Skeletal muscle capillary hemodynamics from rest to contractions: implications for oxygen transfer

The role of the heart in the evolution of aerobic performance

Aerobic metabolism underlies vital traits such as locomotion and thermogenesis, and aerobic capacity influences fitness in many animals. The heart is a key determinant of aerobic capacity, but the relative influence of cardiac output versus other steps in the O2 transport pathway remains contentious. In this Commentary, we consider this issue by examining the mechanistic basis for adaptive increases in aerobic capacity (thermogenic V̇O2,max; also called summit metabolism) in deer mice (Peromyscus maniculatus) native to high altitude. Thermogenic V̇O2,max is increased by acclimation to cold hypoxia (simulating high-altitude conditions), and high-altitude populations generally have greater V̇O2,max than their low-altitude counterparts. This plastic and evolved variation in V̇O2,max is associated with corresponding variation in maximal cardiac output, along with variation in other traits across the O2 pathway (e.g. arterial O2 saturation, blood haemoglobin content and O2 affinity, tissue O2 extraction, tissue oxidative capacity). By applying fundamental principles of gas exchange, we show that the relative influence of cardiac output on V̇O2,max depends on the O2 diffusing capacity of thermogenic tissues (skeletal muscles and brown adipose tissues). Functional interactions between cardiac output and blood haemoglobin content determine circulatory O2 delivery and thus affect V̇O2,max, particularly in high-altitude environments where erythropoiesis can increase haematocrit and blood viscosity. There may also be functional linkages between cardiac output and tissue O2 diffusion due to the role of blood flow in determining capillary haematocrit and red blood cell flux. Therefore, the functional interactions between cardiac output and other traits in the O2 pathway underlie the adaptive evolution of aerobic capacities.

Understanding exercise (in)tolerance in sickle cell disease: impacts of hemolysis and exercise training on skeletal muscle oxygen delivery

Sickle cell disease (SCD) is characterized by central (cardiac) and peripheral vascular dysfunctions, significantly diminishing exercise capacity and quality of life. Although central cardiopulmonary abnormalities in SCD are known to reduce exercise capacity and quality of life; the impact of hemolysis and subsequent cell-free hemoglobin (Hb)-mediated peripheral vascular abnormalities on those outcomes are not fully understood. Despite the recognized benefits of exercise training for cardiovascular health and clinical management in chronic diseases like heart failure, there remains substantial debate on the advisability of regular physical activity for patients with SCD. This is primarily due to concerns that prolonged and/or high-intensity exercise might trigger metabolic shifts leading to vaso-occlusive crises. As a result, exercise recommendations for patients with SCD are often vague or nonexistent, reflecting a gap in knowledge about the mechanisms of exercise intolerance and the impact of exercise training on SCD-related health issues. This mini-review sheds light on recent developments in understanding how SCD affects exercise tolerance, with a special focus on the roles of hemolysis and the release of cell-free hemoglobin in altering cardiovascular and skeletal muscle function. Also highlighted here is the emerging research on the therapeutic effects and safety of exercise training in patients with SCD. In addition, the review identifies future research opportunities to fill existing gaps in our understanding of exercise (in)tolerance in SCD.

Differential changes in blood flow and oxygen utilization in active muscles between voluntary exercise and electrical muscle stimulation in young adults

The physiological effects on blood flow and oxygen utilization in active muscles during and after involuntary contraction triggered by electrical muscle stimulation (EMS) remain unclear, particularly compared with those elicited by voluntary (VOL) contractions. Therefore, we used diffuse correlation and near-infrared spectroscopy (DCS-NIRS) to compare changes in local muscle blood flow and oxygen consumption during and after these two types of muscle contractions in humans. Overall, 24 healthy young adults participated in the study, and data were successfully obtained from 17 of them. Intermittent (2-s contraction, 2-s relaxation) isometric ankle dorsiflexion with a target tension of 20% of maximal VOL contraction was performed by EMS or VOL for 2 min, followed by a 6-min recovery period. DCS-NIRS probes were placed on the tibialis anterior muscle, and relative changes in local tissue blood flow index (rBFI), oxygen extraction fraction (rOEF), and metabolic rate of oxygen (rMRO2) were continuously derived. EMS induced more significant increases in rOEF and rMRO2 than VOL exercise but a comparable increase in rBFI. After EMS, rBFI and rMRO2 decreased more slowly than after VOL and remained significantly higher until the end of the recovery period. We concluded that EMS augments oxygen consumption in contracting muscles by enhancing oxygen extraction while increasing oxygen delivery at a rate similar to the VOL exercise. Under the conditions examined in this study, EMS demonstrated a more pronounced and/or prolonged enhancement in local muscle perfusion and aerobic metabolism compared with VOL exercise in healthy participants.NEW & NOTEWORTHY This is the first study to visualize continuous changes in blood flow and oxygen utilization within contracted muscles during and after electrical muscle stimulation (EMS) using combined diffuse correlation and near-infrared spectroscopy. We found that initiating EMS increases blood flow at a rate comparable to that during voluntary (VOL) exercise but enhances oxygen extraction, resulting in higher oxygen consumption. Furthermore, EMS increased postexercise muscle perfusion and oxygen consumption compared with that after VOL exercise.

Erythrocyte metabolism

Our aim is to present an updated overview of the erythrocyte metabolism highlighting its richness and complexity. We have manually collected and connected the available biochemical pathways and integrated them into a functional metabolic map. The focus of this map is on the main biochemical pathways consisting of glycolysis, the pentose phosphate pathway, redox metabolism, oxygen metabolism, purine/nucleoside metabolism, and membrane transport. Other recently emerging pathways are also curated, like the methionine salvage pathway, the glyoxalase system, carnitine metabolism, and the lands cycle, as well as remnants of the carboxylic acid metabolism. An additional goal of this review is to present the dynamics of erythrocyte metabolism, providing key numbers used to perform basic quantitative analyses. By synthesizing experimental and computational data, we conclude that glycolysis, pentose phosphate pathway, and redox metabolism are the foundations of erythrocyte metabolism. Additionally, the erythrocyte can sense oxygen levels and oxidative stress adjusting its mechanics, metabolism, and function. In conclusion, fine-tuning of erythrocyte metabolism controls one of the most important biological processes, that is, oxygen loading, transport, and delivery.

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

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

Microvascular Adaptations to Muscle Stretch: Findings From Animals and the Elderly

Microcirculation in skeletal muscle is disturbed with advancing aging, causing limited capillary blood flow and exercise incapacity. Muscle stretch has been widely performed in physical therapy, sports medicine, and health promotion. However, the effect of stretch on microvascular reactivity and muscle blood flow remains unknown. This review focuses on stretch-induced microvascular adaptations based on evidence from cultured cells, small animals, and human studies. Vascular endothelium senses and responds to mechanical stimuli including stretch. This endothelial mechanotransduction potentially plays a vital role in the stretch-induced microvascular adaptation alongside hypoxia. Aging impairs microvascular endothelial function, but muscle stretch has the potential to restore it. Muscle stretch may be an alternative to improve vascular function and enhance exercising blood flow, especially for those who have difficulties in participating in exercise due to medical, functional, or psychological reasons.

Capillary Endothelial Insulin Transport: The Rate-limiting Step for Insulin-stimulated Glucose Uptake

The rate-limiting step for skeletal muscle glucose uptake is transport from microcirculation to muscle interstitium. Capillary endothelium poses a barrier that delays the onset of muscle insulin action. Defining physiological barriers that control insulin access to interstitial space is difficult because of technical challenges that confront study of microscopic events in an integrated physiological system. Two physiological variables determine muscle insulin access. These are the number of perfused capillaries and the permeability of capillary walls to insulin. Disease states associated with capillary rarefaction are closely linked to insulin resistance. Insulin permeability through highly resistant capillary walls of muscle poses a significant barrier to insulin access. Insulin may traverse the endothelium through narrow intercellular junctions or vesicular trafficking across the endothelial cell. Insulin is large compared with intercellular junctions, making this an unlikely route. Transport by endothelial vesicular trafficking is likely the primary route of transit. Studies in vivo show movement of insulin is not insulin receptor dependent. This aligns with single-cell transcriptomics that show the insulin receptor is not expressed in muscle capillaries. Work in cultured endothelial cell lines suggest that insulin receptor activation is necessary for endothelial insulin transit. Controversies remain in the understanding of transendothelial insulin transit to muscle. These controversies closely align with experimental approaches. Control of circulating insulin accessibility to skeletal muscle is an area that remains ripe for discovery. Factors that impede insulin access to muscle may contribute to disease and factors that accelerate access may be of therapeutic value for insulin resistance.

