Search

The relationship between pial arteriolar caliber and cerebral blood flow (CBF) was examined in 11 cats subjected to reperfusion for up to 120 min after 10 min of global cerebral ischemia induced by four-vessel occlusion and systemic hypotension. Thirty minutes after reperfusion CBF, as assessed by radiolabeled microsphere injection, had increased to 588% of control in middle cerebral artery (MSEC) cortical gray matter territory. The caliber of MSEC pial arterioles measured using the closed cranial window technique (greater than 33 to less than 213 microns) increased to 172% of baseline. By 60 min of reperfusion, CBF was 76% of basal levels, but pial arterioles remained 133% of baseline. After 120 min, CBF approximated baseline values, but pial dilatation persisted (115% of control). Intracranial pressure measurements did not differ significantly from resting values. At 45 min and beyond, total cerebrovascular resistance did not differ from resting values. The coexistence of vasodilatation within pial arterioles and normal blood flow in cortical gray matter indicates that pial vessels (greater than 33 microns) cannot be responsible for normal blood flow restoration following postocclusive hyperemia. Resistance during the posthyperemic phase must be increased selectively within parenchymal vessels to account for normal total cerebrovascular resistance, pial vessel dilatation, and normal-low parenchymal blood flow. Whether obstruction rather than vasoconstriction explains the resistance changes within intraparenchymal vessels remains for further study.

We tested the effect of certain newly synthesized allosteric modifiers of hemoglobin on the dilation induced by arterial hypoxia, arterial hypotension, and arterial hypercapnia in cerebral arterioles of anesthetized cats equipped with cranial windows for the observation of the cerebral microcirculation. The allosteric modifiers of hemoglobin are isomers of 2-(aryloxy)-2-methylpropionic acid. They shift the oxygen dissociation of hemoglobin to the right, thereby facilitating the local release of oxygen. When these compounds were applied topically by superfusion at a rate of 1 ml/min in a concentration of 0.1 mM, they had no significant effect on baseline arteriolar diameter but reduced significantly the vasodilation from arterial hypoxia and arterial hypotension. They did not influence the vasodilation from arterial hypercapnia. Spectrophotometric measurements of optical densities from pial veins 50-80 microns in diameter indicated that the superfusion with the allosteric compounds reduced hemoglobin oxygen saturation both during room air breathing and during hypoxia. We conclude that the vasodilations from arterial hypoxia and arterial hypotension are mediated by local oxygen-dependent mechanisms. The allosteric modifiers of hemoglobin may be useful as tools in investigating oxygen-dependent mechanisms.

This manuscript considers some of the most prominent consequences of traumatic brain injury (TBI), namely vascular and axonal change, and evaluates the role of damaging oxygen radicals in their pathogenesis. To this end, existing as well as new data derived from traumatically injured experimental animals and humans was employed. Experimental animals were subjected to fluid-percussion brain injury. Some animals were equipped with cranial windows to allow for the functional assessment of the pial vasculature, while others received various exogenous tracers to assess blood-brain barrier status. In order to identify traumatically induced axonal change, some animals were also processed for the light and electron microscopic visualization of antibodies targeted to the neurofilament subunits. Similar immunocytochemical strategies were employed in the postmortem study of humans who had sustained severe TBI. Through these approaches, TBI was recognized to result in vascular abnormalities ranging from impaired vascular responsiveness to altered blood-brain barrier status. Typically, these vascular abnormalities continued for several hours postinjury and showed evolution which correlated with the production of damaging oxygen radicals. Importantly, the use of radical scavengers reversed these vascular abnormalities and provided protection. Traumatically induced axonal damage was also associated with evolving posttraumatic change. This involved the continued posttraumatic disassembly and misalignment of the intra-axonal neurofilament subunits which caused impaired axoplasmic transport leading to axonal swelling and detachment. Although these intra-axonal changes did not appear to be directly caused by oxygen radicals, it is suggested that the presence of oxygen radicals may exacerbate the progression of these reactive events.

