Peripheral vasodilatation determines cardiac output in exercising humans: insight from atrial pacing

Lean body mass and the cardiorespiratory phenotype: An ethnic-specific relationship in Hans Chinese women and men

BACKGROUND

Lean body mass (LBM) and the functional capacity of cardiovascular (CV) and respiratory systems constitute a female-specific relationship in European-American individuals. Whether this recent finding be extrapolated to the world's largest ethnic group, that is, Hans Chinese (HC, a population characterized by low LBM), is unknown.

METHODS

Healthy HC adults (n = 144, 50% ♀) closely matched by sex, age and physical activity were included. Total and regional (leg, arm and trunk) LBM and body composition were measured via dual-energy X-ray absorptiometry. Cardiac structure, stiffness, central/peripheral haemodynamics and peak O2 consumption (VO2peak) were assessed via transthoracic echocardiography and pulmonary gas analyses at rest and during exercise up to peak effort. Regression analyses determined the sex-specific relationship of LBM with cardiac and aerobic phenotypes.

RESULTS

Total and regional LBM were lower and body fat percentage higher in women compared with men (P < 0.001). In both sexes, total LBM positively associated with left ventricular (LV) mass and peak volumes (r ≥ 0.33, P ≤ 0.005) and negatively with LV end-systolic and central arterial stiffness (r ≥ -0.34, P ≤ 0.004). Total LBM strongly associated with VO2peak (r ≥ 0.60, P < 0.001) and peak cardiac output (r ≥ 0.40, P < 0.001) in women and men. Among regional LBM, leg LBM prominently associated with the arterio-venous O2 difference at peak exercise in both sexes (r ≥ 0.43, P < 0.001). Adjustment by adiposity or CV risk factors did not modify the results.

CONCLUSIONS

LBM independently determines internal cardiac dimensions, ventricular mass, distensibility and the capacity to deliver and consume O2 in HC adults irrespective of sex.

Heat-related changes in the velocity and kinetic energy of flowing blood influence the human heart's output during hyperthermia

Passive whole-body hyperthermia increases limb blood flow and cardiac output (Q ̇ $\dot Q$ ), but the interplay between peripheral and central thermo-haemodynamic mechanisms remains unclear. Here we tested the hypothesis that local hyperthermia-induced alterations in peripheral blood flow and blood kinetic energy modulate flow to the heart andQ ̇ $\dot Q$ . Body temperatures, regional (leg, arm, head) and systemic haemodynamics, and left ventricular (LV) volumes and functions were assessed in eight healthy males during: (1) 3 h control (normothermic condition); (2) 3 h of single-leg heating; (3) 3 h of two-leg heating; and (4) 2.5 h of whole-body heating. Leg, forearm, and extracranial blood flow increased in close association with local rises in temperature while brain perfusion remained unchanged. Increases in blood velocity with small to no changes in the conduit artery diameter underpinned the augmented limb and extracranial perfusion. In all heating conditions,Q ̇ $\dot Q$ increased in association with proportional elevations in systemic vascular conductance, related to enhanced blood flow, blood velocity, vascular conductance and kinetic energy in the limbs and head (all R2 ≥ 0.803; P < 0.001), but not in the brain. LV systolic (end-systolic elastance and twist) and diastolic functional profiles (untwisting rate), pulmonary ventilation and systemic aerobic metabolism were only altered in whole-body heating. These findings substantiate the idea that local hyperthermia-induced selective alterations in peripheral blood flow modulate the magnitude of flow to the heart andQ ̇ $\dot Q$ through changes in blood velocity and kinetic energy. Localised heat-activated events in the peripheral circulation therefore affect the human heart's output. KEY POINTS: Local and whole-body hyperthermia increases limb and systemic perfusion, but the underlying peripheral and central heat-sensitive mechanisms are not fully established. Here we investigated the regional (leg, arm and head) and systemic haemodynamics (cardiac output:Q ̇ $\dot Q$ ) during passive single-leg, two-leg and whole-body hyperthermia to determine the contribution of peripheral and central thermosensitive factors in the control of human circulation. Single-leg, two-leg, and whole-body hyperthermia induced graded increases in leg blood flow andQ ̇ $\dot Q$ . Brain blood flow, however, remained unchanged in all conditions. Ventilation, extracranial blood flow and cardiac systolic and diastolic functions only increased during whole-body hyperthermia. The augmentedQ ̇ $\dot Q$ with hyperthermia was tightly related to increased limb and head blood velocity, flow and kinetic energy. The findings indicate that local thermosensitive mechanisms modulate regional blood velocity, flow and kinetic energy, thereby controlling the magnitude of flow to the heart and thus the coupling of peripheral and central circulation during hyperthermia.

The cardiovascular changes underlying a low cardiac output with exercise in patients with type 2 diabetes mellitus

The significant morbidity and premature mortality of type 2 diabetes mellitus (T2DM) is largely associated with its cardiovascular consequences. Focus has long been on the arterial atheromatosis of DM giving rise to early stroke and myocardial infarctions, whereas less attention has been given to its non-ischemic cardiovascular consequences. Irrespective of ischemic changes, T2DM is associated with heart failure (HF) most commonly with preserved ejection fraction (HFpEF). Largely due to increasing population ages, hypertension, obesity and T2DM, HFpEF is becoming the most prevalent form of heart failure. Unfortunately, randomized controlled trials of HFpEF have largely been futile, and it now seems logical to address the important different phenotypes of HFpEF to understand their underlying pathophysiology. In the early phases, HFpEF is associated with a significantly impaired ability to increase cardiac output with exercise. The lowered cardiac output with exercise results from both cardiac and peripheral causes. T2DM is associated with left ventricular (LV) diastolic dysfunction based on LV hypertrophy with myocardial disperse fibrosis and significantly impaired ability for myocardial blood flow increments with exercise. T2DM is also associated with impaired ability for skeletal muscle vasodilation during exercise, and as is the case in the myocardium, such changes may be related to vascular rarefaction. The present review discusses the underlying phenotypical changes of the heart and peripheral vascular system and their importance for an adequate increase in cardiac output. Since many of the described cardiovascular changes with T2DM must be considered difficult to change if fully developed, it is suggested that patients with T2DM are early evaluated with respect to their cardiovascular compromise.

Sympathetic transduction of cardiac sympathetic nerve activity in healthy, conscious sheep

Sympathetic transduction is the study of how impulses of sympathetic nerve activity (SNA) affect end-organ function. Recently, the transduction of resting bursts of muscle SNA (MSNA) has been investigated and shown to have a role in the maintenance of blood pressure through changes in vascular tone in humans. In the present study, we investigate whether directly recorded resting cardiac SNA (CSNA) regulates heart rate (HR), coronary blood flow (CoBF), coronary vascular conductance (CVC), cardiac output (CO) and mean arterial pressure. Instrumentation was undertaken to record CSNA and relevant vascular variables in conscious sheep. Recordings were performed at baseline, as well as after the infusion of a β-adrenoceptor blocker (propranolol) to determine the role of β-adrenergic signalling in sympathetic transduction in the heart. The results show that after every burst of CSNA, there was a significant effect of time on HR (n = 10, ∆: +2.1 ± 1.4 beats min-1 , P = 0.002) and CO (n = 8, ∆: +100 ± 150 mL min-1 , P = 0.002) was elevated, followed by an increase in CoBF (n = 9, ∆: +0.76 mL min-1 , P = 0.001) and CVC (n = 8, ∆: +0.0038 mL min-1  mmHg-1 , P = 0.0028). The changes in HR were graded depending on the size and pattern of CSNA bursts. The HR response was significantly attenuated after the infusion of propranolol. Our study is the first to explore resting sympathetic transduction in the heart, suggesting that CSNA can dynamically change HR mediated by an action on β-adrenoceptors. KEY POINTS: Sympathetic transduction is the study of how impulses of sympathetic nerve activity (SNA) affect end-organ function. Previous studies have examined sympathetic transduction primarily in the skeletal muscle and shown that bursts of muscle SNA alter blood flow to skeletal muscle and mean arterial pressure, although this has not been examined in the heart. We investigated sympathetic transduction in the heart and show that, in the conscious condition, the size of bursts of SNA to the heart can result in incremental increases in heart rate and coronary blood flow mediated by β-adrenoceptors. The pattern of bursts of SNA to the heart also resulted in incremental increases in heart rate mediated by β-adrenoceptors. This is the first study to explore the transduction of bursts of SNA to the heart.