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.

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.

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 V ̇ O 2 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 ( Q ̇ O 2 )-to-O₂ utilization ( V ̇ O 2 ) 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 ( P O 2 is ) 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 ( P a O 2 : 64 (9) vs. 82 (9) mmHg) compared to healthy controls (HC) (P < 0.05). P O 2 is was significantly lower in MCT rats during the first 30 s of SNP- contractions. SNP superfusion elevated P O 2 is in both groups; however, MCT rats demonstrated a lower P O 2 is 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 Q ̇ O 2 -to- V ̇ O 2 ratio in contracting muscle to the same levels as HC. These data support muscle Q ̇ O 2 -to- V ̇ O 2 mismatch as a potential contributor to slow V ̇ O 2 kinetics and therefore exercise intolerance in PH and suggest peripheral vascular dysfunction or remodelling as a possible mechanism.

In vivo cooling-induced intracellular Ca2+ elevation and tension in rat skeletal muscle

It is an open question as to whether cooling‐induced muscle contraction occurs in the in vivo environment. In this investigation, we tested the hypotheses that a rise in intracellular Ca²⁺ concentration ([Ca²⁺]i) and concomitant muscle contraction could be evoked in vivo by reducing muscle temperature and that this phenomenon would be facilitated or inhibited by specific pharmacological interventions designed to impact Ca²⁺‐induced Ca²⁺‐release (CICR). Progressive temperature reductions were imposed on the spinotrapezius muscle of Wistar rats under isoflurane anesthesia by means of cold fluid immersion. The magnitude, location, and temporal profile of [Ca²⁺]i were estimated using fura‐2 loading. Caffeine (1.25–5.0 mM) and procaine (1.6–25.6 mM) loading were applied in separatum to evaluate response plasticity by promoting or inhibiting CICR, respectively. Lowering the temperature of the muscle surface to ~5°C produced active tension and discrete sites with elevated [Ca²⁺]i. This [Ca²⁺]i elevation differed in magnitude from fiber to fiber and also from site to site within a fiber. Caffeine at 1.25 and 5.0 mM reduced the magnitude of cooling necessary to elevate [Ca²⁺]i (i.e., from ~5°C to ~8 and ~16°C, respectively, both p < 0.05) and tension. Conversely, 25.6 mM procaine lowered the temperature at which [Ca²⁺]i elevation and tension were detected to ~2°C (p < 0.05). Herein we demonstrate the spatial and temporal relationship between cooling‐induced [Ca²⁺]i elevation and muscle contractile force in vivo and the plasticity of these responses with CICR promotion and inhibition.

Regulation of capillary hemodynamics by KATP channels in resting skeletal muscle

ATP-sensitive K⁺ channels (K_ATP ) have been implicated in the regulation of resting vascular smooth muscle membrane potential and tone. However, whether K_ATP 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 K_ATP 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 (f_RBC ), velocity (V_RBC ), and capillary tube hematocrit (Hct_cap ) were assessed via intravital microscopy of the rat spinotrapezius muscle (n = 6) under control (CON) and glibenclamide (GLI; K_ATP 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 f_RBC (CON:20.1 ± 1.8, GLI:14.6 ± 1.3 cells/s; p < 0.05) and V_RBC (CON:240 ± 17, GLI:182 ± 17 µm/s; p < 0.05) but not Hct_cap (CON:0.26 ± 0.01, GLI:0.26 ± 0.01; p > 0.05). The absence of GLI effects on the %RBC-flowing capillaries and Hct_cap indicates preserved muscle O₂ diffusing capacity (DO₂ m). In contrast, GLI lowered both f_RBC and V_RBC 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 K_ATP channels regulate capillary hemodynamics and, therefore, microvascular gas exchange in resting skeletal muscle.

Type I diabetes suppresses intracellular calcium ion increase normally evoked by heat stress in rat skeletal muscle

Heat stress, via its effects on muscle intracellular Ca2+ concentrations ([Ca2+]i), has been invoked as a putative therapeutic countermeasure to type 1 diabetes-induced muscle atrophy. Using a circulation- and neurally intact in vivo muscle preparation, we tested the hypothesis that impaired muscle Ca2+ homeostasis in type 1 diabetic rats is due to attenuated heat stress tolerance mediated via transient receptor potential vanilloid 1 (TRPV1). Male Wistar rats were randomly assigned to one of the following four groups: 1) healthy control 30°C (CONT 30°C); 2) CONT 40°C; 3) diabetes 30°C (DIA 30°C); and 4) DIA 40°C. The temperature of 40°C was selected because it exceeds the TRPV1 activation threshold. Spinotrapezius muscles of Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca2+ probe Fura-2 AM. [Ca2+]i was estimated over 20 min using fluorescence microscopy (340/380 nm ratio) in quiescent muscle held at the required temperature, using a calibrated heat source applied to the ventral muscle surface. Western blotting was performed to determine the protein expression levels of TRPV1 in spinotrapezius muscle. After 20 min of heat stress, the CONT 40°C condition induced a 12.3 ± 5% [Ca2+]i (P < 0.05) elevation that was markedly absent in the DIA 40°C or other conditions. Thus, no significant differences were found among DIA 40°C, DIA 30°C, and CONT 30°C. TRPV1 protein expression was decreased by 42.0 ± 9% in DIA compared with CONT (P < 0.05) and, unlike CONT, heat stress did not increase TRPV1 phosphorylation. In conclusion, diabetes suppresses TRPV1 protein expression and function and inhibits the elevated myocyte [Ca2+]i evoked normally by heat stress. These results suggest that capsaicin or other therapeutic strategies to increase Ca2+ accumulation via TRPV1 might be more effective than hyperthermic therapy for type 1 diabetic patients.

The capillary fascicle in skeletal muscle: Structural and functional physiology of RBC distribution in capillary networks

KEY POINTS

The capillary module, consisting of parallel capillaries from arteriole to venule, is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres. Using intravital video microscopy of resting extensor digitorum longus muscle in rats, we demonstrated the capillary fascicle as a series of interconnected modules forming continuous columns that align naturally with the dimensions of the muscle fascicle. We observed structural heterogeneity for module topology, and functional heterogeneity in space and time for capillary-red blood cell (RBC) haemodynamics within a module and between modules. We found that module RBC haemodynamics were independent of module resistance, providing direct evidence for microvascular flow regulation at the level of the capillary module. The capillary fascicle is an updated paradigm for characterizing blood flow and RBC distribution in skeletal muscle capillary networks.

ABSTRACT

Capillary networks are the fundamental site of oxygen exchange in the microcirculation. The capillary module (CM), consisting of parallel capillaries from terminal arteriole (TA) to post-capillary venule (PCV), is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres, requiring co-ordination from numerous modules. It has previously been recognized that TAs and PCVs interact with multiple CMs, creating interconnected networks. Using label-free intravital video microscopy of resting extensor digitorum longus muscle in rats, we found that these networks form continuous columns of linked CMs spanning thousands of microns, herein denoted as the capillary fascicle (CF); this structure aligns naturally with the dimensions of the muscle fascicle. We measured capillary-red blood cell (RBC) haemodynamics and module topology (n = 9 networks, 327 modules, 1491 capillary segments). The average module had length 481 μm, width 157 μm and 9.51 parallel capillaries. We observed structural heterogeneity for CM topology, and functional heterogeneity in space and time for capillary-RBC haemodynamics within a module and between modules. There was no correlation between capillary RBC velocity and lineal density. A passive inverse relationship between module length and haemodynamics was remarkably absent, providing direct evidence for microvascular flow regulation at the level of the CM. In summary, the CF is an updated paradigm for characterizing RBC distribution in skeletal muscle, and strengthens the theory of capillary networks as major contributors to the signal that regulates capillary perfusion.