The importance of trigeminal neuroeffector mechanisms in the regulation of postischemic cerebral blood flow (CBF) was evaluated in cats subjected to chronic unilateral denervation of cortical sensory nerve fibers by trigeminal ganglionectomy or by the topical application of capsaicin to a cortical branch of the middle cerebral artery. CBF was determined using isotopically labeled microspheres before and at intervals after reperfusion following 10 min of global cerebral ischemia induced by four vessel occlusion combined with systemic hypotension. Postocclusive hyperemia 30 min after reperfusion in cortical gray matter ipsilateral to the side of denervation was attenuated by up to 58% (176 vs. 91 ml/100 g per min; p less than 0.05), but resting CBF, the duration of hyperemia, and the cerebrovascular response to hypercapnia were unaffected. These data underline the influence of neurogenic mechanisms in the regulation of postischemic CBF. Blockade of this axon reflex-like mechanism may reduce the morbidity associated with several hyperperfusion syndromes.

The influence of the trigeminal nerve on the cerebral circulation was investigated in chronically denervated cats during and after reversible four-vessel occlusion for 10 minutes combined with controlled hypotension (50 mm Hg). Postocclusive hyperemia 30 minutes after reperfusion was attenuated by up to 48% in cortical gray matter of the anterior, middle, and posterior cerebral artery territories on the side of trigeminal ganglionectomy. Similar results were observed for denervation accomplished by direct surgical ablation and by the topical application of capsaicin to a cortical branch of the middle cerebral artery. Denervation did not alter basal cerebral blood flow or the duration of hyperemia, nor did it impair the cerebrovascular response to hypercapnia. These data demonstrate the importance of neurogenic mechanisms in the development of postischemic hyperperfusion and suggest that strategies directed at blocking axon reflex-like mechanisms may be beneficial in reducing the morbidity that follows severe cortical hyperemia.

We studied the role of prostaglandins and free radicals in the induction of the functional and morphological pial arteriolar abnormalities produced by concussive brain injury. Anesthetized cats equipped with a cranial window for the observation of the pial microcirculation were subjected to concussive brain injury using a fluid-percussion device following administration of cyclooxygenase inhibitors (indomethacin or AHR-5850) or the vehicle for the solution of these agents (NaCl or Na2CO3 solution). Pial arterioles from vehicle-treated animals displayed sustained dilation, reduced responsiveness to the vasoconstrictor effect of arterial hypocapnia, and a high density of endothelial lesions. Animals pretreated with cyclooxygenase inhibitors showed less pronounced vasodilation, normal responsiveness to hypocapnia, and a significantly reduced number of lesions. The vasodilation and reduced responsiveness to the vasoconstrictor effects of hypocapnia after brain injury also were inhibited by topical application of free radical scavengers (nitroblue tetrazolium, superoxide dismutase, or mannitol). The vessels from cats pretreated with free radical scavengers also had a lower density of endothelial lesions than controls. The results support the view that the immediate cause of cerebral arteriolar damage in concussive brain injury is the generation of free oxygen radicals associated with increased prostaglandin synthesis.

Marked hyperemia may develop in brain following temporary cessation of blood flow and is associated with the morbidity following cardiac arrest, stroke, and head injury. Regional cerebral blood flow was measured using radiolabeled microspheres and compared in 10 symmetrical regions after chronic unilateral trigeminal ganglionectomy (n = 8), trigeminal rhizotomy (n = 4), or sham operation (n = 4) following 10 min of combined brachiocephalic-left subclavian occlusion and hypotension (mean arterial blood pressure less than 50 mmHg) in cats. Blood flow was symmetrical at rest in the three groups and was undetectable during the ischemic period. Within 30 min after re-establishing flow, values in cortical gray matter increased symmetrically to approximately 250 ml.100 g-1.min-1 in the rhizotomy and the sham groups. Increases of similar magnitude were measured on the intact side following trigeminal ganglionectomy but flow was attenuated by greater than 50% ipsilateral to the ganglionectomy. Marked hyperemia developed during reperfusion in thalamus, caudate, deep cortical white matter, midbrain, and pons, but no asymmetries were present in the three groups. These data suggest that cortical hyperemia is mediated by trigeminal neurogenic mechanisms via axonal reflexlike mechanisms and suggest the importance of therapeutic strategies based on blockade of this nerve or its constituent neuropeptides.