The heart, a secondary organ in the control of blood circulation

Circulation of the blood is a fundamental physiological function traditionally ascribed to the pressure-generating function of the heart. However, over the past century the 'cardiocentric' view has been challenged by August Krogh, Ernst Starling, Arthur Guyton and others, based on haemodynamic data obtained from isolated heart preparations and organ perfusion. Their research brought forth experimental evidence and phenomenological observations supporting the concept that cardiac output occurs primarily in response to the metabolic demands of the tissues. The basic tenets of Guyton's venous return model are presented and juxtaposed with their critiques. Developmental biology of the cardiovascular system shows that the blood circulates before the heart has achieved functional integrity and that its movement is intricately connected with the metabolic demands of the tissues. Long discovered, but as yet overlooked, negative interstitial pressure may play a role in assisting the flow returning to the heart. Based on these phenomena, an alternative circulation model has been proposed in which the heart functions like a hydraulic ram and maintains a dynamic equilibrium between the arterial (centrifugal) and venous (centripetal) forces which define the blood's circular movement. In this focused review we introduce some of the salient arguments in support of the proposed circulation model. Finally, we present evidence that exercising muscle blood flow is subject to local metabolic control which upholds optimal perfusion in the face of a substantive rise in muscle vascular conductance, thus lending further support to the permissive role of the heart in the overall control of blood circulation.

Stroke volume response during prolonged exercise depends on left ventricular filling: evidence from a β-blockade study

Prolonged moderate-intensity exercise leads to a progressive upward drift in heart rate (HR) that may compromise stroke volume (SV). Alternatively, the HR drift may be related to abated SV due to impaired ventricular function. The aim of this study was to examine the effects of cardiovascular drift on left ventricular volumes and in turn SV. Thirteen healthy young males completed two 60-min cycling bouts on a semirecumbent cycle ergometer at 57% maximal oxygen consumption (V̇o2max) either under placebo condition (CON) or after ingesting a small dose of β1-blockers (BB). Measurements of HR, end-diastolic volume (EDV), and end-systolic volume were obtained by echocardiography and used to calculate SV. Other variables such as ear temperature, skin temperature, blood pressure, and blood volume were measured to assess potential changes in thermoregulatory needs and loading conditions. HR drift was successfully prevented when using BB from min 10 to min 60 (128 ± 9 to 126 ± 8 beats/min, P = 0.29) but not in CON (134 ± 10 to 148 ± 10 beats/min, P < 0.01). Conversely, during the same time, SV increased by 13% when using BB (103 ± 9 to 116 ± 7 mL, P < 0.01), whereas it was unchanged in CON (99 ± 7 to 101 ± 9 mL, P = 0.37). The SV behavior was mediated by a 4% increase in EDV in the BB condition (164 ± 18 to 170 ± 18 mL, P < 0.01), whereas no change was observed in the CON condition (162 ± 18 to 160 ± 18 mL, P = 0.23). In conclusion, blocking HR drift enhances EDV and SV during prolonged exercise. These findings suggest that SV behavior is tightly related to filling time and loading conditions of the left ventricle.

Acute regulation of erythropoietin via lower body negative pressure: Influence of sex and age

The regulation of erythropoiesis via hemodynamic stimuli such as reduced central blood volume (CBV) remains uncertain in women and elderly individuals. This study assessed the acute effects of lower body negative pressure (LBNP) on key endocrine biomarkers regulating erythropoiesis, that is, erythropoietin (EPO) and copeptin, in young and older women and men (n = 87). Transthoracic echocardiography and hemodynamics were assessed throughout incremental LBNP levels for 1 hour, or until presyncope, with established methods. Venous blood samples were collected at baseline and immediately after termination of the orthostatic tolerance (OT) test for subsequent hormone analyses. The average age of young women and men (33.1 ± 6.0 vs. 29.5 ± 6.9 yr) and older women and men (63.8 ± 8.0 vs. 65.3 ± 8.9 yr) as well as their physical activity levels were matched within each age and sex group. CBV, as determined by right atrial volume, was reduced in all individuals at the end of the OT test (p < 0.001). The average OT time ranged from 50.1 to 58.1 min in all individuals. LBNP increased circulating EPO in young women (p = 0.023) but not in young men or older individuals. Copeptin was increased in all individuals with LBNP but was exclusively associated with EPO in men (r = 0.39, p = 0.013). The present study indicates that the acute hemodynamic regulation of EPO production is both sex- and age-dependent.

Lean body mass and the cardiovascular system constitute a female-specific relationship

Recent evidence points toward a link between lean body mass (LBM) and cardiovascular capacity in women. This study aimed at determining the sex-specific relationship of LBM with central and peripheral circulatory variables in healthy women and men (n=70) matched by age (60±12 years versus 58±15 years), physical activity, and cardiovascular risk factors. Regional (legs, arms, and trunk) and whole-body (total) body composition were assessed via dual-energy x-ray absorptiometry. Cardiac structure, function, and central/peripheral hemodynamics were measured via transthoracic echocardiography and the volume-clamp method at rest and peak incremental exercise. Regression analyses determined sex-specific relationships between LBM and cardiovascular variables. Regional and total LBM were lower in women than men (P<0.001), with little overlap between sexes. Leg and arm LBM positively associated with left ventricular (LV) internal resting dimensions in women (r≥0.53, P≤0.002) but not men (P≥0.156). Leg, arm, and total LBM only associated with LV relaxation in women (r≥0.43, P≤0.013). All LBM variables strongly associated with LV volumes at peak exercise in women (r≥0.54, P≤0.001) but not men and negatively associated with total peripheral resistance at peak exercise in women (r≥0.43, P≤0.023). Adjustment by adiposity-related or cardiovascular risk factors did not alter results. In conclusion, leg and arm LBM independently associate with internal cardiac dimensions, ventricular relaxation, and systemic vascular resistance in a sex-specific manner, with these relationships exclusively present in women.

Differences in Cardiac Output and Aerobic Capacity Between Sexes Are Explained by Blood Volume and Oxygen Carrying Capacity

Whether average sex differences in cardiorespiratory fitness can be mainly explained by blood inequalities in the healthy circulatory system remains unresolved. This study evaluated the contribution of blood volume (BV) and oxygen (O₂) carrying capacity to the sex gap in cardiac and aerobic capacities in healthy young individuals. Healthy young women and men (n = 28, age range = 20–43 years) were matched by age and physical activity. Echocardiography, blood pressures, and O₂ uptake were measured during incremental exercise. Left ventricular end-diastolic volume (LVEDV), stroke volume (SV), cardiac output (Q), peak O₂ uptake (VO_2peak), and BV were assessed with precise methods. The test was repeated in men after blood withdrawal and reduction of O₂ carrying capacity, reaching women’s levels. Before blood normalization, exercise cardiac volumes and output (LVEDV, SV, Q) adjusted by body size and VO_2peak (42 ± 9 vs. 50 ± 11 ml⋅min^–1⋅kg^–1, P < 0.05) were lower in women relative to men. Blood normalization abolished sex differences in cardiac volumes and output during exercise (P ≥ 0.100). Likewise, VO_2peak was similar between women and men after blood normalization (42 ± 9 vs. 40 ± 8 ml⋅min^–1⋅kg^–1, P = 0.416). In conclusion, sex differences in cardiac output and aerobic capacity are not present in experimental conditions matching BV and O₂ carrying capacity between healthy young women and men.

No evidence for pericardial restraint in the snapping turtle (Chelydra serpentina) following pharmacologically-induced bradycardia at rest or during exercise

Most animals elevate cardiac output during exercise through a rise in heart rate (f_H), whilst stroke volume (V_S) remains relatively unchanged. Cardiac pacing reveals that elevating f_H alone does not alter cardiac output, which is instead largely regulated by the peripheral vasculature. In terms of myocardial oxygen demand, an increase in f_H is more costly than that which would incur if V_S instead were to increase. We hypothesized that f_H must increase because any substantial rise in V_S would be constrained by the pericardium. To investigate this hypothesis, we explored the effects of pharmacologically-induced bradycardia, with ivabradine treatment, on V_S at rest and during exercise in the common snapping turtle (Chelydra serpentina) with intact or opened pericardium. We first showed that, in isolated myocardial preparations, ivabradine exerted a pronounced positive inotropic effect on atrial tissue, but only minor effects on ventricle. Ivabradine reduced f_H in vivo, such that exercise tachycardia was attenuated. Pulmonary and systemic V_S rose in response to ivabradine. The rise in pulmonary V_S largely compensated for the bradycardia at rest, leaving total pulmonary flow unchanged by ivabradine, although ivabradine reduced pulmonary blood flow during swimming (exercise x ivabradine interaction, P<0.05). Although systemic V_S increased, systemic blood flow was reduced by ivabradine both at rest and during exercise, in spite of ivabradine's potential to increase cardiac contractility. Opening the pericardium had no effect on f_H, V_S or blood flows before or after ivabradine, indicating that the pericardium does not constrain V_S in turtles, even during pharmacologically-induced bradycardia.