Impact of supine versus upright exercise on muscle deoxygenation heterogeneity during ramp incremental cycling is site specific

PURPOSE

We tested the hypothesis that incremental ramp cycling exercise performed in the supine position (S) would be associated with an increased reliance on muscle deoxygenation (deoxy[heme]) in the deep and superficial vastus lateralis (VLd and VLs, respectively) and the superficial rectus femoris (RFs) when compared to the upright position (U).

METHODS

11 healthy men completed ramp incremental exercise tests in S and U. Pulmonary [Formula: see text]O₂ was measured breath-by-breath; deoxy[heme] was determined via time-resolved near-infrared spectroscopy in the VLd, VLs and RFs.

RESULTS

Supine exercise increased the overall change in deoxy[heme] from baseline to maximal exercise in the VLs (S: 38 ± 23 vs. U: 26 ± 15 μM, P < 0.001) and RFs (S: 36 ± 21 vs. U: 25 ± 15 μM, P < 0.001), but not in the VLd (S: 32 ± 23 vs. U: 29 ± 26 μM, P > 0.05).

CONCLUSIONS

The present study supports that the impaired balance between O₂ delivery and O₂ utilization observed during supine exercise is a regional phenomenon within superficial muscles. Thus, deep muscle defended its O₂ delivery/utilization balance against the supine-induced reductions in perfusion pressure. The differential responses of these muscle regions may be explained by a regional heterogeneity of vascular and metabolic control properties, perhaps related to fiber type composition.

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

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

Application of Molecular Hydrogen as an Antioxidant in Responses to Ventilatory and Ergogenic Adjustments during Incremental Exercise in Humans

We investigated effects of molecular hydrogen (H₂) supplementation on acid-base status, pulmonary gas exchange responses, and local muscle oxygenation during incremental exercise. Eighteen healthy, trained subjects in a randomized, double-blind, crossover design received H₂-rich calcium powder (HCP) (1500 mg/day, containing 2.544 µg/day of H₂) or H₂-depleted placebo (1500 mg/day) for three consecutive days. They performed cycling incremental exercise starting at 20-watt work rate, increasing by 20 watts/2 min until exhaustion. Breath-by-breath pulmonary ventilation (V˙_E) and CO₂ output (V˙CO₂) were measured and muscle deoxygenation (deoxy[Hb + Mb]) was determined via time-resolved near-infrared spectroscopy in the vastus lateralis (VL) and rectus femoris (RF). Blood gases' pH, lactate, and bicarbonate (HCO₃^-) concentrations were measured at rest and 120-, 200-, and 240-watt work rates. At rest, the HCP group had significantly lower V˙_E, V˙CO₂, and higher HCO₃^-, partial pressures of CO₂ (PCO₂) versus placebo. During exercise, a significant pH decrease and greater HCO₃^- continued until 240-watt workload in HCP. The V˙_E was significantly lower in HCP versus placebo, but HCP did not affect the gas exchange status of V˙CO₂ or oxygen uptake (V˙O₂). HCP increased absolute values of deoxy[Hb + Mb] at the RF but not VL. Thus, HCP-induced hypoventilation would lead to lower pH and secondarily impaired balance between O₂ delivery and utilization in the local RF during exercise, suggesting that HCP supplementation, which increases the at-rest antioxidant potential, affects the lower ventilation and pH status during incremental exercise. HPC induced a significantly lower O₂ delivery/utilization ratio in the RF but not the VL, which may be because these regions possess inherently different vascular/metabolic control properties, perhaps related to fiber-type composition.

Rapid vasodilation within contracted skeletal muscle in humans: new insight from concurrent use of diffuse correlation spectroscopy and Doppler ultrasound

Previous studies showed that conduit artery blood flow rapidly increases after even a brief contraction of muscles within the dependent limb. Whether this rapid hyperemia occurs within contracted skeletal muscle in humans has yet to be confirmed, however. We therefore used diffuse correlation spectroscopy (DCS) to characterize the rapid hyperemia and vasodilatory responses within the muscle microvasculature induced by single muscle contractions in humans. Twenty-five healthy male volunteers performed single 1-s isometric handgrips at 20%, 40%, 60%, and 80% of maximum voluntary contraction. DCS probes were placed on the flexor digitorum superficialis muscle, and a skeletal muscle blood flow index (SMBFI) was derived continuously. At the same time, brachial artery blood flow (BABF) responses were measured using Doppler ultrasound. Single muscle contractions evoked rapid, monophasic increases in both SMBFI and BABF that occurred within 3 s after release of contraction. The initial and peak responses increased with increases in contraction intensity and were greater for BABF than for SMBFI at all intensities. BABF reached its peak within 5 to 8 s after the end of contraction. The SMBFI continued to increase after the BABF passed its peak and was decreasing toward the resting level and peaked about 10 to 15 s after completion of the contraction. We conclude that single muscle contractions induce rapid, intensity-dependent hyperemia within the contracted skeletal muscle microvasculature. Moreover, the characteristics of the rapid hyperemia and vasodilatory responses of skeletal muscle microvessels differ from those simultaneously evaluated in the upstream conduit artery.NEW & NOTEWORTHY Through the concurrent use of diffuse correlation spectroscopy and Doppler ultrasound, we provide the first evidence in humans that a single brief muscle contraction evokes rapid, intensity-dependent hyperemia within the contracted skeletal muscle microvasculature and the upstream conduit artery. We also show that the magnitude and time course of the contraction-induced rapid hyperemia and vasodilatory responses within skeletal muscle microvessels significantly differ from those in the conduit artery.

August Krogh: Muscle capillary function and oxygen delivery

The capillary bed constitutes the obligatory pathway for almost all oxygen (O₂) and substrate molecules as they pass from blood to individual cells. As the largest organ, by mass, skeletal muscle contains a prodigious surface area of capillaries that have a critical role in metabolic homeostasis and must support energetic requirements that increase as much as 100-fold from rest to maximal exercise. In 1919 Krogh's 3 papers, published in the Journal of Physiology, brilliantly conflated measurements of muscle capillary function at rest and during contractions with Agner K. Erlang's mathematical model of O₂ diffusion. These papers single-handedly changed the perception of capillaries from passive vessels serving at the mercy of their upstream arterioles into actively contracting vessels that were recruited during exercise to elevate blood-myocyte O₂ flux. Although seminal features of Krogh's model have not withstood the test of time and subsequent technological developments, Krogh is credited with helping found the field of muscle microcirculation and appreciating the role of the capillary bed and muscle O₂ diffusing capacity in facilitating blood-myocyte O₂ flux. Today, thanks in large part to Krogh, it is recognized that comprehending the role of the microcirculation, as it supports perfusive and diffusive O₂ conductances, is fundamental to understanding skeletal muscle plasticity with exercise training and resolving the mechanistic bases by which major pathologies including heart failure and diabetes cripple exercise tolerance and cerebrovascular dysfunction predicates impaired executive function.

