Reduced arterial vasodilatation in response to hypoxia impairs cerebral and peripheral oxygen delivery in hypertensive men

The effect of hypoxemia on muscle sympathetic nerve activity and cardiovascular function: a systematic review and meta-analysis

We conducted a systematic review and meta-analysis to determine the effect of acute poikilocapnic, high-altitude, and acute isocapnia hypoxemia on muscle sympathetic nerve activity (MSNA) and cardiovascular function. A comprehensive search across electronic databases was performed until June 2021. All observational designs were included: population (healthy individuals); exposures (MSNA during hypoxemia); comparators (hypoxemia severity and duration); outcomes (MSNA; heart rate, HR; and mean arterial pressure, MAP). Sixty-one studies were included in the meta-analysis. MSNA burst frequency increased by a greater extent during high-altitude hypoxemia [P < 0.001; mean difference (MD), +22.5 bursts/min; confidence interval (CI) = -19.20 to 25.84] compared with acute poikilocapnic hypoxemia (P < 0.001; MD, +5.63 bursts/min; CI = -4.09 to 7.17) and isocapnic hypoxemia (P < 0.001; MD, +4.72 bursts/min; CI = -3.37 to 6.07). MSNA burst amplitude was only elevated during acute isocapnic hypoxemia (P = 0.03; standard MD, +0.46 au; CI = -0.03 to 0.90), and MSNA burst incidence was only elevated during high-altitude hypoxemia [P < 0.001; MD, 33.05 bursts/100 heartbeats; CI = -28.59 to 37.51]. Meta-regression analysis indicated a strong relationship between MSNA burst frequency and hypoxemia severity for acute isocapnic studies (P < 0.001) but not acute poikilocapnia (P = 0.098). HR increased by the same extent across each type of hypoxemia [P < 0.001; MD +13.81 heartbeats/min; 95% CI = 12.59-15.03]. MAP increased during high-altitude hypoxemia (P < 0.001; MD, +5.06 mmHg; CI = 3.14-6.99), and acute isocapnic hypoxemia (P < 0.001; MD, +1.91 mmHg; CI = 0.84-2.97), but not during acute poikilocapnic hypoxemia (P = 0.95). Both hypoxemia type and severity influenced sympathetic nerve and cardiovascular function. These data are important for the better understanding of healthy human adaptation to hypoxemia.

Recent insights into mechanisms of hypoxia-induced vasodilatation in the human brain

The cerebral vasculature manages oxygen delivery by adjusting arterial blood in-flow in the face of reductions in oxygen availability. Hypoxic cerebral vasodilatation, and the associated hypoxic cerebral blood flow reactivity, involve many vascular, erythrocytic and cerebral tissue mechanisms that mediate elevations in cerebral blood flow via micro- and macrovascular dilatation. This contemporary review focuses on in vivo human work - with reference to seminal preclinical work where necessary - on hypoxic cerebrovascular reactivity, particularly where recent advancements have been made. We provide updates with the following information: in humans, hypoxic cerebral vasodilatation is partially mediated via a - likely non-obligatory - combination of: (1) nitric oxide synthases, (2) deoxygenation-coupled S-nitrosothiols, (3) potassium channel-related vascular smooth muscle hyperpolarization, and (4) prostaglandin mechanisms with some contribution from an interrelationship with reactive oxygen species. And finally, we discuss the fact that, due to the engagement of deoxyhaemoglobin-related mechanisms, reductions in O2 content via haemoglobin per se seem to account for ∼50% of that seen with hypoxic cerebral vasodilatation during hypoxaemia. We further highlight the issue that methodological impediments challenge the complete elucidation of hypoxic cerebral reactivity mechanisms in vivo in healthy humans. Future research is needed to confirm recent advancements and to reconcile human and animal findings. Further investigations are also required to extend these findings to address questions of sex-, heredity-, age-, and disease-related differences. The final step is to then ultimately translate understanding of these mechanisms into actionable, targetable pathways for the prevention and treatment of cerebral vascular dysfunction and cerebral hypoxic brain injury.