Effects of Acute Yohimbine Hydrochloride Supplementation on Repeated Supramaximal Sprint Performance

The purpose of this study was to examine the effects of a single acute dose of yohimbine hydrochloride on repeated anaerobic sprint ability. Physically active females (n = 18) completed two separate repeated supramaximal sprint trials each with a different single-dose treatment: placebo (PL; gluten-free corn starch) or yohimbine hydrochloride (YHM; 2.5 mg). For each trial, participants consumed their respective treatment 20 min before exercise. Following a warm-up, participants completed 3 × 15 s Wingate anaerobic tests (WAnTs) separated by 2 min of active recovery. A capillary blood sample was obtained pre- and immediately post-exercise to measure blood concentrations of lactate (LA), epinephrine (EPI), and norepinephrine (NE). Heart rate (HR) and rate of perceived exertion (RPE) were measured following each WAnT. Findings showed that mean power (p < 0.001; η2 = 0.024), total work (p < 0.001; η2 = 0.061), and HR (p < 0.001; η2 = 0.046), were significantly higher with YHM supplementation versus PL. Fatigue index (p < 0.001; η2 = 0.054) and post-exercise LA (p < 0.001; d = 1.26) were significantly lower with YHM compared to PL. YHM resulted in significantly higher EPI concentrations versus PL (p < 0.001; η2 = 0.225) pre- and post-exercise while NE only increased as a function of time (p < 0.001; η2 = 0.227) and was unaffected by treatment. While RPE increased after each WAnT, no differences between treatments were observed (p = 0.539; η2 < 0.001). Together, these results suggest that acute YHM ingestion imparts ergogenic benefits which may be mediated by lower blood LA and fatigue concomitantly occurring with blood EPI increases. Thus, YHM may improve sprint performance although more mechanistic study is warranted to accentuate underlying processes mediating performance enhancement.

Physiological Function during Exercise and Environmental Stress in Humans-An Integrative View of Body Systems and Homeostasis

Claude Bernard’s milieu intérieur (internal environment) and the associated concept of homeostasis are fundamental to the understanding of the physiological responses to exercise and environmental stress. Maintenance of cellular homeostasis is thought to happen during exercise through the precise matching of cellular energetic demand and supply, and the production and clearance of metabolic by-products. The mind-boggling number of molecular and cellular pathways and the host of tissues and organ systems involved in the processes sustaining locomotion, however, necessitate an integrative examination of the body’s physiological systems. This integrative approach can be used to identify whether function and cellular homeostasis are maintained or compromised during exercise. In this review, we discuss the responses of the human brain, the lungs, the heart, and the skeletal muscles to the varying physiological demands of exercise and environmental stress. Multiple alterations in physiological function and differential homeostatic adjustments occur when people undertake strenuous exercise with and without thermal stress. These adjustments can include: hyperthermia; hyperventilation; cardiovascular strain with restrictions in brain, muscle, skin and visceral organs blood flow; greater reliance on muscle glycogen and cellular metabolism; alterations in neural activity; and, in some conditions, compromised muscle metabolism and aerobic capacity. Oxygen supply to the human brain is also blunted during intense exercise, but global cerebral metabolism and central neural drive are preserved or enhanced. In contrast to the strain seen during severe exercise and environmental stress, a steady state is maintained when humans exercise at intensities and in environmental conditions that require a small fraction of the functional capacity. The impact of exercise and environmental stress upon whole-body functions and homeostasis therefore depends on the functional needs and differs across organ systems.

Sex and age interaction in fundamental circulatory volumetric variables at peak working capacity

Background

Whether the fundamental hematological and cardiac variables determining cardiorespiratory fitness and their intrinsic relationships are modulated by major constitutional factors, such as sex and age remains unresolved.

Methods

Transthoracic echocardiography, central hemodynamics and pulmonary oxygen (O₂) uptake were assessed in controlled conditions during submaximal and peak exercise (cycle ergometry) in 85 healthy young (20–44 year) and older (50–77) women and men matched by age-status and moderate-to-vigorous physical activity (MVPA) levels. Main outcomes such as peak left ventricular end-diastolic volume (LVEDV_peak), stroke volume (SV_peak), cardiac output (Q_peak) and O₂ uptake (VO_2peak), as well as blood volume (BV), BV–LVEDV_peak and LVEDV_peak–SV_peak relationships were determined with established methods.

Results

All individuals were non-smokers and non-obese, and MVPA levels were similar between sex and age groups (P ≥ 0.140). BV per kg of body weight did not differ between sexes (P ≥ 0.118), but was reduced with older age in men (P = 0.018). Key cardiac parameters normalized by body size (LVEDV_peak, SV_peak, Q_peak) were decreased in women compared with men irrespective of age (P ≤ 0.046). Older age per se curtailed Q_peak (P ≤ 0.022) due to lower heart rate (P < 0.001). In parallel, VO_2peak was reduced with older age in both sexes (P < 0.001). The analysis of fundamental circulatory relationships revealed that older women require a higher BV for a given LVEDV_peak than older men (P = 0.024).

Conclusions

Sex and age interact on the crucial circulatory relationship between total circulating BV and peak cardiac filling, with older women necessitating more BV to fill the exercising heart than age- and physical activity-matched men.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13293-021-00409-9.

Central Hypovolemia Detection During Environmental Stress-A Role for Artificial Intelligence?

The first step to exercise is preceded by the required assumption of the upright body position, which itself involves physical activity. The gravitational displacement of blood from the chest to the lower parts of the body elicits a fall in central blood volume (CBV), which corresponds to the fraction of thoracic blood volume directly available to the left ventricle. The reduction in CBV and stroke volume (SV) in response to postural stress, post-exercise, or to blood loss results in reduced left ventricular filling, which may manifest as orthostatic intolerance. When termination of exercise removes the leg muscle pump function, CBV is no longer maintained. The resulting imbalance between a reduced cardiac output (CO) and a still enhanced peripheral vascular conductance may provoke post-exercise hypotension (PEH). Instruments that quantify CBV are not readily available and to express which magnitude of the CBV in a healthy subject should remains difficult. In the physiological laboratory, the CBV can be modified by making use of postural stressors, such as lower body "negative" or sub-atmospheric pressure (LBNP) or passive head-up tilt (HUT), while quantifying relevant biomedical parameters of blood flow and oxygenation. Several approaches, such as wearable sensors and advanced machine-learning techniques, have been followed in an attempt to improve methodologies for better prediction of outcomes and to guide treatment in civil patients and on the battlefield. In the recent decade, efforts have been made to develop algorithms and apply artificial intelligence (AI) in the field of hemodynamic monitoring. Advances in quantifying and monitoring CBV during environmental stress from exercise to hemorrhage and understanding the analogy between postural stress and central hypovolemia during anesthesia offer great relevance for healthy subjects and clinical populations.

Blood withdrawal acutely impairs cardiac filling, output and aerobic capacity in proportion to induced hypovolemia in middle-aged and older women

Blood donation entails acute reductions of cardiorespiratory fitness in healthy men. Whether these effects can be extrapolated to blood donor populations comprising women remains uncertain. The purpose of this study was to comprehensively assess the acute impact of blood withdrawal on cardiac function, central hemodynamics and aerobic capacity in women throughout the mature adult lifespan. Transthoracic echocardiography and O2 uptake were assessed at rest and throughout incremental exercise (cycle ergometry) in healthy women (n = 30, age: 47-77 yr). Left ventricular end-diastolic volume (LVEDV), stroke volume (SV), cardiac output (Q̇) and peak O2 uptake (V̇O2peak), and blood volume (BV) were determined with established methods. Measurements were repeated following a 10% reduction of BV within a week period. Individuals were non-smokers, non-obese and moderately fit (V̇O2peak = 31.4 ± 7.3 mL·min-1·kg-1). Hematocrit and BV ranged from 38.0 to 44.8% and from 3.8 to 6.6 L, respectively. The standard 10% reduction in BV resulted in 0.5 ± 0.1 L withdrawal of blood, which did not alter hematocrit (P = 0.953). Blood withdrawal substantially reduced cardiac LVEDV and SV at rest as well as during incremental exercise (≥10% decrements, P ≤ 0.009). Peak Q̇ was proportionally decreased after blood withdrawal (P < 0.001). Blood withdrawal induced a 10% decrement in V̇O2peak (P < 0.001). In conclusion, blood withdrawal impairs cardiac filling, Q̇ and aerobic capacity in proportion to the magnitude of hypovolemia in healthy mature women. Novelty: The filling of the heart and therefore cardiac output are impaired by blood withdrawal in women. Oxygen delivery and aerobic capacity are reduced in proportion to blood withdrawal.