The anaerobic threshold: 50+ years of controversy

The anaerobic threshold (AT) remains a widely recognized, and contentious, concept in exercise physiology and medicine. As conceived by Karlman Wasserman, the AT coalesced the increase of blood lactate concentration ([La^- ]), during a progressive exercise test, with an excess pulmonary carbon dioxide output ( V ̇ C O 2 ). Its principal tenets were: limiting oxygen (O₂ ) delivery to exercising muscle→increased glycolysis, La^- and H⁺ production→decreased muscle and blood pH→with increased H⁺ buffered by blood [HCO₃ ^- ]→increased CO₂ release from blood→increased V ̇ C O 2 and pulmonary ventilation. This schema stimulated scientific scrutiny which challenged the fundamental premise that muscle anoxia was requisite for increased muscle and blood [La^- ]. It is now recognized that insufficient O₂ is not the primary basis for lactataemia. Increased production and utilization of La^- represent the response to increased glycolytic flux elicited by increasing work rate, and determine the oxygen uptake ( V ̇ O 2 ) at which La^- accumulates in the arterial blood (the lactate threshold; LT). However, the threshold for a sustained non-oxidative contribution to exercise energetics is the critical power, which occurs at a metabolic rate often far above the LT and separates heavy from very heavy/severe-intensity exercise. Lactate is now appreciated as a crucial energy source, major gluconeogenic precursor and signalling molecule but there is no ipso facto evidence for muscle dysoxia or anoxia. Non-invasive estimation of LT using the gas exchange threshold (non-linear increase of V ̇ C O 2 versus V ̇ O 2 ) remains important in exercise training and in the clinic, but its conceptual basis should now be understood in light of lactate shuttle biology.

Lean Mass Abnormalities in Heart Failure: The Role of Sarcopenia, Sarcopenic Obesity, and Cachexia

The role of body composition in patients with heart failure (HF) has been receiving much attention in the last few years. Particularly, reduced lean mass (LM), the best surrogate for skeletal muscle mass, is independently associated with abnormal cardiorespiratory fitness (CRF) and muscle strength, ultimately leading to reduced quality of life and worse prognosis. While in the past, reduced CRF in patients with HF was thought to result exclusively from cardiac dysfunction leading to reduced cardiac output at peak exercise, current evidence supports the concept that abnormalities in LM may also play a critical role. Abnormalities in the LM body composition compartment are associated with the development of sarcopenia, sarcopenic obesity, and cachexia. Such conditions have been implicated in the pathophysiology and progression of HF. However, identification of such conditions remains challenging, as universal definitions for sarcopenia, sarcopenic obesity, and cachexia are lacking. In this review article, we describe the most common body composition abnormalities related to the LM compartment, including skeletal and respiratory muscle mass abnormalities, and the consequences of such anomalies on CRF and muscle strength in patients with HF. Finally, we discuss the potential nonpharmacologic therapeutic strategies such as exercise training (ie, aerobic exercise and resistance exercise) and dietary interventions (ie, dietary supplementation and dietary patterns) that have been implemented to target body composition, with a focus on HF.

August Krogh's theory of muscle microvascular control and oxygen delivery: a paradigm shift based on new data

August Krogh twice won the prestigious international Steegen Prize, for nitrogen metabolism (1906) and overturning the concept of active transport of gases across the pulmonary epithelium (1910). Despite this, at the beginning of 1920, the consummate experimentalist was relatively unknown worldwide and even among his own University of Copenhagen faculty. But, in early 1919, he had submitted three papers to Dr Langley, then editor of The Journal of Physiology in England. These papers coalesced anatomical observations of skeletal muscle capillary numbers with O₂ diffusion theory to propose a novel active role for capillaries that explained the prodigious increase in blood-muscle O₂ flux from rest to exercise. Despite his own appraisal of the first two papers as "rather dull" to his friend, the eminent Cambridge respiratory physiologist, Joseph Barcroft, Krogh believed that the third one, dealing with O₂ supply and capillary regulation, was"interesting". These papers, which won Krogh an unopposed Nobel Prize for Physiology or Medicine in 1920, form the foundation for this review. They single-handedly transformed the role of capillaries from passive conduit and exchange vessels, functioning at the mercy of their upstream arterioles, into independent contractile units that were predominantly closed at rest and opened actively during muscle contractions in a process he termed 'capillary recruitment'. Herein we examine Krogh's findings and some of the experimental difficulties he faced. In particular, the boundary conditions selected for his model (e.g. heavily anaesthetized animals, negligible intramyocyte O₂ partial pressure, binary open-closed capillary function) have not withstood the test of time. Subsequently, we update the reader with intervening discoveries that underpin our current understanding of muscle microcirculatory control and place a retrospectroscope on Krogh's discoveries. The perspective is presented that the imprimatur of the Nobel Prize, in this instance, may have led scientists to discount compelling evidence. Much as he and Marie Krogh demonstrated that active transport of gases across the blood-gas barrier was unnecessary in the lung, capillaries in skeletal muscle do not open and close spontaneously or actively, nor is this necessary to account for the increase in blood-muscle O₂ flux during exercise. Thus, a contemporary model of capillary function features most muscle capillaries supporting blood flow at rest, and, rather than capillaries actively vasodilating from rest to exercise, increased blood-myocyte O₂ flux occurs predominantly via elevating red blood cell and plasma flux in already flowing capillaries. Krogh is lauded for his brilliance as an experimentalist and for raising scientific questions that led to fertile avenues of investigation, including the study of microvascular function.

The effect of dietary nitrate supplementation on the speed-duration relationship in mice with sickle cell disease

Sickle cell disease (SCD) causes exercise intolerance likely due to impaired skeletal muscle function and low nitric oxide (NO) bioavailability. Dietary nitrate improves hemodynamic and metabolic control during exercise in humans and animals. The purpose of this investigation was to assess the impact of nitrate supplementation on exercise capacity as measured by the running speed to exercise duration relationship [critical speed (CS)]in mice with SCD. We tested the hypothesis that nitrate supplementation via beetroot juice (BR) would attenuate the exercise intolerance observed in mice with SCD. Ten wild-type (WT) and 18 Berkley sickle-cell mice (BERK) received water (WT: n = 10, BERK: n = 10) or nitrate-rich BR (BERK+BR: n = 8, nitrate dose 1 mmol/kg/day) for 5 days. Following the supplementation period, all mice performed 3-5 constant-speed treadmill tests that resulted in exhaustion within 1.5 to 20 min. Time to exhaustion vs. treadmill speed was fit to a hyperbolic model to determine CS. CS was significantly lower in BERK vs. WT and BERK+BR with no significant difference between WT and BERK+BR (WT: 36.6 ± 1.6, BERK: 23.8 ± 1.5, BERK+BR: 31.1 ± 2.1 m/min, P < 0.05). Exercise tolerance, measured via CS, was significantly lower in BERK mice relative to WT. However, BERK mice receiving 5 days of nitrate supplementation exhibited no difference in exercise tolerance when compared with WT. These results support the potential utility of a dietary nitrate intervention to improve functionality in SCD patients.NEW & NOTEWORTHY Sickle cell disease compromises muscle O₂ delivery resulting in exercise intolerance. Dietary nitrate supplementation increases skeletal muscle blood flow during exercise and may improve exercise capacity in a mouse model of sickle cell disease. We investigated the effects of dietary nitrate supplementation on exercise tolerance in a mouse model of sickle cell disease using the treadmill speed-duration relationship (critical speed). Mice with sickle cell disease provided with a dietary nitrate supplement had a critical speed not significantly different from healthy wild-type mice.

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

KEY POINTS

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

ABSTRACT

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

Vascular reactivity of cutaneous circulation to brief compressive stimuli, in the human forearm

PURPOSE

A brief compressive stimulus is known to induce a rapid hyperemia in skeletal muscles, considered to contribute to the initial phase of functional hyperemia. Whether the same mechano-sensitivity characterizes the cutaneous circulation is debated. This study aims to investigate whether a rapid hyperemic response to compressive stimuli is also expressed by skin blood flow in humans.

METHODS

In 12 subjects, brief compressive stimuli were delivered to the forearm at varying pressures/durations (50/2, 100/2, 200/2, 200/1, 200/5 mmHg/s); the sequence was randomized and repeated with the arm above and below heart level. Laser Doppler flowmetry technique was used to monitor skin blood flow. The response was described in terms of peak skin blood flow normalized to baseline (nSBFpeak), time-to-peak from the release of compression, and excess blood volume (EBV, expressed in terms of seconds of basal flow, s-bf) received during the response.

