The cold receptor TRPM8 activation leads to attenuation of endothelium-dependent cerebral vascular functions during head cooling

Using the cranial window technique, we investigated acute effects of head cooling on cerebral vascular functions in newborn pigs. Head cooling lowered the rectal and extradural brain temperatures to 34.3 ± 0.6°C and 26.1 ± 0.6°C, respectively. During the 3-h hypothermia period, responses of pial arterioles to endothelium-dependent dilators bradykinin and glutamate were reduced, whereas the responses to hypercapnia and an endothelium-independent dilator sodium nitroprusside (SNP) remained intact. All vasodilator responses were restored after rewarming, suggesting that head cooling did not produce endothelial injury. We tested the hypothesis that the cold-sensitive TRPM8 channel is involved in attenuation of cerebrovascular functions. TRPM8 is immunodetected in cerebral vessels and in the brain parenchyma. During normothermia, the TRPM8 agonist icilin produced constriction of pial arterioles that was antagonized by the channel blocker AMTB. Icilin reduced dilation of pial arterioles to bradykinin and glutamate but not to hypercapnia and SNP, thus mimicking the effects of head cooling on vascular functions. AMTB counteracted the impairment of endothelium-dependent vasodilation caused by hypothermia or icilin. Overall, mild hypothermia produced by head cooling leads to acute reversible reduction of selected endothelium-dependent cerebral vasodilator functions via TRPM8 activation, whereas cerebral arteriolar smooth muscle functions are largely preserved.     

Evaluation of cerebrovascular impedance and wave reflection in mouse by ultrasound

Genetic and surgical mouse models are commonly used to study cerebrovascular disease, but their size makes invasive hemodynamic testing technically challenging. The purpose of this study was to demonstrate a noninvasive measurement of cerebrovascular impedance and wave reflection in mice using high-frequency ultrasound in the left common carotid artery (LCCA), and to examine whether microvascular changes associated with hypercapnia could be detected with such an approach. Ten mice (C57BL/6J) were studied using a high-frequency ultrasound system (40 MHz). Lumen area and blood flow waveforms were obtained from the LCCA and used to calculate pulse-wave velocity, input impedance, and reflection amplitude and transit time under both normocapnic and hypercapnic (5% CO₂) ventilation. With hypercapnia, vascular resistance was observed to decrease by 87%±12%. Although the modulus of input impedance was unchanged with hypercapnia, a phase decrease indicative of increased total arterial compliance was observed at low harmonics together with an increased reflection coefficient in both the time (0.57±0.08 versus 0.68±0.08, P=0.04) and frequency domains (0.62±0.08 versus 0.73±0.06, P=0.02). Interestingly, the majority of LCCA blood flow was found to pass into the internal carotid artery (range=76% to 90%, N=3), suggesting that hemodynamic measurements in this vessel are a good metric for intracerebral reactivity in mouse.     

Leptin augments cerebral hemodynamic reserve after three-vessel occlusion: distinct effects on cerebrovascular tone and proliferation in a nonlethal model of hypoperfused rat brain

The adipocytokine leptin has distinct functions regulating vascular tone, inflammation, and collateral artery growth. Arteriogenesis is an inflammatory process and provides a mechanism to overcome the effects of vascular obstruction. We, therefore, tested the effects of leptin in hypoperfused rat brain (three-vessel occlusion). Systemic leptin administration for 1 week after occlusion surgery increased cerebral hemodynamic reserve similar to granulocyte–macrophage colony-stimulating factor (GM-CSF), as indicated by improved CO₂ reactivity (vehicle 0.53%±0.26% versus leptin 1.05%±0.6% per mm Hg arterial pCO₂, P<0.05). Infusion of microspheres under maximal vasodilation failed to show a positive effect of leptin on cerebral perfusion (vehicle 64.9%±4.5% versus leptin 66.3%±7.0%, occluded/nonoccluded hemisphere). Acute treatment with GM-CSF led to a significant increased CO₂ reactivity and cerebral perfusion (79.2%±8.1% versus 64.9%±4.5%, P<0.05). Vasoconstrictive response of isolated rat carotid artery rings, after phenylephrine was attenuated at 24 hours following preincubation with leptin, was unaffected by removal of endothelium but abrogated by coculture with N-(omega)-nitro--arginine methylester, pointing toward an inducible nitric oxide synthase-mediated mechanism. In chronic cerebral hypoperfusion, acute leptin treatment restored the hemodynamic reserve of the cerebral vasculature through its effects on vascular tone, while leaving vascular outward remodeling unaffected. Our results, for the first time, reveal a protective role of leptin on vascular function in hemodynamically compromised brain tissue.     