Reactive oxygen species play a modulatory role in the hyperventilatory response to poikilocapnic hyperoxia in humans

KEY POINTS

The proposed mechanism for the increased ventilation in response to hyperoxia includes a reduced brain CO₂ -[H⁺ ] washout-induced central chemoreceptor stimulation that results from a decrease in cerebral perfusion and the weakening of the CO₂ affinity for haemoglobin. Nonetheless, hyperoxia also results in excessive brain reactive oxygen species (ROS) formation/accumulation, which hypothetically increases central respiratory drive and causes hyperventilation. We then quantified ventilation, cerebral perfusion/metabolism, arterial/internal jugular vein blood gases and oxidant/antioxidant biomarkers in response to hyperoxia during intravenous infusion of saline or ascorbic acid to determine whether excessive ROS production/accumulation contributes to the hyperoxia-induced hyperventilation in humans. Ascorbic acid infusion augmented the antioxidant defence levels, blunted ROS production/accumulation and minimized both the reduction in cerebral perfusion and the increase in ventilation observed during saline infusion. Hyperoxic hyperventilation seems to be mediated by central chemoreceptor stimulation provoked by the interaction between an excessive ROS production/accumulation and reduced brain CO₂ -[H⁺ ] washout.

ABSTRACT

The hypothetical mechanism for the increase in ventilation ( V ̇ E ) in response to hyperoxia (HX) includes central chemoreceptor stimulation via reduced CO₂ -[H⁺ ] washout. Nonetheless, hyperoxia disturbs redox homeostasis and raises the hypothesis that excessive brain reactive oxygen species (ROS) production/accumulation may increase the sensitivity to CO₂ or even solely activate the central chemoreceptors, resulting in hyperventilation. To determine the mechanism behind the HX-evoked increase in V ̇ E , 10 healthy men (24 ± 4 years) underwent 10 min trials of HX under saline and ascorbic acid infusion. V ̇ E , arterial and right internal right jugular vein (ijv) partial pressure for oxygen (PO₂ ) and CO₂ (PCO₂ ), pH, oxidant (8-isoprostane) and antioxidant (ascorbic acid) markers, as well as cerebral blood flow (CBF) (Duplex ultrasonography), were quantified at each hyperoxic trial. HX evoked an increase in arterial partial pressure for oxygen, followed by a hyperventilatory response, a reduction in CBF, an increase in arterial 8-isoprostane, and unchanged PijvCO₂ and ijv pH. Intravenous ascorbic acid infusion augmented the arterial antioxidant marker, blunted the increase in arterial 8-isoprostane and attenuated both the reduction in CBF and the HX-induced hyperventilation. Although ascorbic acid infusion resulted in a slight increase in PijvCO₂ and a substantial decrease in ijv pH, when compared with the saline bout, HX evoked a similar reduction and a paired increase in the trans-cerebral exchanges for PCO₂ and pH, respectively. These findings indicate that the poikilocapnic hyperoxic hyperventilation is likely mediated via the interaction of the acidic brain interstitial fluid and an increase in central chemoreceptor sensitivity to CO₂ , which, in turn, seems to be evoked by the excessive ROS production/accumulation.

Regional differences in cerebrovascular reactivity in response to acute isocapnic hypoxia in healthy humans: Methodological considerations

The cerebrovasculature responds to blood gas challenges. Regional differences (anterior vs. posterior) in cerebrovascular responses to increases in CO₂ have been extensively studied. However, regional cerebrovascular reactivity (CVR) responses to low O₂ (hypoxia) are equivocal, likely due to differences in analysis. We assessed the effects of acute isocapnic hypoxia on regional CVR comparing absolute and relative (%-change) responses in the middle cerebral artery (MCA) and posterior cerebral artery (PCA). We instrumented 14 healthy participants with a transcranial Doppler ultrasound (cerebral blood velocity), finometer (beat-by-beat blood pressure), dual gas analyzer (end-tidal CO₂ and O₂), and utilized a dynamic end-tidal forcing system to elicit a single 5-min bout of isocapnic hypoxia (∼45 Torr P_ETO₂, ∼80 % SpO₂). During exposure to acute hypoxia, absolute responses were larger in the anterior compared to posterior cerebral circulation (P < 0.001), but were not different when comparing relative responses (P = 0.45). Consistent reporting of CVR to hypoxia will aid understanding normative responses, particularly in assessing populations with impaired cerebrovascular function.

