Inorganic nitrate supplementation may improve diastolic function and the O2 cost of exercise in cancer survivors: a pilot study.

In non-cancer populations, inorganic dietary nitrate (NO3-) supplementation is associated with enhanced cardiorespiratory function but remains untested in patients with a history of cancer. Therefore, this pilot study sought to determine if oral NO3- supplementation, as a supportive care strategy, increases left ventricular (LV) function and exercise performance in survivors of cancer treated with anticancer therapy while simultaneously evaluating the feasibility of the methods and procedures required for future large-scale randomized trials. Two cohorts of patients with a history of cancer treated with anticancer chemotherapy were recruited. Patients in cohort 1 (n = 7) completed a randomized, double-blind, crossover study with 7 days of NO3- or placebo (PL) supplementation, with echocardiography. Similarly, patients in cohort 2 (n = 6) received a single, acute dose of NO3- supplementation or PL. Pulmonary oxygen uptake (VO2), arterial blood pressure, and stroke volume were assessed during exercise. In cohort 1, NO3- improved LV strain rate in early filling (mean difference (MD) [95% CI]: - 0.3 1/s [- 0.6 to 0.06]; P = 0.04) and early mitral septal wall annular velocity (MD [95% CI]: 0.1 m/s [- 0.01 to - 0.001]; P = 0.02) compared to placebo. In cohort 2, NO3- decreased the O2 cost of low-intensity steady-state exercise (MD [95% CI]: - 0.5 ml/kg/min [- 0.9 to - 0.09]; P = 0.01). Resting and steady-state arterial blood pressure and stroke volume were not different between conditions. No differences between conditions for peak VO2 (MD [95% CI]: - 0.7 ml/kg/min [- 3.0 to 1.6]; P = 0.23) were observed. The findings from this pilot study warrant further investigation in larger clinical trials targeting the use of long-term inorganic dietary NO3- supplementation as a possible integrative supportive care strategy in patients following anticancer therapy.

Effects of pulmonary hypertension on microcirculatory hemodynamics in rat skeletal muscle.

Pulmonary hypertension (PH) has previously been characterized as a disease of the pulmonary vasculature that subsequently results in myocardial dysfunction. Heart failure compromises skeletal muscle microvascular function, which contributes to exercise intolerance. Therefore, we tested the hypothesis that such changes might be present in PH. Thus, we investigated skeletal muscle oxygen (O2) transport in the rat model of PH to determine if O2 delivery (Q̇O2) is impaired at the level of the microcirculation as evidenced via reduced red blood cell (RBC) flux, velocity, hematocrit, and percentage of capillaries flowing in quiescent muscle. Adult male Sprague-Dawley rats were randomized into healthy (n = 9) and PH groups (n = 9). Progressive PH was induced via a one-time intraperitoneal injection of monocrotaline (MCT; 50 mg/kg) and rats were monitored weekly via echocardiography. Intravital microscopy in the spinotrapezius muscle was performed when echocardiograms confirmed moderate PH (preceding right ventricular (RV) failure). At 25 ± 9 days post-MCT, PH rats displayed RV hypertrophy (RV/(Left ventricle + Septum): 0.28 ± 0.05 vs. 0.44 ± 0.11), pulmonary congestion, and increased right ventricular systolic pressure (21 ± 8 vs. 55 ± 14 mm Hg) compared to healthy rats (all P < 0.05). Reduced capillary RBC velocity (403 ± 140 vs. 227 ± 84 µm/s; P = 0.01), RBC flux (33 ± 12 vs. 23 ± 5 RBCs/s; P = 0.04) and % of capillaries supporting continuous RBC flux at rest (79 ± 8 vs. 56 ± 13%; P = 0.01) were evident in PH rats compared to healthy rats. When Q̇O2 within a given field of view was quantified (RBC flux x % of capillaries supporting continuous RBC flux), PH rats demonstrated lower overall Q̇O2 (↓ 50%; P = 0.002). These data support that microcirculatory hemodynamic impairments (↓ Q̇O2 and therefore altered Q̇O2-to-V̇O2 matching) may compromise blood-myocyte O2 transport in PH. The mechanistic bases for decreased capillary RBC flux, velocity, and percentage of capillaries supporting RBC flow remains an important topic.

