Effect of chemodenervation on the cerebral vascular and microvascular response to hypoxia

Influence of Hypoxia on Cerebral Blood Flow Regulation in Humans

The brain is a vital organ that relies on a constant and adequate supply of blood to match oxygen and glucose delivery with the local metabolic demands of active neurones. It is well established that cerebral blood flow is altered in response to both neural activity and humoral stimuli. Thus, augmented neural activation (e.g. visual stimulation) leads to locally increased cerebral blood flow via functional hyperaemia, whereas humoral stimuli (i.e. alterations in arterial PO2 and PCO2) produce global increases in cerebral blood flow. Perhaps not surprisingly, cerebrovascular responses to neural activity and humoral stimuli may not be highly correlated because they reflect different physiological mechanisms for vasodilation. Exquisite regulation of cerebral blood flow is particularly important under hypoxic conditions when cerebral PO2 can be reduced substantially. Indeed, cerebrovascular reactivity to hypoxia determines the capacity of cerebral vessels to respond and compensate for a reduced oxygen supply. This reactivity is dynamic, changing with prolonged exposure to hypoxic environments, and in patients and healthy individuals exposed to chronic intermittent periods of hypoxia. More recently, a number of animal studies have provided evidence that glial cells (i.e. astrocytes) play an important role in regulating cerebral blood flow under normoxic and hypoxic conditions. This review aims to summarize our current understanding of cerebral blood flow control during hypoxia in humans and put into context the underlying neurovascular mechanisms that may contribute to this regulation.

The apneic threshold during non-REM sleep in dogs: sensitivity of carotid body vs. central chemoreceptors

The relative importance of peripheral vs. central chemoreceptors in causing apnea/unstable breathing during sleep is unresolved. This has never been tested in an unanesthetized preparation with intact carotid bodies. We studied three unanesthetized dogs during normal sleep in a preparation in which intact carotid body chemoreceptors could be reversibly isolated from the systemic circulation and perfused. Apneic thresholds and the CO2 reserve (end-tidal Pco2 eupneic − end-tidal Pco2 apneic threshold) were determined using a pressure support ventilation technique. Dogs were studied when both central and peripheral chemoreceptors sensed transient hypocapnia induced by the pressure support ventilation and again with carotid body isolation such that only the central chemoreceptors sensed the hypocapnia. We observed that the CO2 reserve was ≅4.5 Torr when the carotid chemoreceptors sensed the transient hypocapnia but more than doubled (>9 Torr) when only the central chemoreceptors sensed hypocapnia. Furthermore, the expiratory time prolongations observed when only central chemoreceptors were exposed to hypocapnia differed from those obtained when both the central and peripheral chemoreceptors sensed the hypocapnia in that they 1) were substantially shorter for a given reduction in end-tidal Pco2, 2) showed no stimulus: response relationship with increasing hypocapnia, and 3) often occurred at a time (>45 s) beyond the latency expected for the central chemoreceptors. These findings agree with those previously obtained using an identical pressure support ventilation protocol in carotid body-denervated sleeping dogs (Nakayama H, Smith CA, Rodman JR, Skatrud JB, Dempsey JA. J Appl Physiol 94: 155–164, 2003). We conclude that hypocapnia sensed at the carotid body chemoreceptor is required for the initiation of apnea following a transient ventilatory overshoot in non-rapid eye movement sleep.

Specific carotid body chemostimulation is sufficient to elicit phrenic poststimulus frequency decline in a novel in situ dual-perfused rat preparation