Bilateral regional extracranial blood flow regulation to hypoxia and unilateral duplex ultrasound measurement error

NEW FINDINGS

What is the central question of this study? Is blood flow regulation to hypoxia different between the internal carotid arteries (ICAs) and vertebral arteries (VAs), and what is the measurement error in unilateral extracranial artery assessments compared to bilateral? What is the main finding and its importance? ICA and VA blood flow regulation to hypoxia is comparable when factoring for vessel type and vessel side. Compared to bilateral assessment, vessels assessed unilaterally had individual measurement errors of up to 37%. Assessing the vessel with the larger resting blood flow, not the left or right vessel, reduces unilateral measurement error.

ABSTRACT

Whether blood flow regulation to hypoxia is similar between left and right internal carotid arteries (ICAs) and vertebral arteries (VAs) is unclear. Extracranial blood flow is regularly calculated by doubling a unilateral assessment; however, lateral artery differences may lead to measurement error. This study aimed to determine extracranial blood flow regulation to hypoxia when factoring for vessel type (ICAs or VAs) and vessel side (left or right) effects, and to investigate unilateral assessment measurement error compared to bilateral assessment. In a repeated-measures crossover design, extracranial arteries of 44 participants were assessed bilaterally by duplex ultrasound during 90 min of normoxic and poikilocapnic hypoxic (12.0% fraction of inspired oxygen) conditions. Linear mixed model analyses revealed no Condition × Vessel Type × Vessel Side interaction for blood flow, vessel diameter and flow velocity (all P > 0.05) indicating left and right ICA and VA blood flow regulation to hypoxia was similar. Bilateral hypoxic reactivity was comparable (ICAs, 1.4 (1.0) vs. VAs, 1.7 (1.1) Δ%·Δ S p O 2 ^-1 ; P = 0.12). Compared to bilateral assessment, unilateral mean measurement error of the relative blood flow response to hypoxia was up to 5%, but individual errors reached 37% and were greatest in ICAs and VAs with the smaller resting blood flow due to a ratio-scaling problem. In conclusion, left and right ICA and VA regulation to hypoxia is comparable when factoring for vessel type and vessel side. Assessing the ICA and VA vessels with the larger resting blood flow, not the left or right vessel, reduces unilateral measurement error.

Hypertension impairs hypoxia-induced angiogenesis in men

OBJECTIVE

The inability of the organism to appropriately respond to hypoxia results in abnormal cell metabolism and function. Hypoxia-induced angiogenesis seems to be suppressed in experimental models of hypertension; however, this hypothesis has not been tested in humans. We examined changes in endothelial biomarkers and vascular chemoattraction/angiogenic capacity in response to isocapnic hypoxia in hypertensive men.

METHODS

Twelve normotensive (38 ± 10 years) and nine hypertensive men (45 ± 11 years) were exposed to 5-min trials of normoxia (21% O2) and isocapnic hypoxia (10% O2). During the last minute of each trial, venous blood was drawn. Endothelial progenitor cells (EPCs; CD45/CD34/VEGFR2), endothelial microvesicles (apoptotic EMVs, CD42b/CD31/AnnexinV; endothelial activation, CD62E/CD144), nitrite, vascular endothelial growth factor (VEGF), and stromal cell-derived factor 1 (SDF-1) were measured.

RESULTS

During normoxia, EPCs, nitrite, endothelial activation, and SDF-1 were similar between groups, whereas VEGF was lower (P = 0.02) and apoptotic EMVs tended to increase (P = 0.07) in hypertensive men. During isocapnic hypoxia, endothelial activation increased in both groups (normotensive, P = 0.007 vs. normoxia; hypertensive, P = 0.006 vs. normoxia), whereas EMVs were higher only in the hypertensive group (P = 0.03 vs. normotensive). EPCs (P = 0.01 vs. normoxia; P = 0.03 vs. hypertensive men), NO (P = 0.01 vs. normoxia; P = 0.04 vs. hypertensive), and VEGF (P = 0.02 vs. normoxia; P = 0.0005 vs. hypertensive) increased only in normotensive individuals in response to isocapnic hypoxia. SDF-1 did not change in either group.

CONCLUSION

These results suggest that hypertension-induced impairment in angiogenesis in response to isocapnic hypoxia is related to disrupted NO bioavailability, VEGF chemotactic signaling, and EPC mobilization.

Non-invasive hemodynamic profile of early COVID-19 infection

INTRODUCTION

Little is known about the systemic and pulmonary macrohemodynamics in early COVID-19 infection. Echocardiography may provide useful insights into COVID-19 physiopathology.

METHODS

Twenty-three COVID-19 patients were enrolled in a medical ward. Gas exchange, transthoracic echocardiographic, and hemodynamic variables were collected.