Acetylsalicylic acid,but not clopidogrel,inhibits therapeutically induced cerebral arteriogenesis in the hypoperfused rat brain

This study investigated the effects of acetylsalicylic acid (ASA) and clopidogrel, standardly used in the secondary prevention of vascular occlusions, on cerebral arteriogenesis in vivo and in vitro. Cerebral hypoperfusion was induced by three-vessel occlusion (3-VO) in rats, which subsequently received vehicle, ASA (6.34 mg/kg), or clopidogrel (10 mg/kg). Granulocyte colony-stimulating factor (G-CSF), which enhanced monocyte migration in an additional cell culture model, augmented cerebrovascular arteriogenesis in subgroups (40 μg/kg). Cerebrovascular reactivity and vessel diameters were assessed at 7 and 21 days. Cerebrovascular reserve capacity was completely abolished after 3-VO and remained severely compromised after 7 (−14±14%) and 21 (−5±11%) days in the ASA groups in comparison with controls (4±5% and 10±10%) and clopidogrel (4±13% and 10±8%). It was still significantly decreased when ASA was combined with G-CSF (1±4%) compared with G-CSF alone (20±8%). Posterior cerebral artery diameters confirmed these data. Monocyte migration into the vessel wall, improved by G-CSF, was significantly reduced by ASA. Acetylsalicylic acid, but not clopidogrel, inhibits therapeutically augmented cerebral arteriogenesis.     

The influence of carbon dioxide on brain activity and metabolism in conscious humans

A better understanding of carbon dioxide (CO₂) effect on brain activity may have a profound impact on clinical studies using CO₂ manipulation to assess cerebrovascular reserve and on the use of hypercapnia as a means to calibrate functional magnetic resonance imaging (fMRI) signal. This study investigates how an increase in blood CO₂, via inhalation of 5% CO₂, may alter brain activity in humans. Dynamic measurement of brain metabolism revealed that mild hypercapnia resulted in a suppression of cerebral metabolic rate of oxygen (CMRO₂) by 13.4%±2.3% (N=14) and, furthermore, the CMRO₂ change was proportional to the subject''s end-tidal CO₂ (Et-CO₂) change. When using functional connectivity MRI (fcMRI) to assess the changes in resting-state neural activity, it was found that hypercapnia resulted in a reduction in all fcMRI indices assessed including cluster volume, cross-correlation coefficient, and amplitude of the fcMRI signal in the default-mode network (DMN). The extent of the reduction was more pronounced than similar indices obtained in visual-evoked fMRI, suggesting a selective suppression effect on resting-state neural activity. Scalp electroencephalogram (EEG) studies comparing hypercapnia with normocapnia conditions showed a relative increase in low frequency power in the EEG spectra, suggesting that the brain is entering a low arousal state on CO₂ inhalation.     

Multimodality monitoring during passive tilt and Valsalva maneuver under hypercapnia.