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.

Oxygen flux from capillary to mitochondria: integration of contemporary discoveries.

Resting humans transport ~ 100 quintillion (1018) oxygen (O2) molecules every second to tissues for consumption. The final, short distance (< 50 µm) from capillary to the most distant mitochondria, in skeletal muscle where exercising O2 demands may increase 100-fold, challenges our understanding of O2 transport. To power cellular energetics O2 reaches its muscle mitochondrial target by dissociating from hemoglobin, crossing the red cell membrane, plasma, endothelial surface layer, endothelial cell, interstitial space, myocyte sarcolemma and a variable expanse of cytoplasm before traversing the mitochondrial outer/inner membranes and reacting with reduced cytochrome c and protons. This past century our understanding of O2's passage across the body's final O2 frontier has been completely revised. This review considers the latest structural and functional data, challenging the following entrenched notions: (1) That O2 moves freely across blood cell membranes. (2) The Krogh-Erlang model whereby O2 pressure decreases systematically from capillary to mitochondria. (3) Whether intramyocyte diffusion distances matter. (4) That mitochondria are separate organelles rather than coordinated and highly plastic syncytia. (5) The roles of free versus myoglobin-facilitated O2 diffusion. (6) That myocytes develop anoxic loci. These questions, and the intriguing notions that (1) cellular membranes, including interconnected mitochondrial membranes, act as low resistance conduits for O2, lipids and H+-electrochemical transport and (2) that myoglobin oxy/deoxygenation state controls mitochondrial oxidative function via nitric oxide, challenge established tenets of muscle metabolic control. These elements redefine muscle O2 transport models essential for the development of effective therapeutic countermeasures to pathological decrements in O2 supply and physical performance.

The effects of pulmonary hypertension on skeletal muscle oxygen pressures in contracting rat spinotrapezius muscle.

NEW FINDINGS: What is the central question of this study? Does impairment in the dynamics of O2 transport in skeletal muscle during a series of contractions constitute a potential mechanism underlying reduced exercise capacity in pulmonary hypertension? What is the main finding and its importance? Pulmonary hypertension compromises the dynamic matching of skeletal muscle O2 delivery-to-utilization following contraction onset in the rat spinotrapezius muscle. These results implicate a role for vascular dysfunction in the slow V̇O2 kinetics and exercise intolerance present in pulmonary hypertension. ABSTRACT: Pulmonary hypertension (PH) is characterized by pulmonary vascular dysfunction and exercise intolerance due, in part, to compromised pulmonary and cardiac function. We tested the hypothesis that there are peripheral (i.e., skeletal muscle) aberrations in O2 delivery ( Q̇O2 )-to-O2 utilization ( V̇O2 ) matching and vascular control that might help to explain poor exercise tolerance in PH. Furthermore, we investigated the peripheral effects of nitric oxide (NO) in attenuating these decrements. Male Sprague-Dawley rats (n = 21) were administered monocrotaline (MCT; 50 mg/kg, i.p.) to induce PH. Disease progression was monitored via echocardiography. Phosphorescence quenching determined the O2 partial pressure in the interstitial space ( PO2is ) in the spinotrapezius muscle at rest and during contractions under control (SNP-) and NO-donor (sodium nitroprusside, SNP+) conditions. MCT rats displayed right ventricular (RV) hypertrophy (right ventricle/(left ventricle + septum): 0.44 (0.13) vs. 0.28 (0.05)), pulmonary congestion, increased RV systolic pressure (48 (18) vs. 20 (8) mmHg) and arterial hypoxaemia ( PaO2 : 64 (9) vs. 82 (9) mmHg) compared to healthy controls (HC) (P < 0.05). PO2is was significantly lower in MCT rats during the first 30 s of SNP- contractions. SNP superfusion elevated PO2is in both groups; however, MCT rats demonstrated a lower PO2is throughout SNP+ contractions versus HC (P < 0.05). Thus, for small muscle mass exercise in MCT rats, muscle oxygenation is impaired across the rest-to-contractions transition and exogenous NO does not raise the Q̇O2 -to- V̇O2 ratio in contracting muscle to the same levels as HC. These data support muscle Q̇O2 -to- V̇O2 mismatch as a potential contributor to slow V̇O2 kinetics and therefore exercise intolerance in PH and suggest peripheral vascular dysfunction or remodelling as a possible mechanism.