The appearance of superoxide anion radical in cerebral extracellular space during and after acute hypertension induced by intravenous norepinephrine was investigated in anesthetized cats equipped with cranial windows. Superoxide was detected by demonstrating the presence of superoxide dismutase-inhibitable reduction of nitroblue tetrazolium. The superoxide dismutase-inhibitable rate of reduction of nitroblue tetrazolium was 4.1 +/- 1.61 nM/min per cm2 during hypertension and 4.55 +/- 0.62 nM/min per cm2 one hour after hypertension had subsided. During norepinephrine administration in the absence of hypertension, the superoxide dismutase-inhibitable rate of reduction of nitroblue tetrazolium was 0.44 +/- 0.17 nM/min per cm2. The reduction of nitroblue tetrazolium during hypertension was also inhibited by prior treatment of the brain surface with phenylglyoxal at pH 10, to induce irreversible inhibition of the anion channel. The results show that acute hypertension is associated with the generation of superoxide which enters the extracellular space of the brain via the anion channel. Following hypertension, the sustained vasodilation caused by acute hypertension was inhibited significantly by topical application of superoxide dismutase and catalase, showing that it was due in part to superoxide and other radicals derived from it. The vasodilator response of cerebral arterioles to topical acetylcholine was converted to vasoconstriction following acute hypertension, and restored to vasodilation following topical application of superoxide dismutase and catalase. The results show that superoxide and other radicals generated after acute hypertension interfere with acetylcholine-induced endothelium-dependent vasodilation, probably because they destroy the endothelium-derived relaxant factor.

Acute severe hypertension induced by intravenous norepinephrine or angiotensin in anesthetized cats equipped with a cranial window caused prolonged arteriolar vasodilation associated with reduced responsiveness to arterial hypercapnia or hypocapnia and passive response to changes in arterial blood pressure. Scanning and transmission electron microscopy of such pial arterioles showed discrete destructive endothelial lesions the density of which correlated with the degree of vasodilation. Abnormalities of the vascular smooth muscle were seen in all dilated arterioles but affected only a small number of smooth muscle cells. The oxygen consumption of pial arterioles from cats subjected to hypertension was significantly reduced in comparison to that of vessels from normal animals. The arteriolar abnormalities induced by hypertension were inhibited by pretreatment with inhibitors of cyclooxygenase (indomethacin or AHR-5850) or by topical application on the brain surface of scavengers of free oxygen radicals (mannitol or superoxide dismutase). The results suggest that the mechanism of the arteriolar abnormalities from acute hypertension involves a sudden increase in prostaglandin synthesis that leads to generation of free oxygen radicals.

The effect of changes in cerebrospinal fluid (CSF) concentration of magnesium ion ([Mg2+]) on pial arterioles was investigated in anesthetized cats equipped with acutely implanted cranial windows for the observation of the pial microcirculation. Increased [Mg2+] caused vasodilation, whereas decreased [Mg2+] caused vasoconstriction. The effect of [Mg2+] was dose dependent and was the same in small and larger arterioles. There was an interaction between CSF [Mg2+] and calcium ion concentration ([Ca2+]), such that the vasodilator effect of Mg2+ was greater when the [Ca2+] was lower, especially in larger vessels. The vasodilator effect of Mg2+ on pial arterioles was enhanced in the presence of the calcium antagonist verapamil (0.5 micrograms/ml), despite the fact that verapamil by itself caused a 12-13% arteriolar dilation. These results show that the vasodilator effect of Mg2+ is probably related to an interaction at the cell membrane resulting in reduction in the influx of Ca2+ into vascular smooth muscle.

There is still considerable controversy regarding the influence of blood viscosity upon CBF. We have measured CBF with microspheres in 23 cats. Autoregulation was disturbed in the left caudate nucleus by microsurgical occlusion of the left middle cerebral artery. Induced hypertension or hypotension was used and i.v. mannitol (1 g/kg) administered. In all cats blood viscosity decreased an average of 16% at 15 minutes and, in 16 cats, increased 10% at 75 minutes post-mannitol. CBF in the right caudate was 79 +/- 6 ml/100g/min, in the left 38 +/- 6 (p less than 0.001). Only minor changes of CBF occurred in areas with presumed normal autoregulation, including the right caudate, in conjunction with pressure or viscosity changes. In the left caudate CBF decreased 21% with hypotension and 18% with higher viscosity, more than on the right (p less than 0.01 and p less than 0.2, respectively). CBF increased in the left caudate 56% with hypertension and 47% with lower viscosity, again much more than on the right (p less than 0.001 and p less than 0.01, respectively). In the other area which is (nearly) exclusively supplied by the middle cerebral artery of the cat, i.e., the ectosylvian cortex, results were similar to those in the caudate nucleus. These results show that viscosity changes must result in compensatory readjustments of vessel diameter, but that these adjustments do not occur where autoregulation to pressure changes is known to be defective. The adjustments to viscosity changes might be called blood viscosity autoregulation of CBF. We hypothesize that pressure autoregulation and blood viscosity autoregulation share the same mechanism.