In occlusive cerebrovascular disease cerebral blood flow (CBF) autoregulation can be impaired and constant CBF during fluctuations in blood pressure (BP) cannot be guaranteed. Therefore, an assessment of cerebral autoregulation should consider not only responsiveness to CO2 or Diamox. Passive tilting (PT) and Valsalva maneuver (VM) are established tests for cardiovascular autoregulatory function by provoking BP changes. To develop a comprehensive test for vasomotor reactivity with a potential increase of sensitivity and specificity, the authors combined these maneuvers. Blood pressure, corrected to represent arterial pressure at the level of the circle of Willis, middle cerebral artery Doppler frequencies (DF), heart rate (HR) and endtidal partial pressure of CO2 (PtCO2) were measured continuously and noninvasively in 81 healthy subjects (19-74 years). Passive tilt and Valsalva maneuver were performed under normocapnia (mean, 39 + 4 mmHg CO2) and under hypercapnia (mean, 51 + 5 mm Hg CO2). Resting BP, HR, and DF increased significantly under hypercapnia. Under normocapnia and hypercapnia, PT induced only minor, nonsignificant changes in mean BP at the level of the circle of Willis compared to baseline (normocapnia: + 2 + 15 mm Hg; hypercapnia: -3 +/- 13 mm Hg). This corresponded with a nonsignificant decrease of the mean of DF (normocapnia: -4 +/- 11%; hypercapnia -6 +/- 12%). Orthostasis reduced pulsatility of BP by a predominantly diastolic increase of BP without significant changes in pulsatility of DF. Valsalva maneuver, with its characteristic rapid changes of BP due to elevated intrathoracic pressure, showed no significant BP differences in changes to baseline between normocapnic and hypercapnic conditions. Under both conditions the decrease in BP in phase II was accompanied by significantly increased pulsatility index ratio (PIDF/PIBP). Valsalva maneuver and PT as established tests in autonomic control of circulation provoked not only changes in time-mean of BP but also in pulsatility of BP. The significant increase in pulsatility ratio and decrease of the DF/BP ratio during normocapnia and hypercapnia indicated preserved CBF autoregulation within a wide range of CO2 partial pressures. Hypercapnia did not significantly influence the autoregulatory indices during VM and PT. Physiologically submaximally dilated cerebral arterioles can guarantee unchanged dynamics of cerebral autoregulation. Combined BP and MCA-DF assessment under hypercapnia enables investigating the effect of rapid changes of blood pressure on CO2-induced predilated cerebral arterioles. Assuming no interference of hypercapnia-induced vasodilation, VM, with its rapid, distinct changes in BP, seems especially to be adequate provocation for CBF autoregulation. This combined vasomotor reactivity might provide a more sensitive diagnostic tool to detect impaired cerebral autoregulation very early.     

Mild pericyte deficiency is associated with aberrant brain microvascular flow in aged PDGFRβ+/− mice

The receptor tyrosine kinase PDGFRβ is essential for pericyte migration to the endothelium. In mice lacking one allele of PDGFRβ (PDGFRβ^+/−), previous reports have described an age-dependent loss of pericytes in the brain, leading to cerebrovascular dysfunction and subsequent neurodegeneration reminiscent of that seen in Alzheimer’s disease and vascular dementia. We examined 12–20-month-old PDGFRβ^+/− mice to better understand how pericyte loss affects brain microvascular structure and perfusion in vivo. We observed a mild reduction of cortical pericyte number in PDGFRβ^+/− mice (27% fewer cell bodies) compared to controls, but no decrease in pericyte coverage of the endothelium. This mild degree of pericyte loss caused no discernable change in cortical microvascular density, length, basal diameter or reactivity to hypercapnia. Yet, it was associated with an increase in basal blood cell velocity, primarily in pre-capillary arterioles. Taken together, our results suggest that mild pericyte loss can lead to aberrant cerebral blood flow despite a lack of apparent effect on microvascular structure and reactivity.     

Pial arteries respond earlier than penetrating arterioles to neural activation in the somatosensory cortex in awake mice exposed to chronic hypoxia: an additional mechanism to proximal integration signaling?