Time-dependent ventilatory responses to hypoxic and hypercapnic challenges, such as posthypoxic frequency decline (PHxFD) and posthypercapnic frequency decline (PHcFD), could profoundly affect breathing stability. However, little is known about the mechanisms that mediate these phenomena. To determine the contribution of specific carotid body chemostimuli to PHxFD and PHcFD, we developed a novel in situ arterially perfused, vagotomized, decerebrate rat preparation in which central and peripheral chemoreceptors are perfused separately (i.e., a nonanesthetized in situ dual perfused preparation). We confirmed that 1) the perfusion of central and peripheral chemoreceptor compartments was independent by applying specific carotid body hypoxia and hypercapnia before and after carotid sinus nerve transection, 2) the PCO(2) chemoresponse of the dual perfused preparation was similar to other decerebrate preparations, and 3) the phrenic output was stable enough to allow investigation of time-dependent phenomena. We then applied four 5-min bouts (separated by 5 min) of specific carotid body hypoxia (40 Torr PO(2) and 40 Torr PCO(2)) or hypercapnia (100 Torr PO(2) and 60 Torr PCO(2)) while holding the brain stem PO(2) and PCO(2) constant. We report the novel finding that specific carotid body chemostimuli were sufficient to elicit several phrenic time-dependent phenomena in the rat. Hypoxic challenges elicited PHxFD that increased with bout, leading to progressive augmentation of the phrenic response. Conversely, hypercapnia elicited short-term depression and PHcFD, neither of which was bout dependent. These results, placed in the context of previous findings, suggest multiple physiological mechanisms are responsible for PHxFD and PHcFD, a redundancy that may illustrate that these phenomena have significant adaptive advantages.

S-nitrosothiols signal the ventilatory response to hypoxia

Increased ventilation in response to hypoxia has been appreciated for over a century, but the biochemistry underlying this response remains poorly understood. Here we define a pathway in which increased minute ventilation (&Vdot;E ) is signalled by deoxyhaemoglobin-derived S-nitrosothiols (SNOs). Specifically, we demonstrate that S-nitrosocysteinyl glycine (CGSNO) and S-nitroso-l-cysteine (l-CSNO)-but not S-nitroso-d-cysteine (d-CSNO)-reproduce the ventilatory effects of hypoxia at the level of the nucleus tractus solitarius (NTS). We show that plasma from deoxygenated, but not from oxygenated, blood produces the ventilatory effect of both SNOs and hypoxia. Further, this activity is mediated by S-nitrosoglutathione (GSNO), and GSNO activation by gamma-glutamyl transpeptidase (gamma-GT) is required. The normal response to hypoxia is impaired in a knockout mouse lacking gamma-GT. These observations suggest that S-nitrosothiol biochemistry is of central importance to the regulation of breathing.

Critical oxygen tension in rat brain: a combined (31)P-NMR and EPR oximetry study

The relationship between cerebral interstitial oxygen tension (Pt(O(2))) and cellular energetics was investigated in mechanically ventilated, anesthetized rats during progressive acute hypoxia to determine whether there is a "critical" brain Pt(O(2)) for maintaining steady-state aerobic metabolism. Cerebral Pt(O(2)), measured by electron paramagnetic resonance oximetry, decreased proportionately to inspired oxygen fraction. (31)P-nuclear magnetic resonance measurements revealed no changes in P(i), phosphocreatine (PCr)/P(i) ratio, or intracellular pH when arterial blood oxygen tension (Pa(O(2))) was reduced from 145.1 +/- 11.7 to 56.5 +/- 4.4 mmHg (means +/- SE). Intracellular acidosis, a sharp rise in P(i), and a decline in the PCr/P(i) ratio developed when Pa(O(2)) was reduced further to 40.7 +/- 2.3 mmHg. The corresponding Pt(O(2)) values were 15.1 +/- 1.8, 8.8 +/- 0.4, and 6.8 +/- 0.3 mmHg. We conclude that over a range of decreasing oxygen tensions, cerebral oxidative metabolism is not sensitive to oxygen concentration. Oxygen becomes a regulatory substrate, however, when Pt(O(2)) is decreased to a critical level.

Effect of AMPA receptor blockade on the control of cerebral O2 supply/consumption balance in newborn pigs