Muscle Oxygenation, Neural, and Cardiovascular Responses to Isometric and Workload-matched Dynamic Resistance Exercise

Differences in blood flow patterns and energy cost between isometric and dynamic resistance exercise may result to variant cardiovascular, neural, and muscle metabolic responses. We aimed to compare the cardiovascular, baroreceptor sensitivity, and muscle oxygenation responses between workload-matched, large muscle-mass isometric and dynamic resistance exercises. Twenty-four young men performed an isometric and a dynamic double leg-press protocol (4 sets×2 min) with similar tension time index (workload). Beat-by-beat hemodynamics, baroreceptor sensitivity, muscle oxygenation, and blood lactate were assessed. The increase in blood pressure was greater (p<0.05) in the 1^st set during dynamic than isometric exercise (by ~4.5 mmHg), not different in the 2^nd and 3^rd sets, and greater in the 4^th set during isometric exercise (by ~5 mmHg). Dynamic resistance exercise evoked a greater increase in heart rate, stroke volume, cardiac output, and contractility index (p<0.05), and a greater decline in peripheral resistance, baroreceptor sensitivity, and cardiac function indices than isometric exercise (p<0.05). Participants exhibited a greater reduction in muscle oxyhemoglobin and a greater increase in muscle deoxyhemoglobin in dynamic versus isometric exercise (p<0.001-0.05), with no differences in total hemoglobin and blood lactate. In conclusion, large muscle-mass, multiple-set isometric exercise elicits a relatively similar blood pressure but blunted cardiovascular and baroreceptor sensitivity responses compared to workload-matched dynamic resistance exercise. Differences in blood pressure responses between protocols appear small (~5 mmHg) and are affected by the number of sets. The muscle oxidative stimulus is greater during dynamic resistance exercise than workload-matched isometric exercise.

Effects of Short-Term Golden Root Extract (Rhodiola rosea) Supplementation on Resistance Exercise Performance

The purpose of this study was to examine the effects of short-term Golden Root Extract (GRE; Rhodiola rosea) supplementation on blood lactate, catecholamines, and performance during repeated bench press exercise. Resistance-trained males (n = 10) participated in this study. In a double-blinded, crossover, counterbalanced study design, participants supplemented with either 1500 mg/day of GRE or placebo (PL; gluten-free cornstarch) for 3 days prior to experimentation. An additional 500 mg dose was ingested 30 min prior to exercise testing. During each exercise trial, participants completed 2 repetitions of bench press at 75% of one-repetition maximum (1RM) as explosively as possible. A linear position transducer was used to measure mean concentric velocity. After 5 min of rest, participants completed 3 sets × repetitions to failure (RTF) at 75% 1RM separated by 2 min of rest between each set. A capillary blood sample was obtained pre- (PRE) and immediately post- (POST) exercise to measure blood concentrations lactate (LA), epinephrine (EPI), and norepinephrine (NE). Mean concentric velocity was significantly higher with GRE when compared to PL (p = 0.046). However, total RTF were significantly lower with GRE versus PL (p < 0.001). Regardless of treatment, LA was significantly higher Post versus Pre (p < 0.001), but GRE resulted in greater Post values compared to PL (p = 0.049). EPI and NE increased in both conditions Pre to Post (p < 0.001). However, Pre NE was significantly higher with GRE versus PL (p = 0.008). Findings indicate that short-term GRE supplementation increases mean bench press velocity but decreases bench press repetition volume. Furthermore, GRE resulted in higher NE levels and blood lactate following exercise. Thus, supplementing with GRE may enhance explosive resistance training performance but may also impair upper body strength-endurance.

Possible Effects of Beetroot Supplementation on Physical Performance Through Metabolic, Neuroendocrine, and Antioxidant Mechanisms: A Narrative Review of the Literature

Athletes often seek to use dietary supplements to increase performance during exercise. Among various supplements, much attention has been paid to beetroot in recent years. Beetroot is a source of carbohydrates, fiber, protein, minerals, and vitamins; also, it is a natural source of nitrate and associated with improved sports performance. Nitrates can the modification of skeletal muscle contractile proteins or calcium handling after translation. The time to reach the peak plasma nitrate is between 1 and 3 h after consumption of a single dose of nitrate. Nitrate is metabolized by conversion to nitrite and subsequently nitric oxide. Beetroot can have various effects on athletic performance through nitric oxide. Nitric oxide is an intracellular and extracellular messenger for regulating certain cellular functions and causes vasodilation of blood vessels and increases blood flow. Nitric oxide seems to be effective in improving athletic performance by increasing oxygen, glucose, and other nutrients for better muscle fueling. Nitric oxide plays the main role in anabolic hormones, modulates the release of several neurotransmitters and the major mediators of stress involved in the acute hypothalamic-pituitary-adrenal response to exercise. Beetroot is an important source of compounds such as ascorbic acid, carotenoids, phenolic acids, flavonoids, betaline, and highly active phenolics and has high antioxidant properties. Beetroot supplement provides an important source of dietary polyphenols and due to the many health benefits. Phytochemicals of Beetroot through signaling pathways inhibit inflammatory diseases. In this study, the mechanisms responsible for these effects were examined and the research in this regard was reviewed.

Sex Differences In Cardiorespiratory Fitness Are Explained By Blood Volume And Oxygen Carrying Capacity

AIMS

Intrinsic sex differences in fundamental blood attributes have long been hypothesized to contribute to the gap in cardiorespiratory fitness between men and women. This study experimentally assessed the role of blood volume and oxygen (O2) carrying capacity on sex differences in cardiac function and aerobic power.

METHODS AND RESULTS

Healthy women and men (n = 60) throughout the mature adult lifespan (42-88 yr) were matched by age and physical activity levels. Transthoracic echocardiography, central blood pressure and O2 uptake were assessed throughout incremental exercise (cycle ergometry). Main outcomes such as left ventricular end-diastolic volume (LVEDV), stroke volume (SV), cardiac output (Q), and peak O2 uptake (VO2peak), as well as blood volume (BV) were determined with established methods. Measurements were repeated in men following blood withdrawal and O2 carrying capacity reduction matching women's levels. Prior to blood normalization, BV and O2 carrying capacity were markedly reduced in women compared with men (P < 0.001). Blood normalization resulted in a precise match of BV (82.36 ± 9.83 vs. 82.34 ± 7.70 ml·kg-1, P = 0.993) and O2 carrying capacity (12.0 ± 0.6 vs. 12.0 ± 0.7 g·dl-1, P = 0.562) between women and men. Body size-adjusted cardiac filling and output (LVEDV, SV, Q) during exercise as well as VO2peak (30.8 ± 7.5 vs. 35.6 ± 8.7 ml·min-1·kg-1, P < 0.001) were lower in women compared with men prior to blood normalization. VO2peak did not differ between women and men after blood normalization (30.8 ± 7.5 vs. 29.7 ± 7.4 ml·min-1·kg-1, P = 0.551).

CONCLUSIONS

Sex differences in cardiorespiratory fitness are abolished when blood attributes determining O2 delivery are experimentally matched between adult women and men.

TRANSLATIONAL PERSPECTIVE

Low cardiorespiratory fitness is strongly associated with all-cause and cardiovascular mortality in asymptomatic adults independently of traditional risk factors, relationships seemingly enhanced in middle-aged and older women. Yet, whether the primary hematological determinants of cardiorespiratory fitness that were established in studies comprising men explain the difference between sexes remains uncertain. Importantly, blood attributes are amenable to modification and thus potentially translated into effective targets to improve or preserve cardiovascular health in the general population. The present experimental study demonstrates that blood normalization between men and women eliminate sex differences in cardiorespiratory fitness.

