Elevated extracellular potassium prior to muscle contraction reduces onset and steady-state exercise hyperemia in humans

Global REACH 2018: increased adrenergic restraint of blood flow preserves coupling of oxygen delivery and demand during exercise at high-altitude

Chronic exposure to hypoxia (high-altitude, HA; >4000 m) attenuates the vasodilatory response to exercise and is associated with a persistent increase in basal sympathetic nerve activity (SNA). The mechanism(s) responsible for the reduced vasodilatation and exercise hyperaemia at HA remains unknown. We hypothesized that heightened adrenergic signalling restrains skeletal muscle blood flow during handgrip exercise in lowlanders acclimatizing to HA. We tested nine adult males (n = 9) at sea-level (SL; 344 m) and following 21-28 days at HA (∼4300 m). Forearm blood flow (FBF; duplex ultrasonography), mean arterial pressure (MAP; brachial artery catheter), forearm vascular conductance (FVC; FBF/MAP), and arterial and venous blood sampling (O₂ delivery ( DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ ) and uptake ( V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ )) were measured at rest and during graded rhythmic handgrip exercise (5%, 15% and 25% of maximum voluntary isometric contraction; MVC) before and after local α- and β-adrenergic blockade (intra-arterial phentolamine and propranolol). HA reduced ΔFBF (25% MVC: SL: 138.3 ± 47.6 vs. HA: 113.4 ± 37.1 ml min^-1 ; P = 0.022) and Δ V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ (25% MVC: SL: 20.3 ± 7.5 vs. HA: 14.3 ± 6.2 ml min^-1 ; P = 0.014) during exercise. Local adrenoreceptor blockade at HA restored FBF during exercise (25% MVC: SL_{α-β blockade} : 164.1 ± 71.7 vs. HA_{α-β blockade} : 185.4 ± 66.6 ml min^-1 ; P = 0.947) but resulted in an exaggerated relationship between DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ and V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ( DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ / V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slope: SL: 1.32; HA: slope: 1.86; P = 0.037). These results indicate that tonic adrenergic signalling restrains exercise hyperaemia in lowlanders acclimatizing to HA. The increase in adrenergic restraint is necessary to match oxygen delivery to demand and prevent over perfusion of contracting muscle at HA. KEY POINTS: In exercising skeletal muscle, local vasodilatory signalling and sympathetic vasoconstriction integrate to match oxygen delivery to demand and maintain arterial blood pressure. Exposure to chronic hypoxia (altitude, >4000 m) causes a persistent increase in sympathetic nervous system activity that is associated with impaired functional capacity and diminished vasodilatation during exercise. In healthy male lowlanders exposed to chronic hypoxia (21-28 days; ∼4300 m), local adrenoreceptor blockade (combined α- and β-adrenergic blockade) restored skeletal muscle blood flow during handgrip exercise. However, removal of tonic adrenergic restraint at high altitude caused an excessive rise in blood flow and subsequently oxygen delivery for any given metabolic demand. This investigation is the first to identify greater adrenergic restraint of blood flow during acclimatization to high altitude and provides evidence of a functional role for this adaptive response in regulating oxygen delivery and demand.

ATP and acetylcholine interact to modulate vascular tone and α1-adrenergic vasoconstriction in humans

The vascular endothelium senses and integrates numerous inputs to regulate vascular tone. Recent evidence reveals complex signal processing within the endothelium, yet little is known about how endothelium-dependent stimuli interact to regulate blood flow. We tested the hypothesis that combined stimulation of the endothelium with adenosine triphosphate (ATP) and acetylcholine (ACh) elicits greater vasodilation and attenuates α₁-adrenergic vasoconstriction compared with combination of ATP or ACh with the endothelium-independent dilator sodium nitroprusside (SNP). We assessed forearm vascular conductance (FVC) in young adults (6 women, 7 men) during local intra-arterial infusion of ATP, ACh, or SNP alone and in the following combinations: ATP + ACh, SNP + ACh, and ATP + SNP, wherein the second dilator was coinfused after attaining steady state with the first dilator. By design, each dilator evoked a similar response when infused separately (ΔFVC, ATP: 48 ± 4; ACh: 57 ± 6; SNP: 53 ± 6 mL·min^-1·100 mmHg^-1; P ≥ 0.62). Combined infusion of the endothelium-dependent dilators evoked greater vasodilation than combination of either dilator with SNP (ΔFVC from first dilator, ATP + ACh: 45 ± 9 vs. SNP + ACh: 18 ± 7 and ATP + SNP: 26 ± 4 mL·min^-1·100 mmHg^-1, P < 0.05). Phenylephrine was subsequently infused to evaluate α₁-adrenergic vasoconstriction. Phenylephrine elicited less vasoconstriction during infusion of ATP or ACh versus SNP (ΔFVC, -25 ± 3 and -29 ± 4 vs. -48 ± 3%; P < 0.05). The vasoconstrictor response to phenylephrine was further diminished during combined infusion of ATP + ACh (-13 ± 3%; P < 0.05 vs. ATP or ACh alone) and was less than that observed when either dilator was combined with SNP (SNP + ACh: -26 ± 3%; ATP + SNP: -31 ± 4%; both P < 0.05 vs. ATP + ACh). We conclude that endothelium-dependent agonists interact to elicit vasodilation and limit α₁-adrenergic vasoconstriction in humans.NEW & NOTEWORTHY The results of this study highlight the vascular endothelium as a critical site for integration of vasomotor signals in humans. To our knowledge, this is the first study to demonstrate that combined stimulation of the endothelium with ATP and ACh results in enhanced vasodilation compared with combination of either ATP or ACh with an endothelium-independent dilator. Furthermore, we show that ATP and ACh interact to modulate α₁-adrenergic vasoconstriction in human skeletal muscle in vivo.

