Key points
Preclinical models have demonstrated that nitric oxide is a key component of neurovascular coupling; this has yet to be translated to humans.
We conducted two separate protocols utilizing ...intravenous infusion of a nitric oxide synthase inhibitor and isovolumic haemodilution to assess the influence of nitric oxide on neurovascular coupling in humans.
Isovolumic haemodilution did not alter neurovascular coupling.
Intravenous infusion of a nitric oxide synthase inhibitor reduced the neurovascular coupling response by ∼30%, indicating that nitric oxide is integral to neurovascular coupling in humans.
Nitric oxide is a vital neurovascular signalling molecule in preclinical models, yet the mechanisms underlying neurovascular coupling (NVC) in humans have yet to be elucidated. To investigate the contribution of nitric oxide to NVC in humans, we utilized a visual stimulus paradigm to elicit an NVC response in the posterior cerebral circulation. Two distinct mechanistic interventions were conducted on young healthy males: (1) NVC was assessed during intravenous infusion of saline (placebo) and the non‐selective competitive nitric oxide synthase inhibitor NG‐monomethyl‐l‐arginine (l‐NMMA, 5 mg kg−1 bolus & subsequent 50 μg kg−1 min−1 maintenance dose; n = 10). The order of infusion was randomized, counterbalanced and single blinded. A subset of participants in this study (n = 4) underwent a separate intervention with phenylephrine infusion to independently consider the influence of blood pressure changes on NVC (0.1–0.6 μg kg−1 min−1 constant infusion). (2) NVC was assessed prior to and following isovolumic haemodilution, whereby 20% of whole blood was removed and replaced with 5% human serum albumin to reduce haemoglobin concentration (n = 8). For both protocols, arterial and internal jugular venous blood samples were collected at rest and coupled with volumetric measures of cerebral blood flow (duplex ultrasound) to quantify resting cerebral metabolic parameters. l‐NMMA elicited a 30% reduction in the peak (P = 0.01), but not average (P = 0.11), NVC response. Neither phenylephrine nor haemodilution influenced NVC. Nitric oxide signalling is integral to NVC in humans, providing a new direction for research into pharmacological treatment of humans with dementia.
Key points
Preclinical models have demonstrated that nitric oxide is a key component of neurovascular coupling; this has yet to be translated to humans.
We conducted two separate protocols utilizing intravenous infusion of a nitric oxide synthase inhibitor and isovolumic haemodilution to assess the influence of nitric oxide on neurovascular coupling in humans.
Isovolumic haemodilution did not alter neurovascular coupling.
Intravenous infusion of a nitric oxide synthase inhibitor reduced the neurovascular coupling response by ∼30%, indicating that nitric oxide is integral to neurovascular coupling in humans.
New Findings
What is the central question of this study?
In this study, we investigated intracranial cerebrovascular and ventilatory reactivity to 6% CO2 in children and adults and explored dynamic ...ventilatory and cerebrovascular onset responses.
What is the main finding and its importance?
We showed that cerebrovascular reactivity was similar in children and adults, but the intracranial blood velocity onset response was markedly attenuated in children. Sex differences were apparent, with greater increases in intracranial blood velocity in females and lower ventilatory reactivity in adult females. Our study confirms the importance of investigating dynamic onset responses when assessing the influence of development on cerebrovascular regulation.
The purpose of this study was to compare the integrated intracranial cerebrovascular reactivity (CVR) and hypercapnic ventilatory response between children and adults and to explore the dynamic response of the middle cerebral artery mean velocity (MCAV). Children (n = 20; 9.9 ± 0.7 years of age) and adults (n = 21; 24.4 ± 2.0 years of age) completed assessment of CVR over 240 s using a fixed fraction of inspired CO2 (0.06). Baseline MCAV was higher in the adult females compared with the males (P ≤ 0.05). The MCAV was greater in female children compared with male children (P ≤ 0.05) and in female adults compared with male adults (P ≤ 0.05) with hypercapnia. Relative CVR was similar in children and adults (3.71 ± 1.06 versus 4.12 ± 1.32% mmHg−1; P = 0.098), with absolute CVR being higher in adult females than males (3.27 ± 0.86 versus 2.53 ± 0.70 cm s−1 mmHg−1; P ≤ 0.001). Likewise, the hypercapnic ventilatory response did not differ between the children and adults (1.89 ± 1.00 versus 1.77 ± 1.34 l min−1 mmHg−1; P = 0.597), but was lower in adult females than males (1.815 ± 0.37 versus 2.33 ± 1.66 l min−1 mmHg−1; P ≤ 0.05). The heart rate response to hypercapnia was greater in children than in adults (P = 0.001). A monoexponential regression model was used to characterize the dynamic onset, consisting of a delay term, amplitude and time constant (τ). The results revealed that MCAV τ was faster in adults than in children (34 ± 18 versus 74 ± 28 s; P = 0.001). Our study provides new insight into the impact of age and sex on CVR and the dynamic response of the MCAV to hypercapnia.