Acute Beetroot Juice Supplementation Attenuates Morning-Associated Decrements in Supramaximal Exercise Performance in Trained Sprinters

Diurnal fluctuations in power output have been well established with power loss typically occurring in morning (AM) times. Beetroot juice (BRJ) is a source of dietary nitrate that possess ergogenic properties, but it is unknown if ingestion can mitigate performance decrements in the morning. The purpose of this study was to examine the effects of acute BRJ supplementation on diurnal fluctuations in anaerobic performance in trained sprinters. Male Division 1 National Collegiate Athletic Association (NCAA) sprinters (n = 10) participated. In a double-blinded crossover study design, participants completed three counterbalanced exercise trials under different conditions: Morning–placebo (8:00 HR, AM-PL), Morning–BRJ (8:00 HR, AM-BRJ), and Afternoon–no supplement (15:00 HR, PM). For each trial, participants completed 3 × 15 s Wingate anaerobic tests separated by 2 min of rest. Each trial was separated by a 72 h washout period. Mean power output (p = 0.043), anaerobic capacity (p = 0.023), and total work (p = 0.026) were significantly lower with the AM-PL condition compared to PM. However, BRJ supplementation prevented AM losses of mean power output (p = 0.994), anaerobic capacity (p = 0.941), and total work (p = 0.933) in the AM-BRJ compared to the PM condition. Rate of perceived exertion was not significantly different between any conditions (p = 0.516). Heart rate was significantly lower during the AM-BRJ condition compared to AM-PL (p = 0.030) and PM (p < 0.001). Findings suggest anaerobic capacity suffers during AM versus PM times in trained sprinters, but BRJ ingestion abolishes AM-associated decrements in performance.

Indices of leg resistance artery function are independently related to cycling V̇O2 max

PURPOSE

While maximum blood flow influences one's maximum rate of oxygen consumption (V̇O2 max), with so many indices of vascular function, it is still unclear if vascular function is related to V̇O2 max in healthy, young adults. The purpose of this study was to determine if several common vascular tests of conduit artery and resistance artery function provide similar information about vascular function and the relationship between vascular function and V̇O2 max.

METHODS

Twenty-two healthy adults completed multiple assessments of leg vascular function, including flow-mediated dilation (FMD), reactive hyperemia (RH), passive leg movement (PLM), and rapid onset vasodilation (ROV). V̇O2 max was assessed with a graded exercise test on a cycle ergometer.

RESULTS

Indices associated with resistance artery function (e.g., peak flow during RH, PLM, and ROV) were generally related to each other (r = 0.47-77, p < .05), while indices derived from FMD were unrelated to other tests (p < .05). Absolute V̇O2 max (r = 0.57-0.73, p < .05) and mass-specific V̇O2 max (r = 0.41-0.46, p < .05) were related to indices of resistance artery function, even when controlling for factors like body mass and sex. FMD was only related to mass-specific V̇O2 max after statistically controlling for baseline artery diameter (r = 0.44, p < .05).

CONCLUSION

Indices of leg resistance artery function (e.g., peak flow during RH, PLM, and ROV) relate well to each other and account for ~30% of the variance in V̇O2 max not accounted for by other factors, like body mass and sex. Vascular interventions should focus on improving indices of resistance artery function, not conduit artery function, when seeking to improve exercise capacity.

Dehydration reduces stroke volume and cardiac output during exercise because of impaired cardiac filling and venous return, not left ventricular function

Dehydration accrued during intense prolonged whole-body exercise in the heat compromises peripheral blood flow and cardiac output ( Q ˙ ). A markedly reduced stroke volume (SV) is a key feature of the dehydration-induced cardiovascular strain, but whether the lower output of the heart is mediated by peripheral or cardiac factors remains unknown. Therefore, we repeatedly quantified left ventricular (LV) volumes, LV mechanics (LV twist, a marker of systolic muscle function, and LV untwisting rate, an independent marker of LV muscle relaxation), left intra-ventricular pressure gradients, blood volume and peripheral blood flow during 2 hr of cycling in the heat with and without dehydration (DEH: 4.0 ± 0.2% body mass loss and EUH: euhydration control, respectively) in eight participants (three females and five males). While brachial and carotid blood flow, blood volume, SV, LV end-diastolic volume (LVEDV), cardiac filling time, systemic vascular conductance and Q ˙ were reduced in DEH compared to EUH after 2 hr, LV twist and untwisting rate tended to be higher (p = .09 and .06, respectively) and intra-ventricular pressure gradients were not different between the two conditions (p = .22). Furthermore, LVEDV in DEH correlated strongly with blood volume (r = .995, p < .01), head and forearms beat volume (r = .98, p < .05), and diastolic LV filling time (r = .98, p < .05). These findings suggest that the decline in SV underpinning the blunted Q ˙ with exercise-induced dehydration is caused by compromised LV filling and venous return, but not intrinsic systolic or diastolic LV function.

What determines systemic blood flow in vertebrates?

In the 1950s, Arthur C. Guyton removed the heart from its pedestal in cardiovascular physiology by arguing that cardiac output is primarily regulated by the peripheral vasculature. This is counterintuitive, as modulating heart rate would appear to be the most obvious means of regulating cardiac output. In this Review, we visit recent and classic advances in comparative physiology in light of this concept. Although most vertebrates increase heart rate when oxygen demands rise (e.g. during activity or warming), experimental evidence suggests that this tachycardia is neither necessary nor sufficient to drive a change in cardiac output (i.e. systemic blood flow, Q̇ _sys) under most circumstances. Instead, Q̇ _sys is determined by the interplay between vascular conductance (resistance) and capacitance (which is mainly determined by the venous circulation), with a limited and variable contribution from heart function (myocardial inotropy). This pattern prevails across vertebrates; however, we also highlight the unique adaptations that have evolved in certain vertebrate groups to regulate venous return during diving bradycardia (i.e. inferior caval sphincters in diving mammals and atrial smooth muscle in turtles). Going forward, future investigation of cardiovascular responses to altered metabolic rate should pay equal consideration to the factors influencing venous return and cardiac filling as to the factors dictating cardiac function and heart rate.

Physiological Impact and Clinical Relevance of Passive Exercise/Movement

Passive exercise/movement has a long history in both medicine and physiology. Early clinical applications of passive exercise/movement utilized pneumatic and direct limb compression to stimulate the vasculature and evoke changes in blood flow to avoid complications brought about by stasis and vascular disease. Over the last 50 years, passive exercise/movement has continued to progress and has provided physiologists with a reductionist approach to mechanistically examine the cardiorespiratory, hyperemic, and afferent responses to movement without the confounding influence of metabolism that accompanies active exercise. This review, in addition to providing an historical perspective, focuses on the recent advancements utilizing passive leg movement, and how the hyperemic response at the onset of this passive movement has evolved from a method to evaluate the central and peripheral regulation of blood flow during exercise to an innovative and promising tool to assess vascular function. As an assessment of vascular function, passive leg movement is relatively simple to perform and provides a nitric oxide-dependent evaluation of endothelial function across the lifespan that is sensitive to changes in activity/fitness and disease state (heart failure, peripheral artery disease, sepsis). The continual refinement and characterization of passive leg movement are aimed at improving our understanding of blood flow regulation and the development of a clinically ready approach to predict and monitor the progression of cardiovascular disease.

Weighing the evidence for using vascular conductance, not resistance, in comparative cardiovascular physiology

Vascular resistance and conductance are reciprocal indices of vascular tone that are often assumed to be interchangeable. However, in most animals in vivo, blood flow (i.e. cardiac output) typically varies much more than arterial blood pressure. When blood flow changes at a constant pressure, the relationship between conductance and blood flow is linear, whereas the relationship between resistance and blood flow is non-linear. Thus, for a given change in blood flow, the change in resistance depends on the starting point, whereas the attendant change in conductance is proportional to the change in blood flow regardless of the starting conditions. By comparing the effects of physical activity at different temperatures or between species – concepts at the heart of comparative cardiovascular physiology – we demonstrate that the difference between choosing resistance or conductance can be marked. We also explain here how the ratio of conductance in the pulmonary and systemic circulations provides a more intuitive description of cardiac shunt patterns in the reptilian cardiovascular system than the more commonly used ratio of resistance. Finally, we posit that, although the decision to use conductance or resistance should be made on a case-by-case basis, in most circumstances, conductance is a more faithful portrayal of cardiovascular regulation in vertebrates.

Do interindividual differences in cardiac output during submaximal exercise explain differences in exercising muscle oxygenation and ratings of perceived exertion?