RESULTS

Mean age was 57 ± 17 years. The patients were hypoxemic (PaO2 /FiO2  = 273.0 ± 102.6 mmHg) and mildly hypocapnic (PaCO2  = 36.2 ± 6.3 mmHg, pH = 7.45 ± 0.03). Mean arterial pressure was decreased (86.7 [80.0-88.3] mmHg). Cardiac index was elevated (4.32 ± 0.90 L∙min-1 ∙m-2 ) and the resulting systemic vascular resistance index low (1,458 [1358-1664] dyn∙s∙cm-5 ∙m-2 ). The right heart was morphologically and functionally normal, with pulmonary artery pressure (PAPm, 18.0 ± 2.9 mmHg) and Total Pulmonary Resistances (TPR, 2.3 [2.1-2.7] mmHg∙l-1 ∙min-1 ) within normal limits. When stratifying for SVRI, patients with an SVRI value below the cohort median had also more severe oxygenation impairment and lower TPR, despite a similar degree of CXR infiltrates. Oxygen delivery index in this group resulted supranormal.

CONCLUSIONS

In the early stages of COVID-19 infection the hemodynamic profile is characterized by a hyperdynamic circulatory state with high CI and low SVRI, while the right heart is functionally unaffected. Our findings suggest that hypoxemia, viral sepsis or peripheral shunting are possible mechanisms for the vasodilation that dominates at this stage of the disease and may itself worsen the gas exchange.

Neurovascular coupling is not influenced by lower body negative pressure in humans

Cerebral blood flow is tightly coupled with local neuronal activation and metabolism, i.e., neurovascular coupling (NVC). Studies suggest a role of sympathetic nervous system in the regulation of cerebral blood flow. However, this is controversial, and the sympathetic regulation of NVC in humans remains unclear. Since impaired NVC has been identified in several chronic diseases associated with a heightened sympathetic activity, we aimed to determine whether reflex-mediated sympathetic activation via lower body negative pressure (LBNP) attenuates NVC in humans. NVC was assessed using a visual stimulation protocol (5 cycles of 30 s eyes closed and 30 s of reading) in 11 healthy participants (aged 24 ± 3 yr). NVC assessments were made under control conditions and during LBNP at -20 and -40 mmHg. Posterior (PCA) and middle (MCA) cerebral artery mean blood velocity (V_mean) and vertebral artery blood flow (VA_flow) were simultaneously determined with cardiorespiratory variables. Under control conditions, the visual stimulation evoked a robust increase in PCA_Vmean (∆18.0 ± 4.5%), a moderate rise in VA_flow (∆9.6 ± 4.3%), and a modest increase in MCA_Vmean (∆3.0 ± 1.9%). The magnitude of NVC response was not affected by mild-to-moderate LBNP (all P > 0.05 for repeated-measures ANOVA). Given the small change that occurred in partial pressure of end-tidal CO₂ during LBNP, this hypocapnia condition was matched via voluntary hyperventilation in absence of LBNP in a subgroup of participants (n = 8). The mild hypocapnia during LBNP did not exert a confounding influence on the NVC response. These findings indicate that the NVC is not influenced by LBNP or mild hypocapnia in humans.NEW & NOTEWORTHY Visual stimulation evoked a robust increase in posterior cerebral artery velocity and a modest increase in vertebral artery blood flow, i.e., neurovascular coupling (NVC), which was unaffected by lower body negative pressure (LBNP) in humans. In addition, although LBNP induced a mild hypocapnia, this degree of hypocapnia in the absence of LBNP failed to modify the NVC response.

KATP channels modulate cerebral blood flow and oxygen delivery during isocapnic hypoxia in humans

KEY POINTS

ATP-sensitive K⁺ (K_ATP ) channels mediate hypoxia-induced cerebral vasodilatation and hyperperfusion in animals. We tested whether K_ATP channels blockade affects the increase in human cerebral blood flow (CBF) and the maintenance of oxygen delivery (CDO₂ ) during hypoxia. Hypoxia-induced increases in the anterior circulation and total cerebral perfusion were attenuated under K_ATP channels blockade affecting the relative changes of brain oxygen delivery. Therefore, in humans, K_ATP channels activation modulates the vascular tone in the anterior circulation of the brain, contributing to CBF and CDO₂ responses to hypoxia.