The responses of pial precapillary vessels were examined bilaterally using a closed cranial-window preparation in 11 anesthetized cats after chronic unilateral section of the trigeminal ganglia. During increases or decreases in arterial PCO2 or hypoxia, vasoconstrictor and vasodilator responses were symmetrical and appropriate on the two sides. The superfusion of vessels with norepinephrine (10 micrograms/ml) constricted large and small pial arterioles to a greater extent on the denervated side, and this difference persisted even after washing (P less than 0.02). After severe hypertension, small and large arterioles dilated on both sides, although to a greater extent on the innervated side. In 8 of 10 acutely hypertensive animals, extravasation of intravenously administered iodinated albumin was greater in the intact than deafferented hemisphere. These results suggest that the trigeminal nerve can modify the responses of cerebral arterioles and participate in the regulation of the cerebral vasculature.

The responses of cerebral (pial) arterioles to increased venous pressure were examined in anesthetized cats equipped with cranial windows for the observation of the cerebral microcirculation. Increased venous pressure was induced by occlusion of the superior vena cava. Intracranial pressure was kept constant. Increased venous pressure when the window was filled with stationary cerebrospinal fluid caused 9-12% arteriolar dilation. Cerebral arteriolar dilation of equal magnitude (8-12%) was also seen when the space under the cranial window was perfused with fluorocarbon FC-80 equilibrated with 100% nitrogen. Increased venous pressure when the cranial window space was perfused with fluorocarbon equilibrated with 100% oxygen caused a small (5%) but significant arteriolar constriction. These results show that the dominant mechanism of autoregulation in the cerebral arterioles is metabolic, and that it involves an oxygen-sensitive mechanism. Myogenic vasoconstriction is unmasked during venous hypertension when the dominant metabolic mechanism is eliminated by increased local supply of oxygen.

A modified cranial-window technique was devised that permits direct observation and study of the pial microcirculation of the cerebellum and comparisons of the responses of the microvessels in this location with those in the hemisphere. We used this technique in anesthetized cats to compare the responses of cerebellar and cerebral arterioles with alterations in arterial blood CO2 tension. Arterioles in these two locations responded with similar percentile changes in vessel caliber to both hypercapnia and hypocapnia.

The effect of vasoactive intestinal polypeptide (VIP) on cerebral arterioles was examined in anesthetized cats equipped with an acutely implanted cranial window for the observation of the pial microcirculation. Topical application of VIP on the surface of the brain in doses of 0.01-1.0 microgram/ml produced a dose-related vasodilation that was of equal magnitude in small (< 100 micrometer diam) and large (> 100 micrometer diam) pial arterioles. The maximum dilation averaged 20% of the control diameter. There were no associated changes in arterial blood pressure or in arterial CO2 tension. In additional experiments the vasodilator effect of VIP was completely inhibited by pretreatment with intravenous indomethacin or AHR-5850. These nonsteroidal anti-inflammatory drugs inhibit cyclooxygenase activity and thereby interfere with the synthesis of prostaglandins and related compounds. The vasodilator effect to topical histamine was unaffected by indomethacin and AHR-5850. The results show that VIP has a strong vasodilator effect on pial arterioles of the cat and suggest that this effect is mediated by prostaglandins.

The responses of pial arterioles to increased venous pressure were studied in anesthetized cats equipped with cranial windows for the observation of the pial microcirculation. Stable increases in venous pressure consistently induced arteriolar vasodilation, which averaged 6-12% of the control diameter. The vasodilation occurred when arterial blood pressure was normal and during arterial hypotension induced by bleeding; it also occurred irrespective of whether intracranial pressure was kept constant or was allowed to increase venous hypertension. The results are consistent with the view that autoregulatory adjustments in caliber of pial arterioles are mediated predominantly by metabolic rather than myogenic mechanisms.