New Findings
What is the central question of this study?
Do differing magnitudes of ventilation influence cerebrovascular CO2 reactivity and the cerebral blood flow response to increases in arterial ...carbon dioxide?
What is the main finding and its importance?
While a greater ventilation, through voluntary hyperventilation, is associated with a higher anterior cerebral blood flow during carbon dioxide breathing, this elevated cerebral blood flow is due to a higher blood pressure and not ventilation per se. A greater ventilation, through voluntary hyperventilation, does not influence global or posterior cerebral blood flow during carbon dioxide breathing. Cerebrovascular reactivity to carbon dioxide is not influenced by an individual's ventilatory sensitivity to carbon dioxide.
Recent work demonstrated an influence of ventilation on cerebrovascular reactivity to CO2; however, the concomitant influence of changes in mean arterial blood pressure (MAP) on ventilation‐induced differences in cerebral blood flow (CBF) has yet to be examined in this context. Healthy participants (n = 15; 25 ± 3 years of age; 179 ± 6 cm height; 74 ± 10 kg weight; 3 female) underwent end‐tidal forcing to increase their partial pressure of end‐tidal CO2 by +3, +6 and +9 mmHg above baseline in 5‐min sequential steps while maintaining iso‐oxia. This protocol was then repeated twice, with participants hyperventilating and hypoventilating by ∼30% compared to the first trial. Intra‐cranial and extra‐cranial CBF were measured using ultrasound. The MAP (finger photo‐plethysmography) was higher during the hyperventilation and hypoventilation trials compared to normal ventilation (main effects, P < 0.05 for both). While internal carotid artery blood flow was higher during the hyperventilation trial compared to normal ventilation (P = 0.01), this was due to a higher MAP, as indicated by analysis of conductance values (P = 0.68) or inclusion of MAP in covariate analysis (P = 0.11). Global CBF (P = 0.11) and vertebral artery blood flow (P = 0.93) were unaffected by the magnitude of ventilation. Further, CO2 reactivity was not affected by the different breathing trials (P > 0.05 for all). Retrospective analysis of a larger data set (n = 53) confirmed these observations and demonstrated no relationships between the ventilatory and global CBF response to hypercapnia (r2 = 0.04; P = 0.14). Therefore, when differences in MAP are accounted for, cerebrovascular CO2 reactivity (assessed via end‐tidal forcing) is independent of the magnitude of ventilation.
Key points
Iron acts as a cofactor in the stabilization of the hypoxic‐inducible factor family, and plays an influential role in the modulation of hypoxic pulmonary vasoconstriction.
It is uncertain ...whether iron regulation is altered in lowlanders during either (1) ascent to high altitude, or (2) following partial acclimatization, when compared to high‐altitude adapted Sherpa.
During ascent to 5050 m, the rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders; however, upon arrival to 5050 m, PASP levels were comparable in both groups, but the reduction in iron bioavailability was more prevalent in lowlanders compared to Sherpa.
Following partial acclimatization to 5050 m, there were differential influences of iron status manipulation (via iron infusion or chelation) at rest and during exercise between lowlanders and Sherpa on the pulmonary vasculature.
To examine the adaptational role of iron bioavailability on the pulmonary vascular responses to acute and chronic hypobaric hypoxia, the haematological and cardiopulmonary profile of lowlanders and Sherpa were determined during: (1) a 9‐day ascent to 5050 m (20 lowlanders; 12 Sherpa), and (2) following partial acclimatization (11 ± 4 days) to 5050 m (18 lowlanders; 20 Sherpa), where both groups received an i.v. infusion of either iron (iron (iii)‐hydroxide sucrose) or an iron chelator (desferrioxamine). During ascent, there were reductions in iron status in both lowlanders and Sherpa; however, Sherpa appeared to demonstrate a more efficient capacity to mobilize stored iron, compared to lowlanders, when expressed as a Δhepcidin per unit change in either body iron or the soluble transferrin receptor index, between 3400–5050 m (P = 0.016 and P = 0.029, respectively). The rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders during ascent; however, PASP was comparable in both groups upon arrival to 5050 m. Following partial acclimatization, despite Sherpa demonstrating a blunted hypoxic ventilatory response and greater resting hypoxaemia, they had similar hypoxic pulmonary vasoconstriction when compared to lowlanders at rest. Iron‐infusion attenuated PASP in both groups at rest (P = 0.005), while chelation did not exaggerate PASP in either group at rest or during exaggerated hypoxaemia (PIO2 = 67 mmHg). During exercise at 25% peak wattage, PASP was only consistently elevated in Sherpa, which persisted following both iron infusion or chelation. These findings provide new evidence on the complex interplay of iron regulation on pulmonary vascular regulation during acclimatization and adaptation to high altitude.