RESULTS

The results consistently evidenced the occurrence of a compression-induced hyperemic response, with nSBFpeak = 2.9 ± 1.1, EBV = 17.0 ± 6.6 s-bf, time-to-peak = 7.0 ± 0.7 s (200 mmHg, 2 s, below heart level). Both nSBFpeak and EBV were significantly reduced (by about 50%) above compared to below heart level (p < 0.01). In addition, EBV slightly increased with increasing pressure (p < 0.05) and duration (p < 0.01) of the stimulus.

CONCLUSIONS

For the first time, the rapid dilatator response to compressive stimuli was demonstrated in human cutaneous circulation. The functional meaning of this response remains to be elucidated.

Limb blood flow and muscle oxygenation responses during handgrip exercise above vs. below critical force

This study compared the brachial artery blood flow (Q̇_BA) and microvascular oxygen delivery responses during handgrip exercise above vs. below critical force (CF; the isometric analog of critical power). Q̇_BA and microvascular oxygen delivery are important determinants of oxygen utilization and metabolite accumulation during exercise, both of which increase progressively during exercise above CF. However the Q̇_BA and microvascular oxygen delivery responses above vs. below CF remain unknown. We hypothesized that Q̇_BA, deoxygenated-heme (deoxy-[heme]; an estimate of microvascular fractional oxygen extraction), and total-heme concentrations (total-[heme]; an estimate of changes in microvascular hematocrit) would demonstrate physiological maximums above CF despite increases in exercise intensity. Seven men and six women performed 1) a 5-min rhythmic isometric-handgrip maximal-effort test (MET) to determine CF and 2) two constant target-force tests above (severe-intensity; S1 and S2) and two constant target-force tests below (heavy-intensity; H1 and H2) CF. CF was 189.3 ± 16.7 N (29.7 ± 1.6%MVC). At end-exercise, Q̇_BA was greater for tests above CF (S1: 418 ± 147 mL/min; S2: 403 ± 137 mL/min) compared to tests below CF (H1: 287 ± 97 mL/min; H2: 340 ± 116 mL/min; all p < 0.05) but was not different between S1 and S2. Further, end-test Q̇_BA during both tests above CF was not different from Q̇_BA estimated at CF (392 ± 37 mL/min). At end-exercise, deoxy-[heme] was not different between tests above CF (S1: 150 ± 50 μM; S2: 155 ± 57 μM), but was greater during tests above CF compared to tests below CF (H1: 101 ± 24 μM; H2: 111 ± 21 μM; all p < 0.05). At end-exercise, total-[heme] was not different between tests above CF (S1: 404 ± 58 μM; S2: 397 ± 73 μM), but was greater during tests above CF compared to H1 (352 ± 58 μM; p < 0.01) but not H2 (371 ± 57 μM). These data suggest limb blood flow limitations exist and maximal levels of muscle microvascular oxygen delivery and extraction occur during exercise above, but not below, CF.

Effect of differential muscle activation patterns on muscle deoxygenation and microvascular haemoglobin regulation

NEW FINDINGS

What is the central question of this study? Does the presence and extent of heterogeneity in the ratio of O2 delivery to uptake across human muscles relate specifically to different muscle activation patterns? What is the main finding and its importance? During ramp incremental knee-extension and cycling exercise, the profiles of muscle deoxygenation (deoxy[haemoglobin + myoglobin]) and diffusive O2 potential (total[haemoglobin + myoglobin]) in the vastus lateralis corresponded to different muscle activation strategies. However, this was not the case for the rectus femoris, where muscle activation and deoxygenation profiles were dissociated and might therefore be determined by other structural and/or functional attributes (e.g. arteriolar vascular regulation and control of red blood cell flux).

ABSTRACT

Near-infrared spectroscopy has revealed considerable heterogeneity in the ratio of O2 delivery to uptake as identified by disparate deoxygenation {deoxy[haemoglobin + myoglobin] (deoxy[Hb + Mb])} values in the exercising quadriceps. However, whether this represents a recruitment phenomenon or contrasting vascular and metabolic control, as seen among fibre types, has not been established. We used knee-extension (KE) and cycling (CE) incremental exercise protocols to examine whether differential muscle activation profiles could account for the heterogeneity of deoxy[Hb + Mb] and microvascular haemoconcentration (i.e. total[Hb + Mb]). Using time-resolved near-infrared spectroscopy for the quadriceps femoris (vastus lateralis and rectus femoris) during exhaustive ramp exercise in eight participants, we tested the following hypotheses: (i) the deoxy[Hb + Mb] (i.e. fractional O2 extraction) would relate to muscle activation levels across exercise protocols; and (ii) KE would induce greater total[Hb + Mb] (i.e. diffusive O2 potential) at task failure (i.e. peak O2 uptake) than CE irrespective of muscle site. At a given level of muscle activation, as assessed by the relative integrated EMG normalized to maximal voluntary contraction (%iEMGmax ), the vastus lateralis deoxy[Hb + Mb] profile was not different between exercise protocols. However, at peak O2 uptake and until 20% iEMGmax for CE, rectus femoris exhibited a lower deoxy[Hb + Mb] (83.2 ± 15.5 versus 98.2 ± 19.4 μm) for KE than for CE (P < 0.05). The total[Hb + Mb] at peak O2 uptake was not different between exercise protocols for either muscle site. These data support the hypothesis that the contrasting patterns of convective and diffusive O2 transport correspond to different muscle activation patterns in vastus lateralis but not rectus femoris. Thus, the differential deoxygenation profiles for rectus femoris across exercise protocols might be dependent upon specific facets of muscle architecture and functional haemodynamic events.

Krogh’s capillary recruitment hypothesis, 100 years on: Is the opening of previously closed capillaries necessary to ensure muscle oxygenation during exercise?

In 1919, August Krogh published his seminal work on skeletal muscle oxygenation. Krogh’s observations indicated that muscle capillary diameter is actively regulated, rather than a passive result of arterial blood flow regulation. Indeed, combining a mathematical model with the number of ink-filled capillaries he observed in muscle cross sections taken at different workloads, Krogh was able to account for muscle tissue’s remarkably efficient oxygen extraction during exercise in terms of passive diffusion from nearby capillaries. Krogh was awarded the 1920 Nobel Prize for his account of muscle oxygenation. Today, his observations are engrained in the notion of capillary recruitment: the opening of previously closed capillaries. While the binary distinction between “closed” and “open” was key to Krogh’s model argument, he did in fact report a continuum of capillary diameters, degrees of erythrocyte deformation, and perfusion states. Indeed, modern observations question the presence of closed muscle capillaries. We therefore examined whether changes in capillary flow patterns and hematocrit among open capillaries can account for oxygen extraction in muscle across orders-of-magnitude changes in blood flow. Our four-compartment model of oxygen extraction in muscle confirms this notion and provides a framework for quantifying the impact of changes in microvascular function on muscle oxygenation in health and disease. Our results underscore the importance of capillary function for oxygen extraction in muscle tissue as first proposed by Krogh. While Krogh’s model calculations still hold, our model predictions support that capillary recruitment can be viewed in the context of continuous, rather than binary, erythrocyte distributions among capillaries.

NEW & NOTEWORTHY Oxygen extraction in working muscle is extremely efficient in view of single capillaries properties. The underlying mechanisms have been widely debated. Here, we develop a four-compartment model to quantify the influence of each of the hypothesized mechanisms on muscle oxygenation. Our results show that changes in capillary flow pattern and hematocrit can account for the high oxygen extraction observed in working muscle, while capillary recruitment is not required to account for these extraction properties.