Low-to-Moderate-Intensity Resistance Exercise Is More Effective than High-Intensity at Improving Endothelial Function in Adults: A Systematic Review and Meta-Analysis

Aerobic exercise has been confirmed to improve endothelial function (EF). However, the effect of resistance exercise (RE) on EF remains controversial. We conducted this systematic review and meta-analysis on randomized controlled trials (RCTs) to determine the effect of RE and its intensities on EF. We searched Web of Science, PubMed/MEDLINE, Scopus, and Wiley Online Library, and included 15 articles (17 trials) for the synthesis. Overall, RE intervention significantly improved flow-mediated dilatation (FMD) in brachial artery (SMD = 0.76; 95% CI: 0.47, 1.05; p < 0.00001), which represents improved EF. Meta-regression showed that the RE intensity was correlated with changes in FMD (Coef. = −0.274, T = −2.18, p = 0.045). We found both intensities of RE improved FMD, but the effect size for the low- to moderate-intensity (30–70%1RM) was bigger (SMD = 1.02; 95% CI: 0.60, 1.43; p < 0.0001) than for the high-intensity (≥70%1RM; SMD = 0.48; 95% CI: 0.21, 0.74; p = 0.005). We further noticed that RE had a beneficial effect (SMD = 0.61; 95% CI: 0.13, 1.09; p = 0.01) on the brachial artery baseline diameter at rest (BAD_rest), and the age variable was correlated with the changes in BAD_rest after RE (Coef. = −0.032, T = −2.33, p = 0.038). Young individuals (<40 years) presented with a bigger effect size for BAD_rest (SMD = 1.23; 95% CI: 0.30, 2.15; p = 0.009), while middle-aged to elderly (≥40 years) were not responsive to RE (SMD = 0.07; 95% CI: −0.28, 0.42; p = 0.70). Based on our findings, we conclude that RE intervention can improve the EF, and low- to moderate-intensity is more effective than high-intensity.

Muscle blood flow is independent of conduit artery diameter following prior vasodilation in males

At the onset of exercise in humans, muscle blood flow (MBF) increases to a new steady‐state that closely matches the metabolic demand of exercise. This increase has been attributed to “contraction‐induced vasodilation,” comprised of the skeletal muscle pump and rapid vasodilatory mechanisms. While most research in this area has focused on forearm blood flow (FBF) and vascular conductance, it is possible that separating FBF into diameter and blood velocity can provide more useful information on MBF regulation downstream of the conduit artery. Therefore, we attempted to dissociate the matching of oxygen delivery and oxygen demand by administering glyceryl tri‐nitrate (GTN) prior to handgrip exercise. Eight healthy males (29 ± 9 years) performed two trials consisting of two bouts of rhythmic handgrip exercise (30 contractions·min⁻¹ at 5% of maximum) for 6 min, one for each control and GTN (0.4 mg sublingual) condition. Administration of GTN resulted in a 12% increase in resting brachial artery diameter that persisted throughout the duration of exercise (CON: 0.50 ± 0.01 cm; GTN: 0.56 ± 0.01 cm, p < 0.05). Resting FBF was greater following GTN administration compared to control (p < 0.05); however, differences in FBF disappeared following the onset of muscle contractions. Our results indicate that the matching of FBF to oxygen demand during exercise is not affected by prior vasodilation, so that any over‐perfusion is corrected at the onset of exercise. Additionally, our findings provide further evidence that the regulation of vascular tone within the microvasculature is independent of the conduit artery diameter.

Regulation of muscle potassium: exercise performance, fatigue and health implications

This review integrates from the single muscle fibre to exercising human the current understanding of the role of skeletal muscle for whole-body potassium (K⁺) regulation, and specifically the regulation of skeletal muscle [K⁺]. We describe the K⁺ transport proteins in skeletal muscle and how they contribute to, or modulate, K⁺ disturbances during exercise. Muscle and plasma K⁺ balance are markedly altered during and after high-intensity dynamic exercise (including sports), static contractions and ischaemia, which have implications for skeletal and cardiac muscle contractile performance. Moderate elevations of plasma and interstitial [K⁺] during exercise have beneficial effects on multiple physiological systems. Severe reductions of the trans-sarcolemmal K⁺ gradient likely contributes to muscle and whole-body fatigue, i.e. impaired exercise performance. Chronic or acute changes of arterial plasma [K⁺] (hyperkalaemia or hypokalaemia) have dangerous health implications for cardiac function. The current mechanisms to explain how raised extracellular [K⁺] impairs cardiac and skeletal muscle function are discussed, along with the latest cell physiology research explaining how calcium, β-adrenergic agonists, insulin or glucose act as clinical treatments for hyperkalaemia to protect the heart and skeletal muscle in vivo. Finally, whether these agents can also modulate K⁺-induced muscle fatigue are evaluated.