Considerable interindividual differences in the Q˙-V˙O2 relationship during exercise have been documented but implications for submaximal exercise tolerance have not been considered. We tested the hypothesis that these interindividual differences were associated with differences in exercising muscle deoxygenation and ratings of perceived exertion (RPE) across a range of submaximal exercise intensities. A total of 31 (21 ± 3 years) healthy recreationally active males performed an incremental exercise test to exhaustion 24 h following a resting muscle biopsy. Cardiac output (Q˙ L/min; inert gas rebreathe), oxygen uptake (V˙O2 L/min; breath-by-breath pulmonary gas exchange), quadriceps saturation (near infrared spectroscopy) and exercise tolerance (6-20; Borg Scale RPE) were measured. The Q˙-V˙O2 relationship from 40 to 160 W was used to partition individuals post hoc into higher (n = 10; 6.3 ± 0.4) versus lower (n = 10; 3.7 ± 0.4, P < 0.001) responders. The Q˙-V˙O2 difference between responder types was not explained by arterial oxygen content differences (P = 0.5) or peripheral skeletal muscle characteristics (P from 0.1 to 0.8) but was strongly associated with stroke volume (P < 0.05). Despite considerable Q˙-V˙O2 difference between groups, no difference in quadriceps deoxygenation was observed during exercise (all P > 0.4). Lower cardiac responders had greater leg (P = 0.027) and whole body (P = 0.03) RPE only at 185 W, but this represented a higher %peak V˙O2 in lower cardiac responders (87 ± 15% vs. 66 ± 12%, P = 0.005). Substantially lower Q˙-V˙O2 in the lower responder group did not result in altered RPE or exercising muscle deoxygenation. This suggests substantial recruitment of blood flow redistribution in the lower responder group as part of protecting matching of exercising muscle oxygen delivery to demand.

Effect of increasing heart rate on finger photoplethysmography fitness index (PPGF) in subjects with implanted cardiac pacemakers

Finger photoplethysmography (PPG) is a noninvasive method that measures blood volume changes in the finger. The PPG fitness index (PPGF) has been proposed as an index of vascular risk and vascular aging. The objectives of the study were to determine the effects of heart rate (HR) on the PPGF and to determine whether PPGF is influenced by blood pressure (BP) changes. Twenty subjects (78±8 years, 3 female) with permanent cardiac pacemakers or cardioverter defibrillators were prospectively recruited. HR was changed by pacing, in a random order from 60 to 100 bpm and in 10 bpm increments. At each paced HR, the PPGF was derived from a finger photoplethysmogram. Cardiac output (CO), stroke volume (SV) and total peripheral resistance (TPR) were derived from the finger arterial pressure waveform. Brachial blood pressure (BP) was measured by the oscillometric method. This study found that as HR was increased from 60 to 100 bpm, brachial diastolic BP, brachial mean BP and CO were significantly increased (p<0.01), whilst the PPGF and SV were significantly decreased (p<0.001). The effects of HR on the PPGF were influenced by BP, with a decreasing HR effect on the PPGF that resulted from a higher BP. In conclusion, HR was a significant confounder for PPGF and it must be taken into account in analyses of PPGF, when there are large changes or differences in the HR. The magnitude of this effect was BP dependent.

Exploring nature's natural knockouts: in vivo cardiorespiratory performance of Antarctic fishes during acute warming

We tested the hypothesis that blackfin icefish (Chaenocephalus aceratus), one of the six species in the family Channichthyidae (the icefishes) that do not express haemoglobin and myoglobin, lack regulatory cardiovascular flexibility during acute warming and activity. The experimental protocols were designed to optimize the surgical protocol and minimize stress. First, minimally invasive heart rate (f_H) measurements were made during a thermal ramp until cardiac failure in C. aceratus and compared with those from the closely related red-blooded black rockcod (Notothenia coriiceps). Then, integrative cardiovascular adjustments were more extensively studied using flow probes and intravascular catheters in C. aceratus during acute warming (from 0 to 8°C) at rest and after imposed activity. Chaenocephalus aceratus had a lower routine f_H than N. coriiceps (9 beats min^-1 versus 14 beats min^-1) and a lower peak f_H during acute warming (38 beats min^-1 versus 55 beats min^-1) with a similar cardiac breakpoint temperature (13 and 14°C, respectively). Routine cardiac output (Q̇) for C. aceratus at ∼0°C was much lower (26.6 ml min^-1 kg^-1) than previously reported, probably because fish in the present study had a low f_H (12 beats min^-1) indicative of a high routine vagal tone and low stress. Chaenocephalus aceratus increased oxygen consumption during acute warming and with activity. Correspondingly, Q̇ increased considerably (maximally 86.3 ml min^-1 kg^-1), as did vascular conductance (5-fold). Thus, unlike earlier suggestions, these data provide convincing evidence that icefish can mount a well-developed cardiovascular regulation of heart rate, cardiac output and vascular conductance, and this regulatory capacity provides flexibility during acute warming.

Maximum heart rate does not limit cardiac output at rest or during exercise in the American alligator (Alligator mississippiensis)

In most vertebrates, increases in cardiac output result from increases in heart rate (fH) with little or no change in stroke volume (Vs), and maximum cardiac output (Q̇) is typically attained at or close to maximum fH. We therefore tested the hypothesis that increasing maximum fH may increase maximum Q̇. To this end, we investigated the effects of elevating fH with right atrial pacing on Q̇ in the American alligator ( Alligator mississippiensis) at rest and while swimming. During normal swimming, Q̇ increased entirely by virtue of a tachycardia (29 ± 1 to 40 ± 3 beats/min), whereas Vs remained stable. In both resting and swimming alligators, increasing fH with right atrial pacing resulted in a parallel decline in Vs that resulted in an unchanged cardiac output. In swimming animals, this reciprocal relationship extended to supraphysiological fH (up to ~72 beats/min), which suggests that maximum fH does not limit maximum cardiac output and that fH changes are secondary to the peripheral factors (for example vascular capacitance) that determine venous return at rest and during exercise.

Elevated biomarkers of sympatho-adrenomedullary activity linked to e-waste air pollutant exposure in preschool children

Air pollution is a risk factor for cardiovascular disease (CVD), and cardiovascular regulatory changes in childhood contribute to the development and progression of cardiovascular events at older ages. The aim of the study was to investigate the effect of air pollutant exposure on the child sympatho-adrenomedullary (SAM) system, which plays a vital role in regulating and controlling the cardiovascular system. Two plasma biomarkers (plasma epinephrine and norepinephrine) of SAM activity and heart rate were measured in preschool children (n = 228) living in Guiyu, and native (n = 104) and non-native children (n = 91) living in a reference area (Haojiang) for >1 year. Air pollution data, over the 4-months before the health examination, was also collected. Environmental PM_2.5, PM₁₀, SO₂, NO₂ and CO, plasma norepinephrine and heart rate of the e-waste recycling area were significantly higher than for the non-e-waste recycling area. However, there was no difference in plasma norepinephrine and heart rate between native children living in the non-e-waste recycling area and non-native children living in the non-e-waste recycling area. PM_2.5, PM₁₀, SO₂ and NO₂ data, over the 30-day and the 4-month average of pollution before the health examination, showed a positive association with plasma norepinephrine level. PM_2.5, PM₁₀, SO₂, NO₂ and CO concentrations, over the 24 h of the day of the health examination, the 3 previous 24-hour periods before the health examination, and the 24 h after the health examination, were related to increase in heart rate. At the same time, plasma norepinephrine and heart rate on children in the high air pollution level group (≤50-m radius of family-run workshops) were higher than those in the low air pollution level group. Our results suggest that air pollution exposure in e-waste recycling areas could result in an increase in heart rate and plasma norepinephrine, implying e-waste air pollutant exposure impairs the SAM system in children.

Alpha adrenergic receptor blockade increases capillarization and fractional O2 extraction and lowers blood flow in contracting human skeletal muscle

AIM

To assess the effect of elevated basal shear stress on angiogenesis in humans and the role of enhanced skeletal muscle capillarization on blood flow and O₂ extraction.

METHODS

Limb haemodynamics and O₂ extraction were measured at rest and during one-leg knee-extensor exercise (12 and 24 W) in 10 healthy untrained young men before and after 4-week treatment with an α₁ receptor-antagonist (Terazosin, 1-2 mg day^-1 ). Corresponding biopsies were taken from the m. vastus lateralis.

RESULTS

Resting leg blood flow was increased by 57% 6 h following Terazosin treatment (P < 0.05), while basal capillary-to-fibre ratio was 1.69 ± 0.08 and increased to 1.90 ± 0.08 after treatment (P < 0.05). Leg O₂ extraction during knee-extensor exercise was higher (4-5%; P < 0.05), leg blood flow and venous lactate levels lower (6-7%; P < 0.05), while leg VO₂ was not different after Terazosin treatment.

