Elke M. Golding

The Consequences of Traumatic Brain Injury on Cerebral Blood Flow and Autoregulation: A Review

In this decade, the brain argueably stands as one of the most exciting and challenging organs to study. Exciting in as far as that it remains an area of research vastly unknown and challenging due to the very nature of its anatomical design: the skull provides a formidable barrier and direct observations of intraparenchymal function in vivo are impractical. Moreover, traumatic brain injury (TBI) brings with it added complexities and nuances. The development of irreversible damage following TBI involves a plethora of biochemical events, including impairment of the cerebral vasculature, which render the brain at risk to secondary insults such as ischemia and intracranial hypertension. The present review will focus on alterations in the cerebrovasculature following TBI, and more specifically on changes in cerebral blood flow (CBF), mediators of CBF including local chemical mediators such as K+, pH and adenosine, endothelial mediators such as nitric oxide and neurogenic mediators such as catecholamines, as well as pressure autoregulation. It is emphasized that further research into these mechanisms may help attenuate the prevalence of secondary insults and therefore improve outcome following TBI.

Endothelium-derived hyperpolarizing factor: a cousin to nitric oxide and prostacyclin.

There is now strong evidence that an endothelial mechanism, other than nitric oxide or prostacyclin, exists for dilating arteries and arterioles. This third pathway has been named endothelium-derived hyperpolarizing factor (EDHF) and should not be confused with endothelium-derived relaxing factor, which is nitric oxide. Currently, there are several ideas for the mechanism of EDHF, which may vary among vessels of different organs and species. During some pathologic states, EDHF can be up-regulated. This up-regulation often occurs as the dilator effects of endothelium-derived nitric oxide are suppressed. The up-regulated EDHF may serve in a protective capacity to help maintain blood flow to organs and tissues during these stressful states. Many anesthetics attenuate the dilator actions of EDHF; however, the full clinical implications of this anesthetic-related attenuation are not known. Like its cousins, nitric oxide and prostacyclin, EDHF is an important regulator of blood flow and should prove to be an important clinical consideration as we gain more knowledge of its mechanisms of action.

Cerebrovascular reactivity to CO2 and hypotension after mild cortical impact injury

Cerebrovascular reactivity to CO(2) or hypotension was studied in vivo and in vitro [pressurized arteries ( approximately 82 micrometer) and arterioles ( approximately 30 micrometer)] at 1 h after mild controlled cortical impact (CCI) injury in rats. The cortical perfusion response [assessed using laser-Doppler flowmetry (LDF)] to altered CO(2) was diminished (up to 81%) after mild CCI injury. The responses to CO(2) alterations in arteries and arterioles isolated from the injured cortex were similar to responses in vessels isolated from sham-injured animals. After mild CCI injury, the autoregulatory response to hypotension (measured using LDF) was maintained or even enhanced, depending on the method used to measure the response. Vessels isolated from the injury site showed a response to changes in pressure similar to that in vessels isolated from sham-injured rats. We conclude that mild CCI injury produces complicated alterations in cerebrovascular control. Whereas the autoregulatory response to hypotension was maintained or even enhanced, the in vivo vascular response to CO(2) was severely compromised. The altered response to CO(2) was not caused by an intrinsic vascular perturbation but rather an altered milieu after mild CCI injury.Cerebrovascular reactivity to CO2 or hypotension was studied in vivo and in vitro [pressurized arteries (∼82 μm) and arterioles (∼30 μm)] at 1 h after mild controlled cortical impact (CCI) injury in rats. The cortical perfusion response [assessed using laser-Doppler flowmetry (LDF)] to altered CO2 was diminished (up to 81%) after mild CCI injury. The responses to CO2 alterations in arteries and arterioles isolated from the injured cortex were similar to responses in vessels isolated from sham-injured animals. After mild CCI injury, the autoregulatory response to hypotension (measured using LDF) was maintained or even enhanced, depending on the method used to measure the response. Vessels isolated from the injury site showed a response to changes in pressure similar to that in vessels isolated from sham-injured rats. We conclude that mild CCI injury produces complicated alterations in cerebrovascular control. Whereas the autoregulatory response to hypotension was maintained or even enhanced, the in vivo vascular response to CO2 was severely compromised. The altered response to CO2 was not caused by an intrinsic vascular perturbation but rather an altered milieu after mild CCI injury.