Perfusion controls muscle glucose uptake by altering the rate of glucose dispersion in vivo

These studies test, using intravital microscopy (IVM), the hypotheses that perfusion effects on insulin-stimulated muscle glucose uptake (MGU) are 1) capillary recruitment independent and 2) mediated through the dispersion of glucose rather than insulin. For experiment 1, capillary perfusion was visualized before and after intravenous insulin. No capillary recruitment was observed. For experiment 2, mice were treated with vasoactive compounds (sodium nitroprusside, hyaluronidase, and lipopolysaccharide), and dispersion of fluorophores approximating insulin size (10-kDa dextran) and glucose (2-NBDG) was measured using IVM. Subsequently, insulin and 2[¹⁴C]deoxyglucose were injected and muscle phospho-2[¹⁴C]deoxyglucose (2[C¹⁴]DG) accumulation was used as an index of MGU. Flow velocity and 2-NBDG dispersion, but not perfused surface area or 10-kDa dextran dispersion, predicted phospho-2[¹⁴C]DG accumulation. For experiment 3, microspheres of the same size and number as are used for contrast-enhanced ultrasound (CEU) studies of capillary recruitment were visualized using IVM. Due to their low concentration, microspheres were present in only a small fraction of blood-perfused capillaries. Microsphere-perfused blood volume correlated to flow velocity. These findings suggest that 1) flow velocity rather than capillary recruitment controls microvascular contributions to MGU, 2) glucose dispersion is more predictive of MGU than dispersion of insulin-sized molecules, and 3) CEU measures regional flow velocity rather than capillary recruitment.

Influence of electroacupuncture stimulation on skin temperature, skin blood flow, muscle blood volume and pupil diameter

OBJECTIVE

To examine the effect of electroacupuncture (EA) stimulation on multiple physiological indices and to evaluate both local and systemic physiological responses induced by the stimulation.

METHODS

15 healthy male college students participated in an experimental crossover study. They received two kinds of interventions: one with EA stimulation and one without EA stimulation on different days. Two disposable acupuncture needles were inserted at two traditional acupuncture points (ST36 and ST38), located along the anterior tibialis muscle. EA stimulation was administered for 10 min. Skin temperature (ST), skin blood flow (SBF) and muscle blood volume (MBV) were recorded near the stimulation sites, while the pupil diameter (PD) was measured before, during and after the interventions.

RESULTS

ST, SBF and MBV increased significantly following EA stimulation. PD of the right and left eyes decreased significantly following EA stimulation. There was a significant difference in ST responses between the groups (P=0.001). For SBF, MBV and PD, no significant differences were demonstrated between the groups.

CONCLUSIONS

Our study showed that 10 min of EA stimulation increased ST, SBF and MBV, and decreased PD, compared to baseline, while no significant change was observed in the control group. This suggests that EA stimulation alters local blood flow and ST, and these responses are likely mediated via segmental spinal reflexes, supraspinal reflexes involving parasympathetic activation, and other mechanisms.

Effect of anemia on muscle oxygen saturation during submaximal exercise

BACKGROUND

Objective measures to assess the need for transfusion in chronic anemia are lacking. Near-infrared spectroscopy may be used, but there is wide variability. Assessment of muscle oxygen saturation (SmO₂ ) during exercise could be used to measure the impact of anemia on performance of everyday activities.

STUDY DESIGN AND METHODS

Hematology patients and controls were recruited to undertake a 6-minute walk test (6MWT) and a 20-second isometric handgrip exercise. Muscle oxygen saturation in the exercising muscles was measured before and during exercise. Changes in saturation during exercise were described. Correlations between identified variables, hemoglobin concentration, and 6MWT distance were undertaken. The effect of transfusion was assessed on a transfused subset.

RESULTS

There were 95 sets of exercises conducted in 74 participants. Baseline SmO₂ correlated with hemoglobin concentration and negatively with 6MWT distance. Paradoxically, a higher hemoglobin was associated with a greater SmO₂ fall during the 6MWT, likely due to greater consumption from improved walk distances. The fall in SmO₂ was independent of hemoglobin during isometric contraction, although levels were lower during contraction due to the lower starting SmO₂ . There was a longer time to peak SmO₂ during recovery following isometric exercise in anemia. There were 17 paired tests following a change in hemoglobin, with SmO₂ not predicting improvement in those who had improved exercise capacity.

CONCLUSION

While baseline SmO₂ correlated with hemoglobin concentration, the correlation was not strong enough to predict transfusion requirements. Recovery after isometric forearm contraction correlated with hemoglobin and warrants further investigation.

Hyperinsulinemia does not cause de novo capillary recruitment in rat skeletal muscle

Objective

The effect of insulin on blood flow distribution within muscle microvasculature has been suggested to be important for glucose metabolism. However, the “capillary recruitment” hypothesis is still controversial and relies on studies using indirect contrast‐enhanced ultrasound (CEU) methods.

Methods

We studied how hyperinsulinemia effects capillary blood flow in rat extensor digitorum longus (EDL) muscle during euglycemic hyperinsulinemic clamp using intravital video microscopy (IVVM). Additionally, we modeled blood flow and microbubble distribution within the vascular tree under conditions observed during euglycemic hyperinsulinemic clamp experiments.

Results

Euglycemic hyperinsulinemia caused an increase in erythrocyte (80 ± 25%, P < .01) and plasma (53 ± 12%, P < .01) flow in rat EDL microvasculature. We found no evidence of de novo capillary recruitment within, or among, capillary networks supplied by different terminal arterioles; however, erythrocyte flow became slightly more homogenous. Our computational model predicts that a decrease in asymmetry at arteriolar bifurcations causes redistribution of microbubble flow among capillaries already perfused with erythrocytes and plasma, resulting in 25% more microbubbles flowing through capillaries.

Conclusions

Our model suggests increase in CEU signal during hyperinsulinemia reflects a redistribution of arteriolar flow and not de novo capillary recruitment. IVVM experiments support this prediction showing increases in erythrocyte and plasma flow and not capillary recruitment.

Skeletal muscle interstitial Po2 kinetics during recovery from contractions

The oxygen partial pressure in the interstitial space (Po_2 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 Po_2 is during recovery from contractions remains unknown. We tested the hypothesis that Po_2 is would recover to resting values and display considerable on-off asymmetry (fast on-, slow off-kinetics), reflective of asymmetric capillary hemodynamics. Microvascular Po₂ (Po_2 mv) was also evaluated to test the hypothesis that a significant transcapillary gradient (ΔPo₂ = Po_2 mv - Po_2 is) would be sustained during recovery. Po_2 mv and Po_2 is (expressed in mmHg) were determined via phosphorescence quenching in the exposed rat spinotrapezius muscle during and after submaximal twitch contractions (n = 12). Po_2 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 Po_2 is (18.5 ± 8.1; P > 0.05). Po_2 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 Po_2 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 = Po_2 mv - Po_2 is). We show that Po_2 is demonstrates on-off asymmetry consistent with Po_2 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.

Supplementary Nitric Oxide Donors and Exercise as Potential Means to Improve Vascular Health in People with Type 1 Diabetes: Yes to NO?

Type 1 diabetes (T1D) is associated with a greater occurrence of cardiovascular pathologies. Vascular dysfunction has been shown at the level of the endothelial layers and failure to maintain a continuous pool of circulating nitric oxide (NO) has been implicated in the progression of poor vascular health. Biochemically, NO can be produced via two distinct yet inter-related pathways that involve an upregulation in the enzymatic activity of nitric oxide synthase (NOS). These pathways can be split into an endogenous oxygen-dependent pathway i.e., the catabolism of the amino acid L-arginine to L-citrulline concurrently yielding NO in the process, and an exogenous oxygen-independent one i.e., the conversion of exogenous inorganic nitrate to nitrite and subsequently NO in a stepwise fashion. Although a body of research has explored the vascular responses to exercise and/or compounds known to stimulate NOS and subsequently NO production, there is little research applying these findings to individuals with T1D, for whom preventative strategies that alleviate or at least temper vascular pathologies are critical foci for long-term risk mitigation. This review addresses the proposed mechanisms responsible for vascular dysfunction, before exploring the potential mechanisms by which exercise, and two supplementary NO donors may provide vascular benefits in T1D.