The pial and penetrating arteries have a crucial role in regulating cerebral blood flow (CBF) to meet neural demand in the cortex. Here, we examined the longitudinal effects of chronic hypoxia on the arterial diameter responses to single whisker stimulation in the awake mouse cortex, where activity-induced responses of CBF were gradually attenuated. The vasodilation responses to whisker stimulation under prehypoxia normal conditions were 8.1% and 12% relative to their baselines in the pial arteries and penetrating arterioles, respectively. After 3 weeks of hypoxia, however, these responses were significantly reduced to 5.5% and 4.1%, respectively. The CBF response, measured using laser-Doppler flowmetry (LDF), induced by the same whisker stimulation was also attenuated (14% to 2.6%). A close linear correlation was found for the responses between the penetrating arteriolar diameter and LDF, and their temporal dynamics. After 3 weeks of chronic hypoxia, the initiation of vasodilation in the penetrating arterioles was significantly extended, but the pial artery responses remained unchanged. These results show that vasodilation of the penetrating arterioles followed the pial artery responses, which are not explainable in terms of proximal integration signaling. The findings therefore indicate an additional mechanism for triggering pial artery dilation in the neurovascular coupling.     

The effect of basal vasodilation on hypercapnic and hypocapnic reactivity measured using magnetic resonance imaging

Cerebrovascular reactivity to vasodilatory hypercapnic and vasoconstrictive hypocapnic challenges is known to be altered in several hemodynamic disorders, which is often attributable to changes in smooth muscle-mediated vascular compliance. Recently, attenuated reactivity to hypercapnia but enhanced reactivity to hypocapnia was observed in patients with chronic stroke. We hypothesize that the latter observation could be explained by a change in the basal vascular tone. In particular, reduced cerebral perfusion pressure, as is prevalent in these patients, may cause vasodilation through autoregulatory mechanisms, and this compensatory baseline condition may alter reactivity to vasoconstrictive hypocapnic challenges. To test this hypothesis, a predilated vascular condition was created in young, healthy subjects (n=11; age=23 to 36 years) using inhalation of 4% CO₂. Using blood oxygenation level-dependent functional magnetic resonance imaging at 3 T, breath holding and cued deep breathing respiratory challenges were administered to assess hypercapnia and hypocapnia reactivity, respectively. During the predilated condition, vasoconstrictive reactivity to hypocapnia was significantly (21.1%, P=0.016) enhanced throughout the gray matter, whereas there was no significant change (6.4%, P=0.459) in hypercapnic vasodilatory reactivity. This suggests that baseline vasodilation may explain the enhanced hypocapnia reactivity observed in some stroke patients, and that hypocapnia challenges may help identify the level of vascular compliance in patients with reduced cerebral perfusion pressure.     

Cerebrovascular reactivity is increased with acclimatization to 3,454?m altitude

Controversy exists regarding the effect of high-altitude exposure on cerebrovascular CO₂ reactivity (CVR). Confounding factors in previous studies include the use of different experimental approaches, ascent profiles, duration and severity of exposure and plausibly environmental factors associated with altitude exposure. One aim of the present study was to determine CVR throughout acclimatization to high altitude when controlling for these. Middle cerebral artery mean velocity (MCAv_mean) CVR was assessed during hyperventilation (hypocapnia) and CO₂ administration (hypercapnia) with background normoxia (sea level (SL)) and hypoxia (3,454 m) in nine healthy volunteers (26±4 years (mean±s.d.)) at SL, and after 30 minutes (HA0), 3 (HA3) and 22 (HA22) days of high-altitude (3,454 m) exposure. At altitude, ventilation was increased whereas MCAv_mean was not altered. Hypercapnic CVR was decreased at HA0 (1.16%±0.16%/mm Hg, mean±s.e.m.), whereas both hyper- and hypocapnic CVR were increased at HA3 (3.13%±0.18% and 2.96%±0.10%/mm Hg) and HA22 (3.32%±0.12% and 3.24%±0.14%/mm Hg) compared with SL (1.98%±0.22% and 2.38%±0.10%/mm Hg; P<0.01) regardless of background oxygenation. Cerebrovascular conductance (MCAv_mean/mean arterial pressure) CVR was determined to account for blood pressure changes and revealed an attenuated response. Collectively our results show that hypocapnic and hypercapnic CVR are both elevated with acclimatization to high altitude.     

