Oxygen-dependent mechanisms in cerebral autoregulation

Autoregulatory adjustments in the caliber of cerebral arterioles were studied in anesthetized cats equipped with cranial windows for the direct observation of the pial microcirculation. Increased venous pressure caused slight, but consistent, arteriolar dilation, at normal and at reduced arterial blood pressure and irrespective of whether or not intracranial pressure was kept constant or allowed to increase. Arterial hypotension caused arteriolar dilation which was inhibited partially by perfusion of the space under the cranial window with artificial CSF equilibrated with high concentrations of oxygen. This vasodilation was inhibited to a greater extent by perfusion of the space under the cranial window with fluorocarbon FC-80, equilibrated with high concentrations of oxygen. CSF or fluorocarbon equilibrated with nitrogen did not influence the vasodilation in response to arterial hypotension. The response to increased venous pressure was converted to vasoconstriction when fluorocarbon equilibrated with high concentrations of oxygen was flowing under the cranial window. The vasodilation in response to arterial hypotension was inhibited by topical application of adenosine deaminase. The results show that both metabolic and myogenic mechanisms play a role in cerebral arteriolar autoregulation. Under normal conditions, the metabolic mechanisms predominate. The presence of the myogenic mechanisms may be unmasked by preventing the operation of the metabolic mechanisms. The major metabolic mechanism seems to be dependent on changes in PO2 within the brain with secondary release of adenosine.

Reduction in cerebral arteriolar oxygen consumption by arachidonate

The oxygen consumption of cerebral arterioles from anesthetized cats was measured using the Cartesian diver microrespirometer following in vitro incubation with 200 micrograms/ml of arachidonate or 50 micrograms/ml of 15-hydroperoxy-eicosatetraenoic acid (15-HPETE). Both agents depressed oxygen consumption severely. This effect was inhibited completely by a combination of superoxide dismutase (SOD) and catalase, indicating that it is mediated by oxygen radicals. Similar depression of oxygen consumption was observed during incubation of the vessels with xanthine oxidase and acetaldehyde as substrate. This enzymic system is known to generate superoxide and hydrogen peroxide. The effect of xanthine oxidase was also partially inhibited by SOD and catalase. The effect of arachidonate was partially inhibited by cyclooxygenase inhibitors. The effect of lipoxygenase inhibitors could not be adequately tested because they depressed oxygen consumption by themselves. Prostaglandins H2 and E2 had no effect on arteriolar oxygen consumption. The results show that arachidonate and 15-HPETE in high concentration depress cerebral arteriolar oxygen consumption via an oxygen radical-mediated mechanism. Furthermore, the radical is generated in the vessel wall and does not require either the brain parenchyma or the formed elements of the blood or the meninges for its production.

Oxygen radicals in the adult respiratory distress syndrome, in myocardial ischemia and reperfusion injury, and in cerebral vascular damage

Recent work suggests that oxygen radicals may be important mediators of damage in a wide variety of pathologic conditions. In this review we consider the evidence supporting the participation of oxygen radicals in the adult respiratory distress syndrome, in ischemia reperfusion injury in the myocardium, and in cerebral vascular injury in acute hypertension and traumatic brain injury. In the adult respiratory distress syndrome there is active sequestration of polymorphonuclear neutrophils in the pulmonary vascular system. There is evidence that activation of these neutrophils results in the production of oxygen radicals which injure the capillary membrane and increase permeability, leading to progressive hypoxia and decreased lung compliance which are hallmarks of the syndrome. In acute arterial hypertension or experimental brain injury oxygen radicals are important mediators of vascular damage. The metabolism of arachidonic acid is the source of oxygen free radical production in these conditions. In myocardial ischemia and reperfusion injury, the ischemic myocyte is "primed" for free radical production. With reperfusion and reintroduction of molecular oxygen there is a burst of oxygen radical production resulting in extensive tissue destruction. Myocardial ischemia--reperfusion injury shares in common with the other two syndromes activation of the arachidonic acid cascade and acute inflammation. Thus it would appear that the generation of toxic oxygen species may represent a final common pathway of tissue destruction in several pathophysiologic states.

Effects of oxygen radicals on cerebral arterioles

Xanthine oxidase and xanthine, a combination that produces hydrogen peroxide and superoxide anion radical, applied topically in anesthetized cats equipped with cranial windows caused arteriolar dilation during application, sustained dilation 1 h after washout, and reduced reactivity to the vasoconstrictive effects of arterial hypocapnia, discrete lesions of the endothelium, and morphological abnormalities of the vascular smooth muscle by electron microscopy. Similar effects were seen in small, but not in large, arterioles during topical application of hydrogen peroxide or hydrogen peroxide plus ferrous sulfate, a combination that produces free hydroxyl radical. The functional changes caused by xanthine oxidase plus xanthine were inhibited completely by superoxide dismutase plus catalase. Superoxide dismutase or catalase, each by itself, eliminated the residual effects seen after washout and reduced the dilation during application of xanthine oxidase. The results show that superoxide anion radical and hydrogen peroxide produce reversible arteriolar dilation and that consistent vascular damage is produced in the presence of both superoxide anion radical and hydrogen peroxide.