Comparison of the myogenic response in rat cerebral arteries of different calibers

The myogenic response, the characteristic of blood vessels to contract with increasing pressure, was studied at three different locations along the middle cerebral artery (MCA) vascular tree. We hypothesized that smaller caliber vessels would have a more pronounced myogenic response at lower pressures than larger diameter arteries, corresponding to pressures normally experienced in vivo. Cerebral vessels (MCAs, branches of the MCA, and penetrating arterioles) were isolated from male rats, cannulated with glass micropipettes, and pressurized. Changes in diameter were measured as the transmural pressure was increased from 20-100 mmHg. The MCAs, which had a resting diameter of 202 +/- 10 micron (n = 9) at 50 mmHg, showed its greatest myogenic response between 60-100 mmHg (8+/-2% constriction, n = 9, p < 0.001). The penetrating arterioles [58 +/- 4 micron (n = 8) at 50 mmHg], on the other hand, showed its greatest myogenic response between 20-60 mmHg (10 +/- 4% constriction, n = 8, p < 0.05). Branches of the MCA [118 +/- 14 micron (n = 8) at 50 mmHg] showed a slight constriction over the entire pressure range (5 +/- 9% constriction between 20-100 mmHg, p=ns). Our results suggest that the myogenic response appears to be best developed in the range of pressures found during physiological conditions for a given vessel in the MCA territory. This characteristic is fundamental in the overall control of cerebrovascular resistance.

Potentiated Endothelium-Derived Hyperpolarizing Factor–Mediated Dilations in Cerebral Arteries Following Mild Head Injury

Evidence in the literature suggests that endothelium-derived hyperpolarizing factor (EDHF) may act in a compensatory manner such that during conditions of compromised nitric oxide (NO), EDHF serves as a back-up mechanism. Given that constitutive NO synthase is chronically downregulated after head trauma, we tested the hypothesis that EDHF is potentiated following injury. Male adult rats were subjected to either sham injury (n = 27) or mild controlled cortical impact (CCI) injury (n = 26). Branches of the middle cerebral artery (MCA) directly within the contusion site were harvested either 1 or 24 h later, pressurized to 60 mm Hg in a vessel chamber and allowed to develop spontaneous tone. Relaxation to luminal application of adenosine triphosphate (ATP) was similar in all groups. Relaxation to ATP in the presence of L-NAME (N(G)-nitro-L-arginine methyl ester) and indomethacin was similar in all groups except for vessels isolated at 24 h following mild CCI injury. In this case, L-NAME and indomethacin had no effect on the ATP-mediated dilation. The ATP-mediated dilation in L-NAME and indomethacin-treated MCA branches was inhibited by charybdotoxin, an inhibitor of large conductance Ca2+-sensitive K+ channels. These findings suggest that there is a significant potentiation of the EDHF-mediated dilation to ATP in cerebral arteries isolated at 24 h following mild CCI injury.

Altered Calcium Dynamics Do Not Account for Attenuation of Endothelium-Derived Hyperpolarizing Factor-Mediated Dilations in the Female Middle Cerebral Artery

Background and Purpose— The contribution of endothelium-derived hyperpolarizing factor (EDHF) to ATP-mediated dilations is significantly attenuated in the rat middle cerebral artery of intact and estrogen-treated ovariectomized (OVX) females compared with males and vehicle-treated OVX females. Since an increase in endothelial calcium appears to be a critical prerequisite in the EDHF response, we tested the hypothesis that endothelial cell intracellular calcium ([Ca2+]i) fails to reach sufficient levels to elicit robust EDHF-mediated dilations in females and that this effect is mediated by estrogen. Methods— Vascular diameter and [Ca2+]i were measured concomitantly in perfused middle cerebral artery segments with the use of videomicroscopy and fura 2 fluorescence, respectively. Results— In the presence of NG-nitro-l-arginine methyl ester and indomethacin, the dilation to 10−5 mol/L ATP was significantly reduced (P <0.05) in intact females (42±8%; n=6) and estrogen-treated OVX females (25±6%; n=9) compared with intact males (89±5%; n=6) and vehicle-treated OVX females (92±2%; n=7). Contrary to our initial hypothesis, endothelial cell [Ca2+]i increased to comparable levels in intact females (461±116 nmol/L), estrogen-treated OVX females (417±50 nmol/L), intact males (421±77 nmol/L), and vehicle-treated OVX females (530±92 nmol/L). In response to luminal ATP (10−5 mol/L), smooth muscle cell [Ca2+]i decreased to a greater degree in males (37±4%; n=8) compared with females (21±5%; n=7) and in vehicle-treated OVX females (18±7%; n=7) compared with estrogen-treated OVX females (3±5%; n=9). Conclusions— Our data suggest that loss of a factor coupling EDHF to reduction of ionized smooth muscle cell [Ca2+]i accounts for the attenuated EDHF-mediated dilations in the female middle cerebral artery.

L-arginine partially restores the diminished CO2 reactivity after mild controlled cortical impact injury in the adult rat.