Using 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466), we tested the hypothesis that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors are important controllers of cerebral O2 supply/consumption balance in newborn piglets during both normoxia and hypoxia. Twenty-seven 2- to 7-day-old piglets were anesthetized with alpha-chloralose and were divided into four groups: 1) normoxia (n = 7), 2) GYKI 52466 (10 mg/kg, n = 7), 3) hypoxia (n = 6), and 4) hypoxia + GYKI 52466 (n = 7). We used [14C]iodoantipyrine to measure regional cerebral blood flow (rCBF) in mL/min/100 g, and we determined O2 extraction by microspectrophotometry, calculating cerebral O2 consumption (VO2) in mL O2/min/100 g in the cortex, hypothalamus, and pons. GYKI 52466 had no effect on regional VO2 or rCBF in normoxic piglets compared with controls. Hypoxia resulted in an increase in local VO2 and rCBF in the cortex and hypothalamus compared with controls: rCBF from 50 +/- 10 to 97 +/- 16 and VO2 from 2.4 +/- 0.5 to 3.7 +/- 0.4 in the cortex, and rCBF from 41 +/- 9 to 99 +/- 17 and VO2 from 2.5 +/- 1 to 3.8 +/- 0.5 in the hypothalamus. GYKI 52466 abolished this hypoxic flow effect in both the cortex (68 +/- 14) and hypothalamus (73 +/- 12). GYKI 52466 also blocked the increased VO2 in the cortex (2.5 +/- 0.4) and hypothalamus (3.0 +/- 0.5) of the hypoxic group. These findings suggest that the AMPA receptor is an important controller of VO2 in the cortex and hypothalamus during hypoxia in this immature porcine model.

Effect of N-methyl-D-aspartate receptor blockade on the control of cerebral O2 supply/consumption balance during hypoxia in newborn pigs

Using dizocilpine (MK-801), we tested the hypothesis that N-methyl-D-aspartate (NMDA) receptors are important controllers of cerebral O2 supply/consumption balance in newborn piglets both during normoxia and hypoxia. Twenty-five 2 to 7-day-old piglets were anesthetized and divided into four groups: (1) Normoxia (n = 6), (2) Normoxia + MK-801 (n = 6), (3) Hypoxia (n = 6), and (4) Hypoxia + MK-801 (n = 7). Regional cerebral blood flow (rCBF) in ml/min/100 g was measured using 14C-iodoantipyrine, and we determined arterial and venous O2 saturations by microspectrophotometry, calculating cerebral O2 consumption (VO2) in ml O2/min/100 g in the cortex, hypothalamus and pons. MK-801 did not significantly affect regional VO2 or rCBF in normoxic piglets. Hypoxia resulted in an increase in local rCBF compared to controls: from 41 +/- 6 to 103 +/- 18 in the cortex; 34 +/- 7 to 101 +/- 20 in the hypothalamus; and 45 +/- 10 to 95 +/- 11 in the pons. Pretreatment with MK-801 abolished this hypoxic flow effect in the cortex (51 +/- 2) and hypothalamus (49 +/- 5), but not in the pons (91 +/- 17). Similar results were observed for VO2 with control values of 1.9 +/- 0.3, 1.6 +/- 0.2 and 2.1 +/- 0.3 for the cortex, hypothalamus and pons respectively. Hypoxia resulted in an increase in the VO2 to 3.9 +/- 0.4 (cortex), 3.8 +/- 0.6 (hypothalamus) and 3.9 +/- 0.8 (pons). Pretreatment with MK-801 prior to hypoxia abolished these effects in the cortex (2.1 +/- 0.2) and hypothalamus (2.1 +/- 0.2), but not in the pons (2.9 +/- 0.2). These findings suggest that NMDA receptors may play a role in the control of cerebral metabolism during hypoxia in this immature porcine model.

A model for the coupling between cerebral blood flow and oxygen metabolism during neural stimulation

A general mathematical model for the delivery of O2 to the brain is presented, based on the assumptions that all of the brain capillaries are perfused at rest and that all of the oxygen extracted from the capillaries is metabolized. The model predicts that disproportionately large changes in blood flow are required in order to support small changes in the O2 metabolic rate. Interpreted in terms of this model, previous positron emission tomography (PET) studies of the human brain during neural stimulation demonstrating that cerebral blood flow (CBF) increases much more than the oxygen metabolic rate are consistent with tight coupling of flow and oxidative metabolism. The model provides a basis for the quantitative interpretation of functional magnetic resonance imaging (fMRI) studies in terms of changes in local CBF.