Rapid changes in the microvascular circulation of skeletal muscle impair insulin delivery during sepsis

Sepsis costs the healthcare system $23 billion annually and has a mortality rate between 10 and 40%. An early indication of sepsis is the onset of hyperglycemia, which is the result of sepsis-induced insulin resistance in skeletal muscle. Previous investigations have focused on events in the myocyte (e.g., insulin signaling and glucose transport and subsequent metabolism) as the causes for this insulin-resistant state. However, the delivery of insulin to the skeletal muscle is also an important determinant of insulin action. Skeletal muscle microvascular blood flow, which delivers the insulin to the muscle, is known to be decreased during sepsis. Here we test whether the reduced capillary blood flow to skeletal muscle belies the sepsis-induced insulin resistance by reducing insulin delivery to the myocyte. We hypothesize that decreased capillary flow and consequent decrease in insulin delivery is an early event that precedes gross cardiovascular alterations seen with sepsis. This hypothesis was examined in mice treated with either lipopolysaccharide (LPS) or polymicrobial sepsis followed by intravital microscopy of the skeletal muscle microcirculation. We calculated insulin delivery to the myocyte using two independent methods and found that LPS and sepsis rapidly reduce insulin delivery to the skeletal muscle by ~50%; this was driven by decreases in capillary flow velocity and the number of perfused capillaries. Furthermore, the changes in skeletal muscle microcirculation occur before changes in both cardiac output and arterial blood pressure. These data suggest that a rapid reduction in skeletal muscle insulin delivery contributes to the induction of insulin resistance during sepsis.

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

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

Unaltered V̇o2 kinetics despite greater muscle oxygenation during heavy-intensity two-legged knee extension versus cycle exercise in humans

Relative perfusion of active muscles is greater during knee extension ergometry (KE) than cycle ergometry (CE). This provides the opportunity to investigate the effects of increased O₂ delivery (Q̇o₂) on deoxygenation heterogeneity among quadriceps muscles and pulmonary oxygen uptake (V̇o₂) kinetics. Using time-resolved near-infrared spectroscopy, we hypothesized that compared with CE the superficial vastus lateralis (VL), superficial rectus femoris, and deep VL in KE would have 1) a smaller amplitude of the exercise-induced increase in deoxy[Hb + Mb] (related to the balance between V̇o₂ and Q̇o₂); 2) a greater amplitude of total[Hb + Mb] (related to the diffusive O₂ conductance); 3) a greater homogeneity of regional muscle deoxy[Hb + Mb]; and 4) no difference in pulmonary V̇o₂ kinetics. Eight participants performed square-wave KE and CE exercise from 20 W to heavy work rates. Deoxy[Hb + Mb] amplitude was less for all muscle regions in KE (P < 0.05: superficial, KE 17-24 vs. CE 19-40; deep, KE 19 vs. CE 26 μM). Furthermore, the amplitude of total[Hb + Mb] was greater for KE than CE at all muscle sites (P < 0.05: superficial, KE, 7-21 vs. CE, 1-16; deep, KE, 11 vs. CE, -3 μM). Although the amplitude and heterogeneity of deoxy[Hb + Mb] were significantly lower in KE than CE during the first minute of exercise, the pulmonary V̇o₂ kinetics was not different for KE and CE. These data show that the microvascular Q̇o₂ to V̇o₂ ratio, and thus tissue oxygenation, was greater in KE than CE. This suggests that pulmonary and muscle V̇o₂ kinetics in young healthy humans are not limited by Q̇o₂ during heavy-intensity cycling.

Exercise intensity and middle cerebral artery dynamics in humans

Despite its necessity for understanding healthy brain aging, the influence of exercise intensity on cerebrovascular kinetics is currently unknown. We, therefore characterized middle cerebral artery blood flow velocity (MCAv) kinetics associated with two exercise intensities: low and moderate. We hypothesized that increasing exercise intensity would increase the MCAv amplitude response (Amp) and that age and estimated fitness (V̇O2max) would be related to Amp. Baseline (BL) values were collected for 90-seconds followed by a 6-minute exercise bout. Heart rate, end-tidal CO2, mean arterial pressure and MCAv were recorded throughout. MCAv kinetics were described by Amp, time delay (TD) and time constant (τ). Sixty-four adults completed the study. Amp was greater during moderate compared to low exercise intensity (p < 0.001) while no difference was observed in either TD (p = 0.65) or τ (p = 0.47). Amp was negatively associated with age (p < 0.01) and positively correlated with estimated V̇O2max (p < 0.01). Although Amp declines with age, maintaining higher V̇O2max may benefit the cerebrovascular response to exercise.

Metabolic regulation of exercise-induced angiogenesis

Skeletal muscle relies on an ingenious network of blood vessels, which ensures optimal oxygen and nutrient supply. An increase in muscle vascularization is an early adaptive event to exercise training, but the cellular and molecular mechanisms underlying exercise-induced blood vessel formation are not completely clear. In this review, we provide a concise overview on how exercise-induced alterations in muscle metabolism can evoke metabolic changes in endothelial cells (ECs) that drive muscle angiogenesis. In skeletal muscle, angiogenesis can occur via sprouting and splitting angiogenesis and is dependent on vascular endothelial growth factor (VEGF) signaling. In the resting muscle, VEGF levels are controlled by the estrogen-related receptor γ (ERRγ). Upon exercise, the transcriptional coactivator peroxisome-proliferator-activated receptor-γ coactivator-1α (PGC1α) orchestrates several adaptations to endurance exercise within muscle fibers and simultaneously promotes transcriptional activation of Vegf expression and increased muscle capillary density. While ECs are highly glycolytic and change their metabolism during sprouting angiogenesis in development and disease, a similar role for EC metabolism in exercise-induced angiogenesis in skeletal muscle remains to be elucidated. Nonetheless, recent studies have illustrated the importance of endothelial hydrogen sulfide and sirtuin 1 (SIRT1) activity for exercise-induced angiogenesis, suggesting that EC metabolic reprogramming may be fundamental in this process. We hypothesize that the exercise-induced angiogenic response can also be modulated by metabolic crosstalk between muscle and the endothelium. Defining the underlying molecular mechanisms responsible for skeletal muscle angiogenesis in response to exercise will yield valuable insight into metabolic regulation as well as the determinants of exercise performance.

Understanding near infrared spectroscopy and its application to skeletal muscle research

Near infrared spectroscopy (NIRS) is a powerful noninvasive tool with which to study the matching of oxygen delivery to oxygen utilization and the number of new publications utilizing this technique has increased exponentially in the last 20 yr. By measuring the state of oxygenation of the primary heme compounds in skeletal muscle (hemoglobin and myoglobin), greater understanding of the underlying control mechanisms that couple perfusive and diffusive oxygen delivery to oxidative metabolism can be gained from the laboratory to the athletic field to the intensive care unit or emergency room. However, the field of NIRS has been complicated by the diversity of instrumentation, the inherent limitations of some of these technologies, the associated diversity of terminology, and a general lack of standardization of protocols. This Cores of Reproducibility in Physiology (CORP) will describe in basic but important detail the most common methodologies of NIRS, their strengths and limitations, and discuss some of the potential confounding factors that can affect the quality and reproducibility of NIRS data. Recommendations are provided to reduce the variability and errors in data collection, analysis, and interpretation. The goal of this CORP is to provide readers with a greater understanding of the methodology, limitations, and best practices so as to improve the reproducibility of NIRS research in skeletal muscle.