Using an open cranial window technique, the authors investigated the mechanisms associated with the suppressed CO2 reactivity after mild controlled cortical impact (CCI) injury in rats. The dilation of arterioles (n = 7) to hypercapnia before injury was 38 ± 12%, which was significantly reduced both at 1 hour (23 ± 15% dilation) and at 2 hours after injury (11 ± 19% dilation). In the presence of L-arginine (10 mmol/L topical suffusion, 300 mg/kg intravenous infusion), the dilation of pial arterioles (n = 6) to hypercapnia was partially restored to 30 ± 6% at 2 hours after injury. In the presence of the nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP) (10−8 mol/L topical suffusion), the dilation of pial arterioles (n = 5) to hypercapnia remained diminished (5 ± 7%) at 2 hours after injury. The dilation of pial arterioles (n = 4) to hypercapnia also remained suppressed (5 ± 6%) with topical suffusion of the free radical scavengers, polyethylene glycol-superoxide dismutase (60 units/mL) and polyethylene glycol-catalase (40 units/mL). The authors have shown that L-arginine at least partially restores the diminished response to hypercapnia after mild CCI injury. Furthermore, these data suggest that the beneficial effects of L-arginine are mediated by a combination of providing substrate for NO synthase and scavenging free radicals.

The effects of potassium on the rat middle cerebral artery.

After traumatic brain injury, extracellular K(+) in brain can dramatically increase. We studied the effects of increased K(+) on the isolated pressurized rat middle cerebral artery (MCA). MCAs (200-250 microm OD) were isolated, cannulated with glass micropipettes, and pressurized. K(+) was increased in the extraluminal bath using three paradigms: (1) isotonic K(+) (K(iso)) where increases in K(+) were offset by decreases in Na(+), (2) hypertonic K(+) (K(hyper)) where K(+) was increased without a concomitant adjustment of Na(+), and (3) K(suc), a solution using K(iso) but with the addition of sucrose to obtain a hypertonic solution. Increases in K(+) in the extraluminal bath produced significant dilations (approximately 20%) at 21 mM K(+) in all three groups (K(iso), K(hyper), and K(suc)). With the K(hyper) and K(suc) groups, the magnitude of the dilation diminished with further increases in K(+). L-NAME (10(-5) M), an inhibitor of nitric oxide synthase, had no effect on the response of the K(hyper) and K(suc) groups at 21 mM but significantly enhanced constrictions of the MCAs above 40 mM K(+) compared to the control. The K(iso) group was not affected by L-NAME at any K(+) concentration and showed profound constrictions above 40 mM K(+). We conclude that changes in the K(+) concentration and osmolality of the extracellular fluid may have profound effects on the cerebral vasculature.

Blood Glucose Concentration Does Not Affect Outcome in Brain Trauma: A 31P MRS Study

Effects of blood glucose concentration on biochemical and neurologic outcome following lateral fluid percussion-induced traumatic injury of moderate severity (2.8 atm) in rats were studied using radioactive phosphorus (31P) magnetic resonance spectroscopy (MRS) and a battery of tests designed to evaluate posttraumatic neurologic motor function. Prior to injury, male Sprague-Dawley rats (n = 18) were randomly assigned to receive either dextrose, 2 ml 50% (wt/vol), zinc insulin (10 IU/kg) or no treatment, thus dividing the animals into hyperglycemic, hypoglycemic, and normoglycemic groups, respectively. Animals were then injured, monitored for 4 h by 31P MRS before being allowed to recover, and assessed for posttraumatic motor function. Following brain injury, there was no difference in brain intracellular pH between groups over the 4-h posttraumatic MRS monitoring period. Similarly, intracellular free magnesium, cytosolic phosphorylation potential, and neurologic outcome posttrauma were not significantly different between groups. We conclude that, unlike models of ischemia, blood glucose concentration may not be a significant factor affecting outcome in traumatic brain injury.

Efficacy of competitive vs noncompetitive blockade of the NMDA channel following traumatic brain injury

N-methyl-D-aspartate (NMDA) receptor antagonists have been demonstrated widely to be neuroprotective in cerebral ischemia, hypoxia, and traumatic brain injury. However, although noncompetitive NMDA antagonists have typically proven efficacious under all of these conditions, competitive antagonists have not been shown to be beneficial following moderate traumatic brain injury. The present study has used phosphorus magnetic resonance spectroscopy ([31P]MRS) to examine the effects of the competitive antagonist cis-4-(phosphonomethyl) piperidine-2-carboxylic acid (CGS-19755) and the noncompetitive antagonist dextromethorphan on biochemical outcome following fluid percussion-induced traumatic brain injury in rats. Five minutes prior to induction of moderate (2.8 +/- 0.2 atm) fluid percussion brain injury, animals received either CGS-19755 (10 mg/kg iv), dextromethorphan (10 mg/kg iv), or equal volume saline vehicle. [31P]MRS spectra were then acquired for 4 h post-trauma and intracellular pH, free magnesium concentration, cytosolic phosphorylation potential, and oxidative capacity determined. Both CGS-19755-treated animals and saline treated controls demonstrated significant and sustained declines in intracellular free magnesium concentration and bioenergetic status following trauma. In contrast, administration of dextromethorphan significantly attenuated free magnesium decline and improved bioenergetic state during the post-traumatic monitoring period. These results suggest that the neuroprotective actions of NMDA antagonists following traumatic brain injury are associated with attenuation of free magnesium decline and that such actions seem to be preferentially mediated by noncompetitive blockers.