Cerebral hemodynamic and ventilatory responses to hypoxia,hypercapnia, and hypocapnia during 5 days at 4,350?m

This study investigated the changes in cerebral near-infrared spectroscopy (NIRS) signals, cerebrovascular and ventilatory responses to hypoxia and CO₂ during altitude exposure. At sea level (SL), after 24 hours and 5 days at 4,350 m, 11 healthy subjects were exposed to normoxia, isocapnic hypoxia, hypercapnia, and hypocapnia. The following parameters were measured: prefrontal tissue oxygenation index (TOI), oxy- (HbO₂), deoxy- and total hemoglobin (HbTot) concentrations with NIRS, blood velocity in the middle cerebral artery (MCAv) with transcranial Doppler and ventilation. Smaller prefrontal deoxygenation and larger ΔHbTot in response to hypoxia were observed at altitude compared with SL (day 5: ΔHbO₂−0.6±1.1 versus −1.8±1.3 μmol/cmper mm Hg and ΔHbTot 1.4±1.3 versus 0.7±1.1 μmol/cm per mm Hg). The hypoxic MCAv and ventilatory responses were enhanced at altitude. Prefrontal oxygenation increased less in response to hypercapnia at altitude compared with SL (day 5: ΔTOI 0.3±0.2 versus 0.5±0.3% mm Hg). The hypercapnic MCAv and ventilatory responses were decreased and increased, respectively, at altitude. Hemodynamic responses to hypocapnia did not change at altitude. Short-term altitude exposure improves cerebral oxygenation in response to hypoxia but decreases it during hypercapnia. Although these changes may be relevant for conditions such as exercise or sleep at altitude, they were not associated with symptoms of acute mountain sickness.     

Calibrated MRI to evaluate cerebral hemodynamics in patients with an internal carotid artery occlusion

The purpose of this study was to assess whether calibrated magnetic resonance imaging (MRI) can identify regional variances in cerebral hemodynamics caused by vascular disease. For this, arterial spin labeling (ASL)/blood oxygen level-dependent (BOLD) MRI was performed in 11 patients (65±7 years) and 14 controls (66±4 years). Cerebral blood flow (CBF), ASL cerebrovascular reactivity (CVR), BOLD CVR, oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO₂) were evaluated. The CBF was 34±5 and 36±11 mL/100 g per minute in the ipsilateral middle cerebral artery (MCA) territory of the patients and the controls. Arterial spin labeling CVR was 44±20 and 53±10% per 10 mm Hg ▵EtCO₂ in patients and controls. The BOLD CVR was lower in the patients compared with the controls (1.3±0.8 versus 2.2±0.4% per 10 mm Hg ▵EtCO₂, P<0.01). The OEF was 41±8% and 38±6%, and the CMRO₂ was 116±39 and 111±40 μmol/100 g per minute in the patients and the controls. The BOLD CVR was lower in the ipsilateral than in the contralateral MCA territory of the patients (1.2±0.6 versus 1.6±0.5% per 10 mmHg ▵EtCO₂, P<0.01). Analysis was hampered in three patients due to delayed arrival time. Thus, regional hemodynamic impairment was identified with calibrated MRI. Delayed arrival artifacts limited the interpretation of the images in some patients.     