ABSTRACT

ATP-sensitive K⁺ (K_ATP ) channels mediate hypoxia-induced cerebral vasodilatation and hyperperfusion in animals. We tested whether K_ATP channels blockade affects the increase in cerebral blood flow (CBF) and the maintenance of oxygen delivery (CDO₂ ) during hypoxia in humans. Nine healthy men were exposed to 5-min trials of normoxia and isocapnic hypoxia (IHX, 10% O₂ ) before (BGB) and 3 h after glibenclamide ingestion (AGB). Mean arterial pressure (MAP), arterial saturation ( S a O 2 ), partial pressure of oxygen ( P a O 2 ) and carbon dioxide ( P aC O 2 ), internal carotid artery blood flow (ICABF), vertebral artery blood flow (VABF), total (t)CBF (Doppler ultrasound) and CDO₂ were quantified during the trials. IHX provoked similar reductions in S a O 2 and P a O 2 , while MAP was not affected by oxygen desaturation or K_ATP blockade. A smaller increase in ICABF (ΔBGB: 36 ± 23 vs. ΔAGB 11 ± 18%, p = 0.019) but not in VABF (∆BGB 26 ± 21 vs. ∆AGB 27 ± 27%, p = 0.893) was observed during the hypoxic trial under K_ATP channels blockade. Thus, IHX-induced increases in tCBF (∆BGB 32 ± 19 vs. ∆AGB 14 ± 13%, p = 0.012) and CDO₂ relative changes (∆BGB 7 ± 13 vs. ∆AGB -6 ± 14%, p = 0.048) were attenuated during the AGB hypoxic trial. In a separate protocol, 6 healthy men (5 from protocol 1) underwent a 5-min exposure to normoxia and IHX before and 3 h after placebo (5 mg of cornstarch) ingestion. IHX reduced S a O 2 and P a O 2 , but placebo did not affect the ICABF, VABF, tCBF, or CDO₂ responses. Therefore, in humans, K_ATP channels activation modulates vascular tone in the anterior rather than the posterior circulation of the brain, contributing to tCBF and CDO₂ responses to hypoxia.

Differential vasomotor responses to isocapnic hyperoxia: cerebral versus peripheral circulation

Isocapnic hyperoxia (IH) evokes cerebral and peripheral hypoperfusion via both disturbance of redox homeostasis and reduction in nitric oxide (NO) bioavailability. However, it is not clear whether the magnitude of the vasomotor responses depends on the vessel network exposed to IH. To test the hypothesis that the magnitude of IH-induced reduction in peripheral blood flow (BF) may differ from the hypoperfusion response observed in the cerebral vascular network under oxygen-enriched conditions, nine healthy men (25 ± 3 yr, mean ± SD) underwent 10 min of IH during either saline or vitamin C (3 g) infusion, separately. Femoral artery (FA), internal carotid artery (ICA), and vertebral artery (VA) BF (Doppler ultrasound), as well as arterial oxidant (8-isoprostane), antioxidant [ascorbic acid (AA)], and NO bioavailability (nitrite) markers were simultaneously measured. IH increased 8-isoprostane levels and reduced nitrite levels; these responses were followed by a reduction in both FA BF and ICA BF, whereas VA BF did not change. Absolute and relative reductions in FA BF were greater than IH-induced changes in ICA and VA perfusion. Vitamin C infusion increased arterial AA levels and abolished the IH-induced increase in 8-isoprostane levels and reduction in nitrite levels. Whereas ICA and VA BF did not change during the vitamin C-IH trial, FA perfusion increased and reached similar levels to those observed during normoxia with saline infusion. Therefore, the magnitude of IH-induced reduction in femoral blood flow is greater than that observed in the vessel network of the brain, which might involve the determinant contribution that NO has in the regulation of peripheral vascular perfusion.