Key points
Iron acts as a cofactor in the stabilization of the hypoxic‐inducible factor family, and plays an influential role in the modulation of hypoxic pulmonary vasoconstriction.
It is uncertain whether iron regulation is altered in lowlanders during either (1) ascent to high altitude, or (2) following partial acclimatization, when compared to high‐altitude adapted Sherpa.
During ascent to 5050 m, the rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders; however, upon arrival to 5050 m, PASP levels were comparable in both groups, but the reduction in iron bioavailability was more prevalent in lowlanders compared to Sherpa.
Following partial acclimatization to 5050 m, there were differential influences of iron status manipulation (via iron infusion or chelation) at rest and during exercise between lowlanders and Sherpa on the pulmonary vasculature.
Key points
It was unknown whether respiratory alkalosis impacts the global cerebral metabolic response as well as the cerebral pro‐oxidation and inflammatory response in passive hyperthermia.
This ...study demonstrated that the cerebral metabolic rate was increased by ∼20% with passive hyperthermia of up to +2°C oesophageal temperature, and this response was unaffected by respiratory alkalosis.
Additionally, the increase in cerebral metabolism did not significantly impact the net cerebral release of oxidative and inflammatory markers.
These data indicate that passive heating of up to +2°C core temperature in healthy young men is not enough to confer a major oxidative and inflammatory burden on the brain, but it does markedly increase the cerebral metabolic rate, independently of PaCO2.
There is limited information concerning the impact of arterial PCO2/pH on heat‐induced alteration in cerebral metabolism, as well as on the cerebral oxidative/inflammatory burden of hyperthermia. Accordingly, we sought to address two hypotheses: (1) passive hyperthermia will increase the cerebral metabolic rate of oxygen (CMRO2) consistent with a combined influence of Q10 and respiratory alkalosis; and (2) the net cerebral release of pro‐oxidative and pro‐inflammatory markers will be elevated in hyperthermia, particularly in poikilocapnic hyperthermia. Healthy young men (n = 6) underwent passive heating until an oesophageal temperature of 2°C above resting was reached. At 0.5°C increments in core temperature, CMRO2 was calculated from the product of cerebral blood flow (ultrasound) and the radial artery–jugular venous oxygen content difference (cannulation). Net cerebral glucose/lactate exchange, and biomarkers of oxidative and inflammatory stress were also measured. At +2.0°C oesophageal temperature, arterial PCO2 was restored to normothermic values using end‐tidal forcing. The primary findings were: (1) while CMRO2 was increased (P < 0.05) by ∼20% with hyperthermia of +1.5–2.0°C, this was not influenced by respiratory alkalosis, and (2) although biomarkers of pro‐oxidation and pro‐inflammation were systemically elevated in hyperthermia (P < 0.05), there were no differences in the trans‐cerebral exchange kinetics. These novel data indicate that passive heating of up to +2°C core temperature in healthy young men is not enough to confer a major oxidative and inflammatory burden on the brain, despite it markedly increasing CMRO2, irrespective of arterial pH.
Key points
It was unknown whether respiratory alkalosis impacts the global cerebral metabolic response as well as the cerebral pro‐oxidation and inflammatory response in passive hyperthermia.
This study demonstrated that the cerebral metabolic rate was increased by ∼20% with passive hyperthermia of up to +2°C oesophageal temperature, and this response was unaffected by respiratory alkalosis.
Additionally, the increase in cerebral metabolism did not significantly impact the net cerebral release of oxidative and inflammatory markers.
These data indicate that passive heating of up to +2°C core temperature in healthy young men is not enough to confer a major oxidative and inflammatory burden on the brain, but it does markedly increase the cerebral metabolic rate, independently of PaCO2.