Cerebral perfusion and oxygenation are impaired by folate deficiency in rat: absolute measurements with noninvasive near-infrared spectroscopy

Brain microvascular pathology is a common finding in Alzheimer''s disease and other dementias. However, the extent to which microvascular abnormalities cause or contribute to cognitive impairment is unclear. Noninvasive near-infrared spectroscopy (NIRS) can address this question, but its use for clarifying the role of microvascular dysfunction in dementia has been limited due to theoretical and practical considerations. We developed a new noninvasive NIRS method to obtain quantitative, dynamic measurements of absolute brain hemoglobin concentration and oxygen saturation and used it to show significant cerebrovascular impairments in a rat model of diet-induced vascular cognitive impairment. We fed young rats folate-deficient (FD) and control diets and measured absolute brain hemoglobin and hemodynamic parameters at rest and during transient mild hypoxia and hypercapnia. With respect to control animals, FD rats featured significantly lower brain hemoglobin concentration (72±4 μmol/L versus 95±6 μmol/L) and oxygen saturation (54%±3% versus 65%±2%). By contrast, resting arterial oxygen saturation was the same for both groups (96%±4%), indicating that decrements in brain hemoglobin oxygenation were independent of blood oxygen carrying capacity. Vasomotor reactivity in response to hypercapnia was also impaired in FD rats. Our results implicate microvascular abnormality and diminished oxygen delivery as a mechanism of cognitive impairment.     

Cerebral vasomotor reactivity during hypo- and hypercapnia in sedentary elderly and Masters athletes

Physical activity may influence cerebrovascular function. The objective of this study was to determine the impact of life-long aerobic exercise training on cerebral vasomotor reactivity (CVMR) to changes in end-tidal CO2 (EtCO2) in older adults. Eleven sedentary young (SY, 27±5 years), 10 sedentary elderly (SE, 72±4 years), and 11 Masters athletes (MA, 72±6 years) underwent the measurements of cerebral blood flow velocity (CBFV), arterial blood pressure, and EtCO2 during hypocapnic hyperventilation and hypercapnic rebreathing. Baseline CBFV was lower in SE and MA than in SY while no difference was observed between SE and MA. During hypocapnia, CVMR was lower in SE and MA compared with SY (1.87±0.42 and 1.47±0.21 vs. 2.18±0.28 CBFV%/mm Hg, P<0.05) while being lowest in MA among all groups (P<0.05). In response to hypercapnia, SE and MA exhibited greater CVMR than SY (6.00±0.94 and 6.67±1.09 vs. 3.70±1.08 CBFV1%/mm Hg, P<0.05) while no difference was observed between SE and MA. A negative linear correlation between hypo- and hypercapnic CVMR (R²=0.37, P<0.001) was observed across all groups. Advanced age was associated with lower resting CBFV and lower hypocapnic but greater hypercapnic CVMR. However, life-long aerobic exercise training appears to have minimal effects on these age-related differences in cerebral hemodynamics.     

Bolus arrival time and cerebral blood flow responses to hypercarbia

The purpose of this study was to evaluate how cerebral blood flow and bolus arrival time (BAT) measures derived from arterial spin labeling (ASL) MRI data change for different hypercarbic gas stimuli. Pseudocontinuous ASL (pCASL) was applied (3.0T; spatial resolution=4 × 4 × 7 mm³; repetition time/echo time (TR/TE)=3,600/11 ms) sequentially in healthy volunteers (n=12; age=30±4 years) for separate experiments in which (i) normocarbic normoxia (i.e., room air), hypercarbic normoxia (i.e., 5% CO₂/21% O₂/74% N₂), and hypercarbic hyperoxia (i.e., carbogen: 5% CO₂/95% O₂) gas was administered (12 L/minute). Cerebral blood flow and BAT changes were quantified using models that account for macrovascular signal and partial volume effects in all gray matter and regionally in cerebellar, temporal, occipital, frontal, and parietal lobes. Regional reductions in BAT of 4.6% to 7.7% and 3.3% to 6.6% were found in response to hypercarbic normoxia and hypercarbic hyperoxia, respectively. Cerebral blood flow increased by 8.2% to 27.8% and 3.5% to 19.8% for hypercarbic normoxia and hypercarbic hyperoxia, respectively. These findings indicate that changes in BAT values may bias functional ASL data and thus should be considered when choosing appropriate experimental parameters in calibrated functional magnetic resonance imaging or ASL cerebrovascular reactivity experiments that use hypercarbic gas stimuli.     