Acid-sensing ion channels blockade attenuates pressor and sympathetic responses to skeletal muscle metaboreflex activation in humans

In animals, the blockade of acid-sensing ion channels (ASICs), cation pore-forming membrane proteins located in the free nerve endings of group IV afferent fibers, attenuates increases in arterial pressure (AP) and sympathetic nerve activity (SNA) during muscle contraction. Therefore, ASICs play a role in mediating the metabolic component (skeletal muscle metaboreflex) of the exercise pressor reflex in animal models. Here we tested the hypothesis that ASICs also play a role in evoking the skeletal muscle metaboreflex in humans, quantifying beat-by-beat mean AP (MAP; finger photoplethysmography) and muscle SNA (MSNA; microneurography) in 11 men at rest and during static handgrip exercise (SHG; 35% of the maximal voluntary contraction) and postexercise muscle ischemia (PEMI) before (B) and after (A) local venous infusion of either saline or amiloride (AM), an ASIC antagonist, via the Bier block technique. MAP (BAM +30 ± 6 vs. AAM +25 ± 7 mmHg, P = 0.001) and MSNA (BAM +14 ± 9 vs. AAM +10 ± 6 bursts/min, P = 0.004) responses to SHG were attenuated under ASIC blockade. Amiloride also attenuated the PEMI-induced increases in MAP (BAM +25 ± 6 vs. AAM +16 ± 6 mmHg, P = 0.0001) and MSNA (BAM +16 ± 9 vs. AAM +8 ± 8 bursts/min, P = 0.0001). MAP and MSNA responses to SHG and PEMI were similar before and after saline infusion. We conclude that ASICs play a role in evoking pressor and sympathetic responses to SHG and the isolated activation of the skeletal muscle metaboreflex in humans. NEW & NOTEWORTHY We showed that regional blockade of the acid-sensing ion channels (ASICs), induced by venous infusion of the antagonist amiloride via the Bier block anesthetic technique, attenuated increases in arterial pressure and muscle sympathetic nerve activity during both static handgrip exercise and postexercise muscle ischemia. These findings indicate that ASICs contribute to both pressor and sympathetic responses to the activation of the skeletal muscle metaboreflex in humans.

GABAA receptors modulate sympathetic vasomotor outflow and the pressor response to skeletal muscle metaboreflex activation in humans

KEY POINTS

The activation of the group III/IV skeletal muscle afferents is one of the principal mediators of cardiovascular responses to exercise; however, the neuronal circuitry mechanisms that are involved during the activation of group III/IV muscle afferents in humans remain unknown. Recently, we showed that GABAergic mechanisms are involved in the cardiac vagal withdrawal during the activation of mechanically sensitive (predominantly mediated by group III fibres) skeletal muscle afferents in humans. In the present study, we found that increases in muscle sympathetic nerve activity and mean blood pressure during isometric handgrip exercise and postexercise ischaemia were significantly greater after the oral administration of diazepam, a benzodiazepine that increases GABA_A activity, but not after placebo administration in young healthy subjects. These findings indicate for the first time that GABA_A receptors modulate sympathetic vasomotor outflow and the pressor responses to activation of metabolically sensitive (predominantly mediated by group IV fibres) skeletal muscle afferents in humans.

ABSTRACT

Animal studies have indicated that GABA_A receptors are involved in the neuronal circuitry of the group III/IV skeletal muscle afferent activation-induced neurocardiovascular responses to exercise. In the present study, we aimed to determine whether GABA_A receptors modulate the neurocardiovascular responses to activation of metabolically sensitive (predominantly mediated by group IV fibres) skeletal muscle afferents in humans. In a randomized, double-blinded, placebo-controlled and cross-over design, 17 healthy subjects (eight women) performed 2 min of ischaemic isometric handgrip exercise at 30% of the maximal voluntary contraction followed by 2 min of postexercise ischaemia (PEI). Muscle sympathetic nerve activity (MSNA), blood pressure (BP) and heart rate (HR) were continuously measured and trials were conducted before and 60 min after the oral administration of either placebo or diazepam (10 mg), a benzodiazepine that enhances GABA_A activity. At rest, MSNA was reduced, whereas HR and BP did not change after diazepam administration. During ischaemic isometric handgrip, greater MSNA (pre: ∆13 ± 9 bursts min^-1 vs. post: ∆29 ± 15 bursts min^-1 , P < 0.001), HR (pre: ∆23 ± 11 beats min^-1 vs. post: ∆31 ± 17 beats min^-1 , P < 0.01) and mean BP (pre: ∆33 ± 12 mmHg vs. post: ∆37 ± 12 mmHg, P < 0.01) responses were observed after diazepam. During PEI, MSNA and mean BP remained elevated from baseline before diazepam (∆10 ± 8 bursts min^-1 and ∆25 ± 14 mmHg, respectively) and these elevations were increased after diazepam (∆17 ± 12 bursts min^-1 and ∆28 ± 13 mmHg, respectively) (P ≤ 0.05). Importantly, placebo pill had no effect on neural, cardiac and pressor responses. These findings demonstrate for the first time that GABA_A receptors modulate MSNA and the pressor responses to skeletal muscle metaboreflex activation in humans.