CONCLUSIONS

These results demonstrate that daily treatment with an α-adrenergic receptor blocker induces capillary growth in human skeletal muscle, likely due to increased shear stress. The increase in capillarization resulted in an increased fractional O₂ extraction, a lower blood flow and venous lactate levels in the exercising leg. The increase in capillarization, and concomitant functional readouts in the exercising leg, may provide a basis for novel angiotherapy.

Face cooling increases blood pressure during central hypovolemia

A reduction in central blood volume can lead to cardiovascular decompensation (i.e., failure to maintain blood pressure). Cooling the forehead and cheeks using ice water raises blood pressure. Therefore, face cooling (FC) could be used to mitigate decreases in blood pressure during central hypovolemia. We tested the hypothesis that FC during central hypovolemia induced by lower-body negative pressure (LBNP) would increase blood pressure. Ten healthy participants (22 ± 2 yr, three women, seven men) completed two randomized LBNP trials on separate days. Trials began with 30 mmHg of LBNP for 6 min. Then, a 2.5-liter plastic bag of ice water (0 ± 0°C) (LBNP+FC) or thermoneutral water (34 ± 1°C) (LBNP+Sham) was placed on the forehead, eyes, and cheeks during 15 min of LBNP at 30 mmHg. Forehead temperature was lower during LBNP+FC than LBNP+Sham, with the greatest difference at 21 min of LBNP (11.1 ± 1.6 vs. 33.9 ± 1.4°C, P < 0.001). Mean arterial pressure was greater during LBNP+FC than LBNP+Sham, with the greatest difference at 8 min of LBNP (98 ± 15 vs. 80 ± 8 mmHg, P < 0.001). Cardiac output was higher during LBNP+FC than LBNP+Sham with the greatest difference at 18 min of LBNP (5.9 ± 1.4 vs. 4.9 ± 1.0 liter/min, P = 0.005). Forearm cutaneous vascular resistance was greater during LBNP+FC than LBNP+Sham, with the greatest difference at 15 min of LBNP (7.2 ± 3.4 vs. 4.9 ± 2.7 mmHg/perfusion units (PU), P < 0.001). Face cooling during LBNP increases blood pressure through increases in cardiac output and vascular resistance.

Nicotine effects on exercise performance and physiological responses in nicotine-naïve individuals: a systematic review

The purpose of this systematic review was to evaluate the effects of smokeless forms of nicotine on physiological responses and exercise performance. Methodology and reporting were based on the PRISMA statement. The intervention was defined as any product containing nicotine that did not require smoking. Searches were conducted across two electronic databases with supplementary approaches utilized. Studies were selected following set inclusion and exclusion criteria and checked by two independent authors. A modified PEDro scale was utilized to rate study quality with studies averaging 9·3/13. Six studies assessed exercise performance with endurance-based parameters reported as significantly improved with nicotine in one study, while anaerobic parameters were unaffected or decreased compared to placebo except in one study which reported enhanced leg extensor torque but no effect on countermovement jump or Wingate anaerobic capacity. Sixteen of 28 studies investigating physiological responses reported that nicotine significantly increased heart rate compared to placebo or control. Blood pressure and blood flow were also reported as significantly increased in multiple studies. While there is strong evidence of nicotine-induced changes in physiological function that would benefit physical performance, beneficial effects have only been reported on leg extensor torque and endurance performance by one study each. Subsequently, there is need for more research with strong methodological quality to definitively evaluate nicotine's potential as an ergogenic aid.

Linking skeletal muscle blood flow and metabolism to the limits of human performance

Over the last 50 years, Bengt Saltin's contributions to our understanding of physiology of the circulation, the matching of the circulation to muscle metabolism, and the underlying mechanisms that set the limits for exercise performance were enormous. His research addressed the key questions in the field using sophisticated experimental methods including field expeditions. From the Dallas Bedrest Study to the 1-leg knee model to the physiology of lifelong training, his prodigious body of work was foundational in the field of exercise physiology and his leadership propelled integrative human physiology into the mainstream of biological sciences.

Cardiovascular Adaptations to Exercise Training

Aerobic exercise training leads to cardiovascular changes that markedly increase aerobic power and lead to improved endurance performance. The functionally most important adaptation is the improvement in maximal cardiac output which is the result of an enlargement in cardiac dimension, improved contractility, and an increase in blood volume, allowing for greater filling of the ventricles and a consequent larger stroke volume. In parallel with the greater maximal cardiac output, the perfusion capacity of the muscle is increased, permitting for greater oxygen delivery. To accommodate the higher aerobic demands and perfusion levels, arteries, arterioles, and capillaries adapt in structure and number. The diameters of the larger conduit and resistance arteries are increased minimizing resistance to flow as the cardiac output is distributed in the body and the wall thickness of the conduit and resistance arteries is reduced, a factor contributing to increased arterial compliance. Endurance training may also induce alterations in the vasodilator capacity, although such adaptations are more pronounced in individuals with reduced vascular function. The microvascular net increases in size within the muscle allowing for an improved capacity for oxygen extraction by the muscle through a greater area for diffusion, a shorter diffusion distance, and a longer mean transit time for the erythrocyte to pass through the smallest blood vessels. The present article addresses the effect of endurance training on systemic and peripheral cardiovascular adaptations with a focus on humans, but also covers animal data.

Repeated double-poling sprint training in hypoxia by competitive cross-country skiers

PURPOSE

Repeated-sprint training in hypoxia (RSH) was recently shown to improve repeated-sprint ability (RSA) in cycling. This phenomenon is likely to reflect fiber type-dependent, compensatory vasodilation, and therefore, our hypothesis was that RSH is even more beneficial for activities involving upper body muscles, such as double poling during cross-country skiing.

METHODS

In a double-blinded fashion, 17 competitive cross-country skiers performed six sessions of repeated sprints (each consisting of four sets of five 10-s sprints, with 20-s intervals of recovery) either in normoxia (RSN, 300 m; FiO2, 20.9%; n = 8) or normobaric hypoxia (RSH, 3000 m; FiO2, 13.8 %; n = 9). Before (pre) and after (post) training, performance was evaluated with an RSA test (10-s all-out sprints-20-s recovery, until peak power output declined by 30%) and a simulated team sprint (team sprint, 3 × 3-min all-out with 3-min rest) on a double-poling ergometer. Triceps brachii oxygenation was measured by near-infrared spectroscopy.

RESULTS

From pretraining to posttraining, peak power output in the RSA was increased (P < 0.01) to the same extent (29% ± 13% vs 26% ± 18%, nonsignificant) in RSH and in RSN whereas the number of sprints performed was enhanced in RSH (10.9 ± 5.2 vs 17.1 ± 6.8, P < 0.01) but not in RSN (11.6 ± 5.3 vs 11.7 ± 4.3, nonsignificant). In addition, the amplitude in total hemoglobin variations during sprints throughout RSA rose more in RSH (P < 0.01). Similarly, the average power output during all team sprints improved by 11% ± 9% in RSH and 15% ± 7% in RSN.

CONCLUSIONS

Our findings reveal greater improvement in the performance of repeated double-poling sprints, together with larger variations in the perfusion of upper body muscles in RSH compared with those in RSN.

Impaired cerebral blood flow and oxygenation during exercise in type 2 diabetic patients

Endothelial vascular function and capacity to increase cardiac output during exercise are impaired in patients with type 2 diabetes (T2DM). We tested the hypothesis that the increase in cerebral blood flow (CBF) during exercise is also blunted and, therefore, that cerebral oxygenation becomes affected and perceived exertion increased in T2DM patients. We quantified cerebrovascular besides systemic hemodynamic responses to incremental ergometer cycling exercise in eight male T2DM and seven control subjects. CBF was assessed from the Fick equation and by transcranial Doppler-determined middle cerebral artery blood flow velocity. Cerebral oxygenation and metabolism were evaluated from the arterial-to-venous differences for oxygen, glucose, and lactate. Blood pressure was comparable during exercise between the two groups. However, the partial pressure of arterial carbon dioxide was lower at higher workloads in T2DM patients and their work capacity and increase in cardiac output were only ~80% of that established in the control subjects. CBF and cerebral oxygenation were reduced during exercise in T2DM patients (P < 0.05), and they expressed a higher rating of perceived exertion (P < 0.05). In contrast, CBF increased ~20% during exercise in the control group while the brain uptake of lactate and glucose was similar in the two groups. In conclusion, these results suggest that impaired CBF and oxygenation responses to exercise in T2DM patients may relate to limited ability to increase cardiac output and to reduced vasodilatory capacity and could contribute to their high perceived exertion.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