Acute exercise stress reveals cerebrovascular benefits associated with moderate gains in cardiorespiratory fitness

Elevated cardiorespiratory fitness improves resting cerebral perfusion, although to what extent this is further amplified during acute exposure to exercise stress and the corresponding implications for cerebral oxygenation remain unknown. To examine this, we recruited 12 moderately active and 12 sedentary healthy males. Middle cerebral artery blood velocity (MCAv) and prefrontal cortical oxyhemoglobin (cO₂Hb) concentration were monitored continuously at rest and throughout an incremental cycling test to exhaustion. Despite a subtle elevation in the maximal oxygen uptake (active: 52±9 ml/kg per minute versus sedentary: 33±5 ml/kg per minute, P<0.05), resting MCAv was not different between groups. However, more marked increases in both MCAv (+28±13% versus +18±6%, P<0.05) and cO₂Hb (+5±4% versus −2±3%, P<0.05) were observed in the active group during the transition from low- to moderate-intensity exercise. Collectively, these findings indicate that the long-term benefits associated with moderate increase in physical activity are not observed in the resting state and only become apparent when the cerebrovasculature is challenged by acute exertional stress. This has important clinical implications when assessing the true extent of cerebrovascular adaptation.     

Enhanced parenchymal arteriole tone and astrocyte signaling protect neurovascular coupling mediated parenchymal arteriole vasodilation in the spontaneously hypertensive rat

Functional hyperemia is the regional increase in cerebral blood flow upon increases in neuronal activity which ensures that the metabolic demands of the neurons are met. Hypertension is known to impair the hyperemic response; however, the neurovascular coupling mechanisms by which this cerebrovascular dysfunction occurs have yet to be fully elucidated. To determine whether altered cortical parenchymal arteriole function or astrocyte signaling contribute to blunted neurovascular coupling in hypertension, we measured parenchymal arteriole reactivity and vascular smooth muscle cell Ca²⁺ dynamics in cortical brain slices from normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. We found that vasoconstriction in response to the thromboxane A₂ receptor agonist U46619 and basal vascular smooth muscle cell Ca²⁺ oscillation frequency were significantly increased in parenchymal arterioles from SHR. In perfused and pressurized parenchymal arterioles, myogenic tone was significantly increased in SHR. Although K⁺-induced parenchymal arteriole dilations were similar in WKY and SHR, metabotropic glutamate receptor activation-induced parenchymal arteriole dilations were enhanced in SHR. Further, neuronal stimulation-evoked parenchymal arteriole dilations were similar in SHR and WKY. Our data indicate that neurovascular coupling is not impaired in SHR, at least at the level of the parenchymal arterioles.     

Role of nitric oxide in cerebrovascular reactivity to NMDA and hypercapnia during prenatal development in sheep

Cerebral vasodilatory responses evoked by activation of NMDA receptors and by hypercapnia are important factors in the integrated vascular response to perinatal cerebral ischemia. Cerebral vasodilation to NMDA is mediated by nitric oxide in adult and newborn animals, whereas vasodilation to hypercapnia is thought to become modulated by nitric oxide, at least in swine, after the newborn period. The developmental role of nitric oxide in the cerebral blood flow response to NMDA and hypercapnia was investigated at mid- and late gestation in fetal sheep. Superfusion of 300microM NMDA over the cerebral cortex through a closed cranial window on the exteriorized head of an anesthetized fetus increased laser-Doppler flow by 41+/-7% (+/-S.E.) at 0.65 gestation. The increase was reduced by superfusion of a nitric oxide synthase inhibitor (18+/-8%). At 0.9 gestation, the response to NMDA was augmented (85+/-24%) compared to that at 0.65 gestation and was reduced by a nitric oxide synthase inhibitor (32+/-6%). In unanesthetized fetal sheep, hypercapnic reactivity of microsphere-determined cerebral blood flow was not significantly attenuated by nitric oxide synthase inhibition at 0.65 gestation (4.6+/-0.7 to 3.7+/-1.0% change/mmHg pCO2) or at 0.9 gestation (4.0+/-0.7 to 3.5+/-0.9% change/mmHg pCO2). Therefore, nitric oxide-dependent cerebrovascular dilation to NMDA-receptor activation is present as early as 0.65 gestation in fetal sheep and increases further during the last trimester, whereas vasodilation to hypercapnia remains unchanged and independent of nitric oxide during the last trimester. Hence, cerebrovascular reactivities to different stimuli do not mature concurrently.     