The age-related reduction in cerebral blood flow affects vertebral artery more than internal carotid artery blood flow

Ageing reduces cerebral blood flow (CBF), while mean arterial pressure (MAP) becomes elevated. According to 'the selfish brain' hypothesis of hypertension, a reduction in vertebral artery blood flow (VA) leads to increased sympathetic activity and thus increases MAP. In twenty-two young (24 ± 3 years; mean ± SD) and eleven elderly (70 ± 5 years) normotensive men, duplex ultrasound evaluated whether the age-related reduction in CBF affects VA more than internal carotid artery (ICA) blood flow. Pulse-contour analysis evaluated MAP while near-infrared spectroscopy determined frontal lobe oxygenation and transcranial Doppler middle cerebral artery mean blood velocity (MCA V_mean ). During supine rest, MAP (90 ± 13 versus 78 ± 9 mmHg; P<0·001) was elevated in the older subjects while their frontal lobe oxygenation (68 ± 7% versus 77 ± 7%; P<0·001), MCA V_mean (49 ± 9 versus 60 ± 12 cm s^-1 ; P = 0·016) and CBF (754 ± 112 versus 900 ± 144 ml min^-1 ; P = 0·004) were low reflected in VA (138 ± 48 versus 219 ± 50 ml min^-1 ; P<0·001) rather than in ICA flow (616 ± 96 versus 680 ± 120 ml min^-1 ; P = 0·099). In conclusion, blood supply to the brain and its oxygenation are affected by ageing and the age-related decline in VA flow appears to be four times as large as that in ICA and could be important for the age-related increase in MAP.

Muscle sympathetic nerve activity and hemodynamic responses to venous distension: does sex play a role?

Peripheral venous distension mechanically stimulates type III/IV sensory fibers in veins and evokes pressor and sympathoexcitatory reflex responses in humans. As young women have reduced venous compliance and impaired sympathetic transduction, we tested the hypothesis that pressor and sympathoexcitatory responses to venous distension may be attenuated in women compared with men. Mean arterial pressure (photoplethysmography), heart rate (HR), stroke volume (SV; Modelflow), cardiac output (CO = HR × SV), muscle sympathetic nerve activity (MSNA), femoral artery blood flow, and femoral artery conductance (Doppler ultrasound) were quantified in eight men (27 ± 4 yr) and nine women (28 ± 4 yr) before [control (CON)], during (INF), and immediately after (post-INF) a local infusion of saline [5% of the total forearm volume (30 ml/min); the infusion time was 2 ± 1 and 1 ± 1 min ( P = 0.0001) for men and women, respectively] through a retrograde catheter inserted into an antecubital vein, to which venous drainage and arterial supply had been occluded. Mean arterial pressure increased during and after infusion in both groups (vs. the CON group, P < 0.05), but women showed a smaller pressor response in the post-INF period (Δ+7.2 ± 2.0 vs. Δ+18.3 ± 3.9 mmHg in men, P = 0.019). MSNA increased and femoral artery conductance decreased similarly in both groups (vs. the CON group, P < 0.05) at post-INF. Although HR changes were similar, increases in SV (Δ+20.4 ± 8.6 vs. Δ+2.6 ± 2.7 ml, P = 0.05) and CO (Δ+0.84 ± 0.17 vs. Δ+0.34 ± 0.10 l/min, P = 0.024) were greater in men compared with women. Therefore, venous distension evokes a smaller pressor response in young women due to attenuated cardiac adjustments rather than reduced venous compliance or sympathetic transduction. NEW & NOTEWORTHY We found that the pressor response to venous distension was attenuated in young women compared with age-matched men. This was due to attenuated cardiac adjustments rather than reduced venous compliance, sympathetic activation, or impaired transduction and vascular control. Collectively, these findings suggest that an attenuated venous distension reflex could be involved in orthostatic intolerance in young women.