Accumulation of intramyocyte TRPV1-mediated calcium during heat stress is inhibited by concomitant muscle contractions

Heat stress promotes intramyocyte calcium concentration ([Ca²⁺]_i) accumulation via transient receptor potential vanilloid 1 (TRPV1) channels. We tested the hypothesis that muscle contractile activity concomitant with heat stress would accelerate the increase in [Ca²⁺]_i via TRPV1, further impairing [Ca²⁺]_i homeostasis. Spinotrapezius muscles of adult Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca²⁺ probe fura 2-AM. Heat stress (muscle surface temperature 40°C) was used as TRPV1 activator. An isometric contraction (100 Hz, 5-10 V, 30 s) was induced electrically concomitant with heat stress. [Ca²⁺]_i was determined for 20 min using in vivo fluorescence microscopy, and the phosphorylation response of TRPV1 was determined by Western blotting. Heat stress induced a significant [Ca²⁺]_i increase of 18.5 ± 8.1% at 20 min and TRPV1 phosphorylation (+231%), which was inhibited by addition of the TRPV1 inhibitor (capsazepine). However, contrary to expectations, the heat stress and isometric contraction condition almost completely inhibited TRPV1 phosphorylation and the consequent [Ca²⁺]_i elevation (<2.8% accumulation during heat stress, P > 0.05). In conclusion, this in vivo physiological model demonstrated that isometric muscle contraction(s) can suppress the phosphorylation response of TRPV1 and maintain [Ca²⁺]_i homeostasis during heat stress. NEW & NOTEWORTHY This investigation is the first document the dynamics of intramyocyte calcium concentration ([Ca²⁺]_i) increase in the myoplasm of skeletal muscle fibers in response to heat stress where the muscle blood flow is preserved. Heat stress at 40°C drives a myoplasmic [Ca²⁺]_i accumulation in concert with transient receptor potential vanilloid 1 (TRPV1) phosphorylation. However, muscle contraction caused TRPV1 channel deactivation by dephosphorylation of TRPV1. TRPV1 inactivation via isometric contraction(s) permits maintenance of [Ca²⁺]_i homeostasis even under high imposed muscle temperature.

"Work-to-Work" exercise slows pulmonary oxygen uptake kinetics, decreases critical power, and increases W' during supine cycling

We have previously demonstrated that the phase II time constant of pulmonary oxygen uptake kinetics ( τ v ˙ o 2 ) is an independent determinant of critical power (CP) when O2 availability is not limiting, that is, during upright cycle exercise in young, healthy individuals. Whether this causative relationship remains when O2 availability is impaired remains unknown. During supine exercise, which causes an O2 availability limitation during the exercise transition, we therefore determined the impact of a raised baseline work rate on τ v ˙ o 2 and CP. CP, τ v ˙ o 2 , and muscle oxygenation status (the latter via near-infrared spectroscopy) were determined via four severe-intensity constant-power exercise tests completed in two conditions: (1) with exercise initiated from an unloaded cycling baseline (U→S), and (2) with exercise initiated from a moderate-intensity baseline work rate of 90% of the gas exchange threshold (M→S). In M→S, critical power was lower (U→S = 146 ± 39 W vs. M→S = 132 ± 33 W, P = 0.023) and τ v ˙ o 2 was greater (U→S = 45 ± 16 sec, vs. M→S = 69 ± 129 sec, P = 0.001) when compared to U→S. There was no difference in tissue oxyhemoglobin concentration ([HbO2  + MbO2 ]) at baseline or during exercise. The concomitant increase in τ v ˙ o 2 and reduction in CP during M→S compared to U→S shows for the first time that τ v ˙ o 2 is an independent determinant of CP in conditions where O2 availability is limiting.

On the Physics Underlying Longitudinal Capillary Recruitment

Numerous researchers have found that capillary vessel haematocrit depends on the vasodilatory state of the arterioles. At rest, vessel haematocrit is down to 15 %, suggesting a red blood cell velocity three times higher than the plasma velocity. This finding is analysed in the context of present understanding of propulsion of red blood cells (RBCs) and plasma by means of the arteriovenous pressure gradient. Interfacial forces between the red blood cells and the plasma are proposed as a rational explanation of the observed red blood cell velocities. While the arteriovenous pressure gradient across the capillaries propels the red blood cell and the plasma jointly, interfacial forces along the red blood cell membrane can propel RBCs at the cost of the plasma. Different options are explored for the physical origin of these interfacial forces and oxygen gradients are found to be the most probable source.

Skeletal muscle interstitial O2 pressures: bridging the gap between the capillary and myocyte

The oxygen transport pathway from air to mitochondria involves a series of transfer steps within closely integrated systems (pulmonary, cardiovascular, and tissue metabolic). Small and finite O₂ stores in most mammalian species require exquisitely controlled changes in O₂ flux rates to support elevated ATP turnover. This is especially true for the contracting skeletal muscle where O₂ requirements may increase two orders of magnitude above rest. This brief review focuses on the mechanistic bases for increased microvascular blood-myocyte O₂ flux (V̇O₂ ) from rest to contractions. Fick's law dictates that V̇O₂ elevations driven by muscle contractions are produced by commensurate changes in driving force (ie, O₂ pressure gradients; ΔPO₂ ) and/or effective diffusing capacity (DO₂ ). While previous evidence indicates that increased DO₂ helps modulate contracting muscle O₂ flux, up until recently the role of the dynamic ΔPO₂ across the capillary wall was unknown. Recent phosphorescence quenching investigations of both microvascular and novel interstitial PO₂ kinetics in health have resolved an important step in the O₂ cascade between the capillary and myocyte. Specifically, the significant transmural ΔPO₂ at rest was sustained (but not increased) during submaximal contractions. This supports the contention that the blood-myocyte interface provides a substantial effective resistance to O₂ diffusion and underscores that modulations in erythrocyte hemodynamics and distribution (DO₂ ) are crucial to preserve the driving force for O₂ flux across the capillary wall (ΔPO₂ ) during contractions. Investigation of the O₂ transport pathway close to muscle mitochondria is key to identifying disease mechanisms and develop therapeutic approaches to ameliorate dysfunction and exercise intolerance.

Microvascular permeability of skeletal muscle after eccentric contraction-induced muscle injury: in vivo imaging using two-photon laser scanning microscopy

Via modulation of endothelial integrity and vascular permeability in response to damage, skeletal muscle microvessels play a crucial permissive role in tissue leukocyte invasion. However, direct visual evidence of altered microvascular permeability of skeletal muscle has not been technically feasible, impairing mechanistic understanding of these responses. Two-photon laser scanning microscopy (TPLSM) allows three-dimensional in vivo imaging of skeletal muscle microcirculation. We hypothesized that the regulation of microvascular permeability in vivo is temporally related to acute inflammatory and regenerative processes following muscle injury. To test our hypothesis, tibialis anterior muscles of anesthetized male Wistar rats were subjected to eccentric contractions (ECCs) via electrical stimulation. The skeletal muscle microcirculation was imaged by an intravenously infused fluorescent dye (rhodamine B isothiocyanate-dextran) to assess microvascular permeability via TPLSM 1, 3, and 7 days after ECC. Immunohistochemistry on serial muscle sections was performed to determine the proportion of VEGF-A-positive muscle fibers in the damaged muscle. Compared with control rats, the volumetrically determined interstitial leakage of fluorescent dye (5.1 ± 1.4, 5.3 ± 1.2 vs. 0.51 ± 0.14 μm3 × 106; P < 0.05, days 1 and 3, respectively, vs. control) and percentage of VEGF-A-positive fibers in the damaged muscle (10 ± 0.4%, 22 ± 1.1% vs. 0%; days 1 and 3, respectively, vs. control) were significantly higher on days 1 and 3 after ECC. The interstitial leakage volume returned to control by day 7. These results suggest that microvascular hyperpermeability assessed by in vivo TPLSM imaging is associated with ECC-induced muscle damage and increased VEGF expression.

NEW & NOTEWORTHY This investigation employed a novel in vivo imaging technique for skeletal muscle microcirculation using two-photon laser scanning microscopy that enabled microvascular permeability to be assessed by four-dimensional image analysis. By combining in vivo imaging and histological analysis, we found the temporal profile of microvascular hyperpermeability to be related to that of eccentric contraction-induced skeletal muscle injury and pronounced novel myocyte VEGF expression.