Key points
Changes in haematocrit influence nitric oxide signalling through alterations in shear stress stimuli and haemoglobin scavenging of nitric oxide; these two regulatory factors have not been ...assessed simultaneously
Isovolumic haemodilution led to a marked increase in brachial artery flow‐mediated dilatation in humans
The increase in flow‐mediated dilatation occurred in the face of an unaltered shear stress stimulus for vasodilatation and reduced resting steady‐state nitric oxide levels in the blood
Collectively, our data point towards haemoglobin scavenging of nitric oxide as a key regulatory factor of brachial flow‐mediated dilatation and highlight the importance of the simultaneous consideration of nitric oxide production and inactivation when investigating vascular function in humans
Haemoglobin (Hb) may impact the transduction of endothelium‐dependent and nitric oxide (NO)‐mediated vasodilator activity, given its contribution to shear stress stimuli and diverse biochemical reactions with NO. We hypothesized that an acute reduction in Hb and haematocrit (Hct) would increase brachial artery flow‐mediated dilatation (FMD). In 11 healthy males (28 ± 7 years; 23 ± 2 kg m−2), FMD (Duplex ultrasound), arterial blood gases, Hct and Hb, blood viscosity, and NO metabolites (ozone‐based chemiluminescence) were measured before and after isovolumic haemodilution, where ∼20% of whole blood was removed and replaced with 5% human serum albumin. Haemodilution reduced Hct by 18 ± 2% (P < 0.001) and whole blood viscosity by 22 ± 5% (P < 0.001). Plasma nitrite (P = 0.01), S‐nitrosothiols (P = 0.03) and total red blood cell NO (P = 0.001) were collectively reduced by ∼15–40%. Brachial artery FMD increased by ∼160% from 3.8 ± 2.1 to 9.7 ± 4.5% (P = 0.004). Statistical covariation for the shear stress stimulus did not alter FMD, indicating that the increase in FMD was not directly related to alterations in whole blood viscosity and the shear stimulus. Collectively, these findings indicate that haemoglobin scavenging of NO appears to be an important factor in the regulation of FMD under normal conditions through constraint of endothelium‐dependent NO‐mediated vasodilatation in healthy humans.
Key points
Changes in haematocrit influence nitric oxide signalling through alterations in shear stress stimuli and haemoglobin scavenging of nitric oxide; these two regulatory factors have not been assessed simultaneously
Isovolumic haemodilution led to a marked increase in brachial artery flow‐mediated dilatation in humans
The increase in flow‐mediated dilatation occurred in the face of an unaltered shear stress stimulus for vasodilatation and reduced resting steady‐state nitric oxide levels in the blood
Collectively, our data point towards haemoglobin scavenging of nitric oxide as a key regulatory factor of brachial flow‐mediated dilatation and highlight the importance of the simultaneous consideration of nitric oxide production and inactivation when investigating vascular function in humans
The purpose of this study was to examine the effect of an acute bout of prolonged sitting with and without exercise breaks on cerebrovascular function in 7‐ to 13‐year‐old children. Forty‐two ...children and adolescents were recruited to a crossover trial, with 15 girls (mean age 10.1 ± 2.5 years) and 16 boys (mean age 10.5 ± 1.3 years) completing the two trial conditions: SIT, uninterrupted sitting for 3 h and CYCLE, 3 h of sitting interrupted hourly with a 10‐min moderate intensity exercise break. Cerebrovascular function was measured Pre and Post SIT and CYCLE from blood flow (Q̇${\dot{Q}}$), diameter, and shear rate of the internal carotid artery (ICA) at rest and in response to CO2. Blood velocity in the middle (MCA) and posterior (PCA) cerebral arteries was assessed at rest, during a neurovascular coupling task (NVC) and in response to CO2. We demonstrate that SIT but not CYCLE reduced ICA cerebrovascular reactivity to CO2 (%Δ ICA Q̇${\dot{Q}}$/Δ end‐tidal CO2: SIT: Pre 5.0 ± 2.4%/mmHg to Post 3.3 ± 2.8%/mmHg vs. CYCLE: Pre 4.4 ± 2.3%/mmHg to Post 5.3 ± 3.4%/mmHg, P = 0.05) and slowed the MCA blood velocity onset response time to hypercapnia (SIT: Pre 57.2 ± 32.6 s to Post 76.6 ± 55.2 s, vs. CYCLE: Pre 64.1 ± 40.4 s to Post 52.3 ± 28.8 s, P = 0.05). There were no changes in NVC. Importantly, breaking up prolonged sitting with hourly exercise breaks prevented the reductions in cerebrovascular reactivity to CO2 and the slowed intracranial blood velocity onset response time to hypercapnia apparent with uninterrupted sitting in children.