Does wearing a facemask decrease arterial blood oxygenation and impair exercise tolerance?

INTRODUCTION: Concerns have been raised that COVID-19 face coverings compromise lung function and pulmonary gas exchange to the extent that they produce arterial hypoxemia and hypercapnia during high intensity exercise resulting in exercise intolerance in recreational exercisers. This study therefore aimed to investigate the effects of a surgical, flannel or vertical-fold N95 masks on cardiorespiratory responses to incremental exercise. METHODS: This investigation studied 11 adult males and females at rest and while performing progressive cycle exercise to exhaustion. We tested the hypotheses that wearing a surgical (S), flannel (F) or horizontal-fold N95 mask compared to no mask (control) would not promote arterial deoxygenation or exercise intolerance nor alter primary cardiovascular variables during submaximal or maximal exercise. RESULTS: Despite the masks significantly increasing end-expired peri-oral %CO2 and reducing %O2, each ∼0.8-2% during exercise (P < 0.05), our results supported the hypotheses. Specifically, none of these masks reduced sub-maximal or maximal exercise arterial O2 saturation (P = 0.744), but ratings of dyspnea were significantly increased (P = 0.007). Moreover, maximal exercise capacity was not compromised nor were there any significant alterations of primary cardiovascular responses (mean arterial pressure, stroke volume, cardiac output) found during sub-maximal exercise. CONCLUSION: Whereas these results are for young healthy recreational male and female exercisers and cannot be applied directly to elite athletes, older or patient populations, they do support that arterial hypoxemia and exercise intolerance are not the obligatory consequences of COVID-19-indicated mask-wearing at least for cycling exercise.

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.

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.

Angiotensin converting enzyme inhibition improves cerebrovascular control during exercise in male rats with heart failure.

We investigated the effects of chronic (∼7 weeks) treatment with the angiotensin converting enzyme (ACE) inhibitor Captopril in rats with heart failure with reduced ejection fraction (HF-rEF) on brain blood flow (BF; radiolabeled microspheres) at rest and during submaximal exercise. We hypothesized that middle cerebral, posterior cerebral, and cerebellar BF during submaximal exercise (20 m/min, 5% incline) would be reduced in rats with HF-rEF (n = 10) compared to healthy (SHAM, n = 10) controls and HF-rEF rats chronically treated with Captopril (HF-rEF + Cap., n = 20). During submaximal exercise middle cerebral (HF-rEF + Cap.: 274 ± 12; HF-rEF: 234 ± 23; SHAM: 248 ± 24 ml/min/100 g) and cerebellar (HF-rEF + Cap.: 222 ± 14; HF-rEF: 243 ± 22; SHAM: 214 ± 23 ml/min/100 g) BF increased from rest in all groups with no difference among groups (P > 0.24). Posterior cerebral BF increased from rest in all groups but was lower than SHAM (394 ± 46 ml/min/100 g; P = 0.03) in HF-rEF (298 ± 19 ml/min/100 g) but not HF-rEF + Cap. (356 ± 18 ml/min/100 g; P = 0.14), supporting the concept that ACE inhibition in HF-rEF elevates brain BF increases, at least to the posterior cerebral region, during moderate intensity exercise/physical activity.

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.

Effects of elevated positive end-expiratory pressure on diaphragmatic blood flow and vascular resistance during mechanical ventilation.