Local temperature-sensitive mechanisms are important mediators of limb tissue hyperemia in the heat-stressed human at rest and during small muscle mass exercise

Limb tissue and systemic blood flow increases with heat stress, but the underlying mechanisms remain poorly understood. Here, we tested the hypothesis that heat stress-induced increases in limb tissue perfusion are primarily mediated by local temperature-sensitive mechanisms. Leg and systemic temperatures and hemodynamics were measured at rest and during incremental single-legged knee extensor exercise in 15 males exposed to 1 h of either systemic passive heat-stress with simultaneous cooling of a single leg (n = 8) or isolated leg heating or cooling (n = 7). Systemic heat stress increased core, skin and heated leg blood temperatures (T_b), cardiac output, and heated leg blood flow (LBF; 0.6 ± 0.1 l/min; P < 0.05). In the cooled leg, however, LBF remained unchanged throughout (P > 0.05). Increased heated leg deep tissue blood flow was closely related to T_b (R² = 0.50; P < 0.01), which is partly attributed to increases in tissue V̇O₂ (R² = 0.55; P < 0.01) accompanying elevations in total leg glucose uptake (P < 0.05). During isolated limb heating and cooling, LBFs were equivalent to those found during systemic heat stress (P > 0.05), despite unchanged systemic temperatures and hemodynamics. During incremental exercise, heated LBF was consistently maintained ∼0.6 l/min higher than that in the cooled leg (P < 0.01), with LBF and vascular conductance in both legs showing a strong correlation with their respective local T_b (R² = 0.85 and 0.95, P < 0.05). We conclude that local temperature-sensitive mechanisms are important mediators in limb tissue perfusion regulation both at rest and during small-muscle mass exercise in hyperthermic humans.

The association of cardiorespiratory fitness with endothelial or smooth muscle vasodilator function

BACKGROUND

Maximal oxygen consumption (VO2max) is strongly associated with peripheral vasodilator function as determined by exercise-induced vasodilation. However, findings with regard to its relation with non-exercise-stimulated vasodilation are unclear. The purpose of this study was to systematically review published literature reporting associations between VO2max and endothelial function (EF) or smooth muscle function (SMF).

DESIGN AND METHODS

We conducted a systematic search of MEDLINE, Cochrane and Web of Science, since their inceptions until April 2014 for articles reporting the association between (a) VO2max during incremental exercise and (b) endothelium-dependent or -independent vasodilator function, by means of correlation and/or regression analysis.

RESULTS

Fifty-six articles exploring 88 associations between VO2max and vascular EF or SMF were included, involving a total of 4159 healthy and diseased subjects. VO2max was determined by incremental cycle ergometer (64%), treadmill (33%) and cycle ergometer/treadmill (3%) exercise. Vasodilator function variables were assessed in the upper limb (86%), lower limb (10%) and both upper and lower limbs (3%). Most of the evaluated bivariate associations involved EF stimuli such as flow-mediated dilation (FMD) (n = 29) or blood flow occlusion (BFO) (n = 18). VO2max was significantly associated with FMD and BFO in 59% and 67% of bivariate associations and 46% and 33% of age-independent associations, respectively. Explored bivariate associations regarding SMF involved sodium nitroprusside (SNP) iontophoresis (n = 7) and nitrate-mediated dilation (NMD) (n = 4). VO2max was associated with NMD in 50% of bivariate associations and 50% of age-independent associations. VO2max was not associated with SNP iontophoresis. Results were similar for associations including only healthy subjects.

CONCLUSIONS

The association between VO2max and EF or SMF is moderately frequent and independent of health status, despite very few studies having assessed vasodilator function in the lower limb.

Greater autonomic modulation during post-exercise hypotension following high-intensity interval exercise in endurance-trained men and women

PURPOSE

An acute reduction in blood pressure observed after a single bout of exercise is termed post-exercise hypotension (PEH). In contrast to moderate intensity aerobic exercise, little is known about the PEH response following high-intensity interval exercise. The present purpose is to assess how sex and training status impact PEH following high-intensity interval exercise.

METHODS

Cardiac volumes and function via echocardiography were measured in 40 normotensive, endurance-trained (ET) and normally active (NA) men and women (Age ± SD = 30.5 ± 5.7) following high-intensity interval cycle exercise. Continuous measurements of ECG and beat-by-beat blood pressure were collected before and 30 min post-exercise for determination of cardiovagal baroreflex function (BRS and αLF), spectral analysis of heart rate and systolic blood pressure (SBP LF).

RESULTS

Post-exercise systolic BP was significantly reduced from baseline, occurring to a greater degree in ET compared with NA (-12.9 vs. -5.3 mmHg, P = 0.008), while mean arterial pressure was similarly reduced in all groups (-4.6 mmHg, P = 0.003). Despite reduced SVI and TPRI, CI was increased post-exercise (P < 0.01). ET experienced a greater decrease in αLF (P = 0.037) and increase in SBP LF (P = 0.017) than NA. Lean body mass was a significant predictor of change in SBP LF (Std. β = 0.735, P = 0.008).

CONCLUSIONS

These results characterize greater depressions in cardiovagal baroreflex function, and increased sympathetic activity, following vigorous exercise in endurance-trained individuals compared with normally active participants. This heightened sympathovagal balance after high-intensity exercise may be a compensatory mechanism in response to greater peripheral blood flow demands following vigorous exercise.

Effect of muscle metaboreflex activation on central hemodynamics and cardiac function in humans

We sought to determine how the mode of muscle metaboreflex activation influences the central hemodynamic response and cardiac inotropic and lusotropic function in healthy humans. Ten healthy males performed (i) isometric handgrip (IHG) with and without post-exercise ischemia (PEI) to examine the influence of isolated muscle metaboreflex activation and (ii) rhythmic handgrip (RHG) with and without ischemia to examine the influence of enhanced muscle metaboreflex activation. Heart rate (HR) and blood pressure (BP) were continuously monitored. Stroke volume (SV, Doppler echocardiography) was measured, cardiac output (CO = HR × SV) and total peripheral resistance (TPR = mean BP/CO) calculated, and indices of left ventricular systolic and diastolic function were obtained (tissue Doppler imaging). During isolated muscle metaboreflex activation with PEI following IHG, mean BP (+23 ± 3 mm Hg) and TPR were elevated from baseline (p < 0.05), whereas HR, SV, and CO were unchanged. Enhanced muscle metaboreceptor activation during ischemic RHG augmented the increase in mean BP, CO, and HR (p < 0.05 ischemic vs. free-flow RHG), whereas SV and TPR were unchanged from baseline. Neither isolated (PEI) nor enhanced muscle metaboreflex activation altered left ventricular systolic function (systolic myocardial velocity), but left atrial systolic function (late diastolic myocardial velocity) was enhanced. These findings indicate that the mode of muscle metaboreceptor activation (during vs. post handgrip) determines whether the resultant pressor response is flow (CO) or vasoconstriction (TPR) mediated, and that although left ventricular systolic function is unchanged, enhanced left atrial systolic function likely aids the preservation of SV during muscle metaboreflex engagement.

Anti-hypotensive treatment and endothelin blockade synergistically antagonize exercise fatigue in rats under simulated high altitude

Rapid ascent to high altitude causes illness and fatigue, and there is a demand for effective acute treatments to alleviate such effects. We hypothesized that increased oxygen delivery to the tissue using a combination of a hypertensive agent and an endothelin receptor A antagonist drugs would limit exercise-induced fatigue at simulated high altitude. Our data showed that the combination of 0.1 mg/kg ambrisentan with either 20 mg/kg ephedrine or 10 mg/kg methylphenidate significantly improved exercise duration in rats at simulated altitude of 4,267 m, whereas the individual compounds did not. In normoxic, anesthetized rats, ephedrine alone and in combination with ambrisentan increased heart rate, peripheral blood flow, carotid and pulmonary arterial pressures, breathing rate, and vastus lateralis muscle oxygenation, but under inspired hypoxia, only the combination treatment significantly enhanced muscle oxygenation. Our results suggest that sympathomimetic agents combined with endothelin-A receptor blockers offset altitude-induced fatigue in rats by synergistically increasing the delivery rate of oxygen to hypoxic muscle by concomitantly augmenting perfusion pressure and improving capillary conductance in the skeletal muscle. Our findings might therefore serve as a basis to develop an effective treatment to prevent high-altitude illness and fatigue in humans.