KIR channel activation links local vasodilatation with muscle fibre recruitment during exercise in humans

KEY POINTS

During exercise, blood flow to working skeletal muscle increases in parallel with contractile activity such that oxygen delivery is sufficient to meet metabolic demand. K⁺ released from active skeletal muscle fibres could facilitate vasodilatation in proportion to the degree of muscle fibre recruitment. Once released, K⁺ stimulates inwardly rectifying K⁺ (K_IR ) channels on the vasculature to elicit an increase in blood flow. In the present study, we demonstrate that K_IR channels mediate the rapid vasodilatory response to an increase in exercise intensity. We also show that K_IR channels augment vasodilatation during exercise which demands greater muscle fibre recruitment independent of the total amount of work performed. These results suggest that K⁺ plays a key role in coupling the magnitude of vasodilatation to the degree of contractile activity. Ultimately, the findings from this study help us understand the signalling mechanisms that regulate muscle blood flow in humans.

ABSTRACT

Blood flow to active skeletal muscle is augmented with greater muscle fibre recruitment. We tested whether activation of inwardly rectifying potassium (K_IR ) channels underlies vasodilatation with elevated muscle fibre recruitment when work rate is increased (Protocol 1) or held constant (Protocol 2). We assessed forearm vascular conductance (FVC) during rhythmic handgrip exercise under control conditions and during local inhibition of K_IR channels (intra-arterial BaCl₂ ). In Protocol 1, healthy volunteers performed mild handgrip exercise for 3 min, then transitioned to moderate intensity for 30 s. BaCl₂ eliminated vasodilatation during the first contraction at the moderate workload (ΔFVC, BaCl₂ : -1 ± 17 vs. control: 30 ± 28 ml min^-1  100 mmHg^-1 ; n = 9; P = 0.004) and attenuated the 30 s area under the curve by 56 ± 14% (n = 9; P < 0.0001). In Protocol 2, participants performed two exercise bouts in which muscle fibre recruitment was manipulated while total contractile work was held constant via reciprocal changes in contraction frequency: (1) low fibre recruitment, with contractions at 12.5% maximal voluntary contraction once every 4 s and (2) high fibre recruitment, with contractions at 25% maximal voluntary contraction once every 8 s. Under control conditions, steady-state FVC was augmented in high vs. low fibre recruitment (211 ± 90 vs. 166 ± 73 ml min^-1 ⋅100 mmHg^-1 ; n = 10; P = 0.0006), whereas BaCl₂ abolished the difference between high and low fibre recruitment (134 ± 59 vs. 134 ± 63 ml min^-1  100 mmHg^-1 ; n = 10; P = 0.85). These findings demonstrate that K_IR channel activation is a key mechanism linking local vasodilatation with muscle fibre recruitment during exercise.

Sustained exercise hyperemia during prolonged adenosine infusion in humans

The contribution of Adenosine (ADO) to exercise hyperemia remains controversial and it is unknown whether ADO can evoke the prolonged vasodilation seen during exercise bouts. Therefore, we tested hypotheses in the human forearm during 3 h of intra-arterial high dose ADO infusion: (1) skeletal muscle blood flow would wane over time; (2) exercise hyperemic responses during ADO administration would be unaffected compared to baseline. Using sodium nitroprusside (SNP), we tested parallel hypotheses regarding nitric oxide (NO) in a separate group of participants. Seventeen young healthy participants (ADO: n = 9; SNP: n = 8) performed multiple rhythmic handgrip exercise bouts (20% of maximum), two during saline and five during 3 h of continuous drug infusion. Five minutes of ADO infusion resulted in a ~5-fold increase in forearm vascular conductance (FVC; 4.8 ± 0.6 vs. 24.2 ± 3.2 mL/min/100 mmHg, P < 0.05). SNP caused a ~4-fold increase (4.4 ± 0.6 vs. 16.6 ± 2 mL/min/100 mmHg, P < 0.05). FVC did not wane over time with ADO (24.2 ± 3.2 and 22 ± 1.2 mL/min/100 mmHg [P > 0.05]) or SNP (16.6 ± 2 and 14.1 ± 2.4 mL/min/ 100 mmHg [P > 0.05]) at 5 versus 150 min. Superimposed exercise during ADO or SNP infusions evoked marked and consistent additional dilation over the course of the infusions. Our findings demonstrate that in humans there is no reduction in endothelial or vascular smooth muscle responsiveness to the exogenous vasodilatory metabolites ADO and NO. Additionally, even in the presence of an exogenous vasodilator, superimposed exercise can cause significant hyperemia.