Although mechanical ventilation (MV) is a life-saving intervention, prolonged MV can lead to deleterious effects on diaphragm function, including vascular incompetence and weaning failure. During MV, positive end-expiratory pressure (PEEP) is used to maintain small airway patency and mitigate alveolar damage. We tested the hypothesis that increased intrathoracic pressure with high levels of PEEP would increase diaphragm vascular resistance and decrease perfusion. Female Sprague-Dawley rats (~6 mo) were randomly divided into two groups receiving low PEEP (1 cmH2O; n = 10) or high PEEP (9 cmH2O; n = 9) during MV. Blood flow, via fluorescent microspheres, was determined during spontaneous breathing (SB), low-PEEP MV, high-PEEP MV, low-PEEP MV + surgical laparotomy (LAP), and high-PEEP MV + pneumothorax (PTX). Compared with SB, both low-PEEP MV and high-PEEP MV increased total diaphragm and medial costal vascular resistance (P ≤ 0.05) and reduced total and medial costal diaphragm blood flow (P ≤ 0.05). Also, during MV medial costal diaphragm vascular resistance was greater and blood flow lower with high-PEEP MV vs. low-PEEP MV (P ≤ 0.05). Diaphragm perfusion with high-PEEP MV+PTX and low-PEEP MV were not different (P > 0.05). The reduced total and medial costal diaphragmatic blood flow with low-PEEP MV appears to be independent of intrathoracic pressure changes and is attributed to increased vascular resistance and diaphragm quiescence. Mechanical compression of the diaphragm vasculature may play a role in the lower diaphragmatic blood flow at higher levels of PEEP. These reductions in blood flow to the quiescent diaphragm during MV could predispose critically ill patients to weaning complications.NEW & NOTEWORTHY This is the first study, to our knowledge, demonstrating that mechanical ventilation, with low and high positive-end expiratory pressure (PEEP), increases vascular resistance and reduces total and regional diaphragm perfusion. The rapid reduction in diaphragm perfusion and increased vascular resistance may initiate a cascade of events that predispose the diaphragm to vascular and thus contractile dysfunction with prolonged mechanical ventilation.

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.

Impact of high sodium intake on blood pressure and functional sympatholysis during rhythmic handgrip exercise.

High dietary sodium intake is a risk factor for arterial hypertension; given that the ability to overcome sympathetically mediated vasoconstriction (functional sympatholysis) is attenuated in individuals with hypertension, we investigated the cardiovascular responses to high salt (HS) intake in healthy humans. We hypothesized that a HS intake of 15 g/day for 7 days would attenuate functional sympatholysis and augment the blood pressure response to handgrip exercise (HGE). Thirteen participants (6 males, 7 females) underwent 2 individual days of testing. Beat-by-beat blood pressure and heart rate were recorded throughout the trial on the non-exercising limb. Forearm blood flow was derived from ultrasonography on the brachial artery of the exercising limb. Participants then underwent a flow-mediated dilation (FMD) test. Next, a submaximal HGE was performed for 7 min with lower body negative pressure initiated during minutes 5-7. A single spot urine sample revealed a significant increase in sodium excretion during the HS conditions (p < 0.01). FMD was reduced during the HS condition. Mean arterial pressure was significantly higher during HS intake. No alteration to functional sympatholysis was found between conditions (p > 0.05). In summary, HS intake increases blood pressure without impacting functional sympatholysis or blood pressure responsiveness during HGE. These findings indicate that brachial artery dysfunction precedes an inefficient functional sympatholysis. Novelty Functional sympatholysis was not impacted by 1 week of high sodium intake. High sodium intake augmented the rate pressure product during handgrip exercise in healthy humans.

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.

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, Po2 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 Po2 is at rest and during contractions (both P < 0.01) but had no effect on white gastrocnemius Po2 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.

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 (Po2is; 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 Po2is 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 Po2is was measured via phosphorescence quenching during electrically induced twitch contractions (180 s; 1 Hz). HF reduced resting Po2is 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 Po2is 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 Po2is to values not different from healthy rats and removed the sex difference. Female HF + Nitrate rats expressed greater resting Po2is 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 (Po2is; indicative of O2 delivery-to-utilization matching) determines, in part, O2 flux into skeletal muscle. We show that heart failure (HF) reduces Po2is 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 Po2is and alters the dynamic response with greater efficacy in female HF rats, particularly at rest and following the onset of muscle contractions.