Human brain blood flow and metabolism during isocapnic hyperoxia: the role of reactive oxygen species

KEY POINTS

It is unknown whether excessive reactive oxygen species (ROS) production drives the isocapnic hyperoxia (IH)-induced decline in human cerebral blood flow (CBF) via reduced nitric oxide (NO) bioavailability and leads to disruption of the blood-brain barrier (BBB) or neural-parenchymal damage. Cerebral metabolic rate for oxygen (CMR O 2 ) and transcerebral exchanges of NO end-products, oxidants, antioxidants and neural-parenchymal damage markers were simultaneously quantified under IH with intravenous saline and ascorbic acid infusion. CBF and CMR O 2 were reduced during IH, responses that were followed by increased oxidative stress and reduced NO bioavailability when saline was infused. No indication of neural-parenchymal damage or disruption of the BBB was observed during IH. Antioxidant defences were increased during ascorbic acid infusion, while CBF, CMR O 2 , oxidant and NO bioavailability markers remained unchanged. ROS play a role in the regulation of CBF and metabolism during IH without evidence of BBB disruption or neural-parenchymal damage.

ABSTRACT

To test the hypothesis that isocapnic hyperoxia (IH) affects cerebral blood flow (CBF) and metabolism through exaggerated reactive oxygen species (ROS) production, reduced nitric oxide (NO) bioavailability, disturbances in the blood-brain barrier (BBB) and neural-parenchymal homeostasis, 10 men (24 ± 1 years) were exposed to a 10 min IH trial (100% O₂ ) while receiving intravenous saline and ascorbic acid (AA, 3 g) infusion. Internal carotid artery blood flow (ICABF), vertebral artery blood flow (VABF) and total CBF (tCBF, Doppler ultrasound) were determined. Arterial and right internal jugular venous blood was sampled to quantify the cerebral metabolic rate of oxygen (CMR O 2 ), transcerebral exchanges (TCE) of NO end-products (plasma nitrite), antioxidants (AA and AA plus dehydroascorbic acid (AA+DA)) and oxidant biomarkers (thiobarbituric acid-reactive substances (TBARS) and 8-isoprostane), and an index of BBB disruption and neuronal-parenchymal damage (neuron-specific enolase; NSE). IH reduced ICABF, tCBF and CMR O 2 , while VABF remained unchanged. Arterial 8-isoprostane and nitrite TCE increased, indicating that CBF decline was related to ROS production and reduced NO bioavailability. AA, AA+DA and NSE TCE did not change during IH. AA infusion did not change the resting haemodynamic and metabolic parameters but raised antioxidant defences, as indicated by increased AA/AA+DA concentrations. Negative AA+DA TCE, unchanged nitrite, reductions in arterial and venous 8-isoprostane, and TBARS TCE indicated that AA infusion effectively inhibited ROS production and preserved NO bioavailability. Similarly, AA infusion prevented IH-induced decline in regional and total CBF and re-established CMR O 2 . These findings indicate that ROS play a role in CBF regulation and metabolism during IH without evidence of BBB disruption or neural-parenchymal damage.

Absent increase in vertebral artery blood flow during l-arginine infusion in hypertensive men

Endothelial dysfunction is observed in the peripheral vasculature of hypertensive patients, but it is unclear how the cerebral circulation is affected. More specifically, little is known about the impact of human hypertension on vertebral artery (VA) endothelial function. This study evaluated whether the endothelial function of the VA is impaired in hypertensive men. For 13 male hypertensive subjects (46 ± 3 yr) and eight age-matched male controls (46 ± 4 yr), blood pressure (BP; photoplethysmography), VA, and common carotid (CC) blood flow (duplex ultrasound) were determined at rest and during 30 min of intravenous l-arginine (30 g; a precursor of nitric oxide) or isotonic saline infusion. Controls and hypertensive subjects demonstrated a similar resting CC (601 ± 30 vs. controls 570 ± 43 ml/min; P = 0.529) and VA blood flow (119 ± 11 vs. controls 112 ± 9 ml/min; P = 0.878). During administration of l-arginine, CC blood flow increased similarly between groups (hypertensive 12 ± 3%, controls 13 ± 2%; P = 0.920). In contrast, the increase in VA blood flow was nonexistent in the hypertensive subjects (0.8 ± 3% vs. controls: 16 ± 4%; P = 0.015) with no significant change in BP. Both CC and VA flow returned to near-resting values within 30 min after the infusion, and for four hypertensive subjects and three controls, time-control experiments using 0.9% saline did not affect VA or CC blood flow significantly. The results demonstrate endothelial dysfunction in the posterior cerebral circulation of middle-aged hypertensive men.