinetics during recovery from contractions.

The oxygen partial pressure in the interstitial space (Po2 is) drives O2 into the myocyte via diffusion, thus supporting oxidative phosphorylation. Although crucial for metabolic recovery and the capacity to perform repetitive tasks, the time course of skeletal muscle Po2 is during recovery from contractions remains unknown. We tested the hypothesis that Po2 is would recover to resting values and display considerable on-off asymmetry (fast on-, slow off-kinetics), reflective of asymmetric capillary hemodynamics. Microvascular Po2 (Po2 mv) was also evaluated to test the hypothesis that a significant transcapillary gradient (ΔPo2 = Po2 mv - Po2 is) would be sustained during recovery. Po2 mv and Po2 is (expressed in mmHg) were determined via phosphorescence quenching in the exposed rat spinotrapezius muscle during and after submaximal twitch contractions (n = 12). Po2 is rose exponentially (P < 0.05) from end-contraction (11.1 ± 5.1), such that the end-recovery value (17.9 ± 7.9) was not different from resting Po2 is (18.5 ± 8.1; P > 0.05). Po2 is off-kinetics were slower than on-kinetics (mean response time: 53.1 ± 38.3 versus 18.5 ± 7.3 s; P < 0.05). A significant transcapillary ΔPo2 observed at end-contraction (16.6 ± 7.4) was maintained throughout recovery (end-recovery: 18.8 ± 9.6; P > 0.05). Consistent with our hypotheses, muscle Po2 is recovered to resting values with slower off-kinetics compared with the on-transient in line with the on-off asymmetry for capillary hemodynamics. Maintenance of a substantial transcapillary ΔPo2 during recovery supports that the microvascular-interstitium interface provides considerable resistance to O2 transport. As dictated by Fick's law (V̇o2 = Do2 × ΔPo2), modulation of O2 flux (V̇o2) during recovery must be achieved via corresponding changes in effective diffusing capacity (Do2; mainly capillary red blood cell hemodynamics and distribution) in the face of unaltered ΔPo2.NEW & NOTEWORTHY Capillary blood-myocyte O2 flux (V̇o2) is determined by effective diffusing capacity (Do2; mainly erythrocyte hemodynamics and distribution) and microvascular-interstitial Po2 gradients (ΔPo2 = Po2 mv - Po2 is). We show that Po2 is demonstrates on-off asymmetry consistent with Po2 mv and erythrocyte kinetics during metabolic transitions. A substantial transcapillary ΔPo2 was preserved during recovery from contractions, indicative of considerable resistance to O2 diffusion at the microvascular-interstitium interface. This reveals that effective Do2 declines in step with V̇o2 during recovery, as per Fick's law.