New Findings
What is the central question of this study?
What are the effects of interrupting prolonged sitting on cerebrovascular function in children?
What is the main finding and its importance?
Prolonged sitting results in declines in cerebrovascular reactivity, a valuable index of cerebrovascular health. Breaking up prolonged sitting with hourly 10 min exercise breaks prevented these changes. These initial findings suggest excessive sedentary behaviour does impact cerebrovascular function in childhood, but taking exercise breaks prevents declines.
Intracranial blood velocity reactivity to a steady‐state hypercapnic stimulus has been shown to be similar in children and adults, but the onset response to hypercapnia is slower in the child. Given ...the vasodilatory effect of hypercapnia on the cerebrovasculature, assessment of vessel diameter, and blood flow are vital to fully elucidate whether the temporal hypercapnic response differs in children versus adults. Assessment of internal carotid artery (ICA) vessel diameter (ICAd), blood velocity (ICAv), volumetric blood flow (QICA), and shear rate (ICASR) in response to a 4 min hypercapnic challenge was completed in children (n = 14, 8 girls; 9.8 ± 0.7 years) and adults (n = 17, 7 females; 24.7 ± 1.8 years). The dynamic onset responses of partial pressure of end‐tidal CO2 (PETCO2), QICA, ICAv, and ICASR to hypercapnia were modeled, and mean response time (MRT) was computed. Following 4 min of hypercapnia, ICA reactivity and ICAd were comparable between the groups. Despite a similar MRT in PETCO2 in children and adults, children had slower QICA (children 108 ± 60 s vs. adults 66 ± 37 s; p = 0.023), ICAv (children 120 ± 52 s vs. adults 52 ± 31 s; p = 0.001), and ICASR (children 90 ± 27 s vs. adults 47 ± 36 s; p = 0.001) MRTs compared with adults. This is the first study to show slower hypercapnic hyperemic kinetic responses of the ICA in children. The mechanisms determining these differences and the need to consider the duration of hypercapnic exposure when assessing CVR in children should be considered in future studies.
Differences in the temporality of cerebrovascular vasomotion between children and adults may explain previously noted distinctions in the dynamic middle cerebral artery onset response to hypercapnia. This study shows slower hypercapnic onset mean response times for internal carotid artery velocity, flow and shear rate in children compared to adults. Although dilation of the internal carotid artery was similar in children and adults after 4 minutes of hypercapnia, the onset diameter response could not be modelled.
With exposure to acute normobaric hypoxia, global cerebral oxygen delivery is maintained via increases in cerebral blood flow (CBF); therefore, regional and localized changes in oxygen tension may ...explain neurocognitive impairment. Neurovascular coupling (NVC) is the close temporal and regional relationship of CBF to changes in neural activity and may aid in explaining the localized CBF response with cognitive activation. High-altitude related cognitive impairment is likely affected by hypocapnic cerebral vasoconstriction that may influence regional CBF regulation independent of hypoxia. We assessed neurocognition and NVC following 30 min of acute exposure to isocapnic hypoxia (decreased partial pressure of end-tidal oxygen; PETO2) during moderate hypoxia (MOD HX; 55 mm Hg PETO2), and severe hypoxia (SEV HX; 45 mm Hg PETO2) in 10 healthy individuals (25.5 ± 3.3 yrs). Transcranial Doppler ultrasound was used to assess mean posterior and middle cerebral blood velocity (PCAv and MCAv, respectively) and neurocognitive performance was assessed via validated computerized tests. The main finding was that reaction time (i.e., kinesthetic and visual-motor ability via Stroop test) was selectively impaired in SEV HX (−4.6 ± 5.2%, P = 0.04), but not MOD HX, while complex cognitive performance (e.g., psychomotor speed, cognitive flexibility, processing speed, executive function, and motor speed) was unaffected with hypoxia (P > 0.05). Additionally, severity of hypoxia had no effect on NVC (PCAv CON vs. SEV HX relative peak response 13.7 ± 6.4% vs. 16.2 ± 11.5%, P = 0.71, respectively). In summary, severe isocapnic hypoxia impaired reaction time, but not complex cognitive performance or NVC. These findings have implications for recreational and military personnel who may experience acute hypoxia.
•We assessed cognition and NVC with acute normobaric isocapnic hypoxia.•Reaction time is selectively impaired during acute severe isocapnic hypoxia.•Complex cognitive abilities and NVC are unaffected with acute isocapnic hypoxia.
PDF