Hyperlipidemia disrupts cerebrovascular reflexes and worsens ischemic perfusion defect

Hyperlipidemia is a highly prevalent risk factor for coronary and cervical atherosclerosis and stroke. However, even in the absence of overt atherosclerosis, hyperlipidemia disrupts endothelial and smooth muscle function. We investigated the impact of hyperlipidemia on resting-brain perfusion, fundamental cerebrovascular reflexes, and dynamic perfusion defect during acute focal ischemia in hyperlipidemic apolipoprotein E knockout mice before the development of flow-limiting atherosclerotic stenoses. Despite elevated blood pressures, absolute resting cerebral blood flow was reduced by 20% in apolipoprotein E knockout compared with wild type when measured by [¹⁴C]-iodoamphetamine technique. Noninvasive, high spatiotemporal resolution laser speckle flow imaging revealed that the lower autoregulatory limit was elevated in apolipoprotein E knockout mice (60 vs. 40 mm Hg), and cortical hyperemic responses to hypercapnia and functional activation were attenuated by 30% and 64%, respectively. Distal middle cerebral artery occlusion caused significantly larger perfusion defects and infarct volumes in apolipoprotein E knockout compared with wild type. Cerebrovascular dysfunction showed a direct relationship to the duration of high-fat diet. These data suggest that hyperlipidemia disrupts cerebral blood flow regulation and diminishes collateral perfusion in acute stroke in the absence of hemodynamically significant atherosclerosis.     

Impaired dynamic cerebral autoregulation at extreme high altitude even after acclimatization

Cerebral blood flow (CBF) increases and dynamic cerebral autoregulation is impaired by acute hypoxia. We hypothesized that progressive hypocapnia with restoration of arterial oxygen content after altitude acclimatization would normalize CBF and dynamic cerebral autoregulation. To test this hypothesis, dynamic cerebral autoregulation was examined by spectral and transfer function analyses between arterial pressure and CBF velocity variabilities in 11 healthy members of the Danish High-Altitude Research Expedition during normoxia and acute hypoxia (10.5% O₂) at sea level, and after acclimatization (for over 1 month at 5,260 m at Chacaltaya, Bolivia). Arterial pressure and CBF velocity in the middle cerebral artery (transcranial Doppler), were recorded on a beat-by-beat basis. Steady-state CBF velocity increased during acute hypoxia, but normalized after acclimatization with partial restoration of SaO₂ (acute, 78%±2% chronic, 89%±1%) and progression of hypocapnia (end-tidal carbon dioxide: acute, 34±2 mm Hg; chronic, 21±1 mm Hg). Coherence (0.40±0.05 Units at normoxia) and transfer function gain (0.77±0.13 cm/s per mm Hg at normoxia) increased, and phase (0.86±0.15 radians at normoxia) decreased significantly in the very-low-frequency range during acute hypoxia (gain, 141%±24% coherence, 136%±29% phase, −25%±22%), which persisted after acclimatization (gain, 136%±36% coherence, 131%±50% phase, −42%±13%), together indicating impaired dynamic cerebral autoregulation in this frequency range. The similarity between both acute and chronic conditions suggests that dynamic cerebral autoregulation is impaired by hypoxia even after successful acclimatization to an extreme high altitude.