The effect of inhibition of prostaglandin synthesis on the pial arteriolar responses to arterial hypercapnia, hypocapnia, and hypoxia was studied in anesthetized cats equipped with a cranial window for the observation of the pial microcirculation of the parietal cortex. Inhibition of prostaglandin synthesis was achieved by intravenous administration of indomethacin (3 mg/kg) or AHR-5850 (2-amino-3-benzoylbenzeneacetic acid, 50 mg/kg). It was shown that the administration of these agents inhibited substantially the vasodilation in response to topical application of arachidonic acid (100--200 micrograms/ml). Inhibition of prostaglandin synthesis did not modify significantly the vasodilator responses to arterial hypercapnia or arterial hypoxia, nor the vasoconstrictor response to arterial hypocapnia. We conclude that endogenous prostaglandins are not mediators of these vascular responses in the pial microcirculation.

A method is described whereby specific segments of the pial vasculature of cats can be both physiologically and morphologically studied. This is accomplished by the insertion of a cranial window through which specific pial vessels can be visualized, illustrated, numbered in a simple sketch, and physiologically evaluated under both control and experimental conditions. Following these procedures, perfusion of aldehydes is initiated and the pia and associated vasculature are stripped from the cortical mantle. The thin velum of pia so obtained permits the direct visualization of the pial vasculature whose unchanged configuration can be matched to the appropriate previously sketched vessels whose individual physiological status is known. In turn these vessels are divided into alternate segments and are prepared for either scanning or transmission electron microscopy. The advantages of this methodology is exemplified by the fact that it provides the physiological status and morphological characteristics of the same microvascular segment. Although designed for the pial vasculature, this protocol may also prove of use in the study of other superficially coursing microvascular beds.

The effects of topical application of agents which produce oxygen radicals on cerebral arterioles were studied in anesthetized cats. Xanthine oxidase plus xanthine, which produced superoxide anion radical, hydrogen peroxide, and hydrogen peroxide plus ferrous sulfate, which produced the free hydroxyl radical, induced sustained dilation, reduced responsiveness to the vasoconstrictor effect of hypocapnia, and destructive lesions of the endothelium and of the vascular smooth muscle. Similar effects were produced by arachidonate, 15-HPETE, and PGG2. The effect of arachidonate was inhibited by mannitol, a free hydroxyl radical scavenger, the effect of PGG2 was inhibited by SOD, the effect of 15-HPETE was inhibited by either catalase or SOD. These results suggest that these cerebral vascular abnormalities were produced by a single destructive free radical, probably the hydroxyl free radical, generated via interaction of superoxide and hydrogen peroxide. Cerebral vascular abnormalities similar to those produced by oxygen radicals were also seen after experimental concussive brain injury or after acute hypertension. After brain injury, activation of phospholipase C and increased brain prostaglandin concentration were demonstrated. The vascular effects of brain injury and acute hypertension were inhibited by free radical scavengers. The results suggest that, in these conditions, vascular damage is induced by oxygen radicals generated from arachidonate in association with increased prostaglandin synthesis.

✓ There is no proof that osmotic agents such as mannitol lower intracranial pressure (ICP) by decreasing brain water content. An alternative mechanism might be a reduction in cerebral blood volume through vasoconstriction. Mannitol, by decreasing blood viscosity, would tend to enhance cerebral blood flow (CBF), but the cerebral vessels would constrict to keep CBF relatively constant, analogous to pressure autoregulation. The cranial window technique was used in this study to measure the pial arteriolar diameter in cats, together with blood viscosity and ICP changes after an intravenous bolus of 1 gm/kg of mannitol. Blood viscosity decreased immediately; the greatest decrease (23%) occurred at 10 minutes, and at 75 minutes there was a “rebound” increase of 10%. Vessel diameters decreased concomitantly, the largest decrease being 12% at 10 minutes, which is exactly the same as the 12% decrease in diameter associated with pronounced hyperventilation (PaCO2 30 to 19 mm Hg) in the same vessels; at 75 minutes vessel diameter increased by 12%. With hyperventilation, ICP was decreased by 26%; 10 minutes after mannitol was given, ICP decreased by 28%, and at 75 minutes it showed a rebound increase of 40%. The correlation between blood viscosity and vessel diameter and between vessel diameter and ICP was very high. An alternative explanation is offered for the effect of mannitol on ICP, the time course of ICP changes, the “rebound effect,” and the absence of influence on CBF, all with one mechanism.