Brain capillary perfusion during sleep

Brain capillary perfusion was evaluated in the different states of the wake-sleep cycle-quiet wakefulness (QW), quiet sleep (QS), and active sleep (AS)-in rats. The extent of the perfused capillary network was determined by intravascular distribution of a fluorescent marker. Evans blue (EB); it remained unchanged across the three behavioral conditions, QW, QS, and AS. The anatomical network was assessed by alkaline phosphatase (AP) endothelial staining, which is known to underestimate the number of existing capillaries. The resulting number of AP profiles were, therefore, significantly lower than the number of EB profiles, but the percentage of AP-stained capillaries that were perfused (96%) was also unchanged across the behavioral conditions. The results indicate that no capillary recruitment accompanies the wake-sleep cycle. Capillary surface area is a relevant factor in determining exchanges across the blood-brain barrier. In the absence of capillary recruitment (relative constancy of the surface area), the CBF changes during sleep should preferentially affect flow-limited with respect to diffusion-limited transport.

Local cerebral blood flow responses in rats to hypercapnia and hypoxia in the rostral ventrolateral medulla and in the cortex

The effects of hypercapnia and hypoxia on two local cerebral blood flows in the parietal cortex (PC-BF) and rostral ventrolateral medulla (RVLM-BF) were examined using laser Doppler flowmetry in anesthetized rats. Hypercapnia for 45 s duration at the end-tidal CO2 between 5% and 10%, induced by increasing inspiratory CO2, increased both cerebral blood flows and systemic blood pressure in a degree-dependent manner. The response of RVLM-BF was significantly stronger than that of PC-BF. Both cerebral blood flow responses to hypercapnia were not influenced by cutting peripheral chemoreceptor afferent nerves. Hypoxia for 45 s duration at the end-tidal O2 between 12% and 6%, induced by decreasing inspiratory O2, produced an increase of similar magnitude in both RVLM and PC local blood flows in a degree-dependent manner and a decrease in systemic blood pressure. The responses of both PC-BF and RVLM-BF to hypoxia were significantly diminished after cutting peripheral chemoreceptor afferent nerves. It is concluded that: (1) the RVLM-BF is much more sensitive to hypercapnia than the PC-BF; and (2) activation of peripheral arterial chemoreceptors possibly contributes to hypoxia-induced increase in the RVLM-BF and PC-BF.

Fluorescence labeling of the capillary network in rat brains

To assess the reliability of fluorescence methods for a quantitative staining of brain capillaries, three different immunohistochemical fluorescent markers were used in the rat brain. Staining of the basement membrane by antibodies directed against fibronectin was compared, in the same brain section, with simultaneous staining of the vascular endothelium constituents nonmuscle myosin or von Willebrand factor (factor VIII). These stainings all resulted in identical patterns, which demonstrates their suitability for capillary staining in the brain. It has been claimed that fixation of the tissue results in the appearance of spurious capillary spots. Such a fixation artifact could be excluded using nonmuscle myosin staining. These results validate the methods of quantitative fluorescent microscopical staining of capillary morphology in the brain and therefore support our concept of a continuous perfusion of all capillaries in the brains of conscious rats.

Effect of amphetamine on cerebral blood flow and capillary perfusion

The purpose of this study was to determine the cerebral regional microvascular and vascular responses to amphetamine sulfate at a dose (5 mg/kg) known to affect neuronal function. Cerebral blood flow (14C-iodoantipyrine method) and percent of perfused capillaries (fluorescein isothiocyanate-dextran and alkaline phosphatase staining method) were determined during control and after intravenous administration of amphetamine in conscious Long-Evans rats. Amphetamine caused an increase in blood pressure (34%) and heart rate (31%). There was a significant increase in averaged cerebral blood flow from 98 +/- 8 to 166 +/- 9 ml/min/100 g after amphetamine. This flow increase was significant in the cortex, basal ganglia, pons and medulla, however the increase was not significant in the hypothalamus. In control rats, there were approximately 325 +/- 17 capillaries/mm2 of brain tissue and 52 +/- 1% of them were perfused. Amphetamine increased the percent perfused significantly to 72 +/- 1% in all examined regions. There was a similar significant increase in the percent of perfused cerebral capillary volume fraction. There were both vascular and microvascular responses to amphetamine, increasing cerebral blood flow as well as reducing the diffusion distance for oxygen.