Continuing axonal and vascular change following experimental brain trauma

The course of axonal and vascular change following trauma was investigated in an animal model of fluid-percussion brain injury. To assess axonal change, the anterograde transport of horseradish peroxidase in selected cerebral and cerebellar efferents was studied in cats that had sustained minor to moderate injuries and had survived the traumatic episode for periods ranging from several hours to several months. To assess vascular change, cats were equipped with cranial windows, which allowed for both the direct functional study of the pial vasculature following injury and the postmortem harvesting of the studied vessels for morphologic analyses. Following fluid-percussion brain injury, a subtle focal perturbation of the axon occurred, and over a 12 to 24 hour period, this perturbation became progressively severe, with the result that the axon swelled, separated from its distal segment, and thereby formed an enlarged reactive swelling. With continued survival, some swellings persisted intact, others degenerated, and others demonstrated a dramatic regenerative response. This regenerative response, characterized by regenerative sprouting and growth conelike outgrowths, persisted through all survival periods considered. Immediately following the induction of the fluid-percussion injury, the pial arterioles dilated, manifested morphologic change, and displayed functional abnormalities. These vascular abnormalities appeared mediated by an accelerated metabolism of arachidonate via cyclooxygenase, which results in the generation of oxygen radicals. Radicals, such as the superoxide anion, continue to be produced within the first hour following injury and thus, similar to the observed axonal responses, continue to contribute to the brain's response to trauma. Although these axonal and vascular changes do not appear to be causally related, they both appear as a continuum of the initial insult and may become interlinked should a secondary insult ensue.

Inhibition by ketanserin of serotonin induced cerebral arteriolar constriction

We studied the effects of serotonin on pial arterioles in anesthetized cats equipped with acutely implanted cranial window for the observation of the pial microcirculation. Serotonin topically applied caused cerebral arteriolar constriction. Ketanserin, a specific 5-HT2 inhibitor, completely blocked the vascular response of serotonin. Aggregated platelet supernatant was topically applied and caused generalized cerebral arteriolar constriction that could be blocked with ketanserin. We conclude that serotonin causes generalized cerebral arteriolar constriction that is due to the stimulation of 5-HT2 receptor. Aggregating platelets release serotonin, which mediates the vasoconstrictive action of the supernatant solution.

Effects of 15-hydroperoxy-eicosatetraenoic acid (15-HPETE) on cerebral arterioles of cats

The effects of topical application of 15-hydroperoxy-eicosatetraenoic acid (15-HPETE, 200 micrograms/ml) on cerebral arterioles were studied in anesthetized cats equipped with cranial windows. 15-HPETE induced arteriolar dilation during application, sustained dilation 1 h after washout, and reduced responsiveness to the vasoconstrictive effects of hypocapnia. Electron microscopy of cerebral arterioles disclosed discrete endothelial lesions and focal morphological abnormalities of the vascular smooth muscle. Topical application of superoxide dismutase or catalase or the combination of the two inhibited the functional and morphological abnormalities induced by 15-HPETE. The results show that the vascular effects of 15-HPETE are mediated by superoxide anion radical and hydrogen peroxide or by other radicals derived from them, such as the hydroxyl radical. The results, together with earlier findings, support the view that the oxygen radicals responsible for these cerebral vascular effects are generated via the prostaglandin hydroperoxidase reaction.

Oxygen radicals mediate the cerebral arteriolar dilation from arachidonate and bradykinin in cats

Topical application of sodium arachidonate (50-200 micrograms/ml) or bradykinin (0.1-10 micrograms/ml) on the brain surface of anesthetized cats caused dose-dependent cerebral arteriolar dilation. This dilation was blocked by 67-100% in the presence of superoxide dismutase and catalase. These enzymes did not affect the changes in arteriolar diameter caused by alterations in arterial blood PCO2, or the arteriolar dilation from topical acetylcholine. Enzymes inactivated by heat had no effect on the vasodilation from arachidonate or bradykinin. Superoxide dismutase alone or catalase alone reduced the dilation during application of 200 micrograms/ml of arachidonate for 15 minutes; they also completely prevented the residual dilation seen 1 hour after washout, as well as the reduction in the vasoconstrictive effects of arterial hypocapnia observed at this time. The results show that superoxide anion radical and hydrogen peroxide, or radicals derived from them, such as the hydroxyl radical, are mediators of the cerebral arteriolar dilation from sodium arachidonate or bradykinin. These radicals are not the endothelium-derived relaxant factor released by acetylcholine. The presence of both superoxide anion radical and hydrogen peroxide is required for the production of the vascular damage seen during prolonged application of high concentrations of sodium arachidonate.

Increased venous pressure causes myogenic constriction of cerebral arterioles during local hyperoxia

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.

Free radical mediation of the effects of acidosis on calcium transport by cardiac sarcoplasmic reticulum in whole heart homogenates

Generation of oxygen free radicals by xanthine acting on xanthine oxidase as a substrate significantly depressed calcium transport by sarcoplasmic reticulum in canine whole heart homogenates at 37 degrees C. At pH 7.0, this effect was completely inhibited by the addition of superoxide dismutase (SOD), a scavenger of the superoxide anion radical. At pH 6.4, SOD (5 to 20 micrograms X ml-1) was ineffective but catalase (20 micrograms X ml-1) was able to inhibit the effects of the xanthine-xanthine oxidase system. SOD + catalase (20 micrograms X ml-1) and SOD + mannitol, a scavenger of the hydroxyl free radical, inhibited the effects of the xanthine-xanthine oxidase system at pH 6.4. Preincubation at pH 6.4, in the absence of an exogenous free radical generating system, depressed calcium transport. This depression was more severe the longer the duration of incubation. However, return of the pH to 7.0 after preincubation at pH 6.4 partially restored calcium uptake velocity. The degree of reversibility was decreased the longer the period of incubation at pH 6.4. SOD reversed the effects of incubation at pH 6.4 for 5 min, but not those for incubations of 10 and 15 min. Mannitol alone was ineffective. The combinations of SOD and mannitol significantly reversed the effects of pH 6.4 up to 15 min. These results demonstrate that both exogenously generated and endogenously generated free oxygen radicals are capable of depressing calcium transport by cardiac sarcoplasmic reticulum in the whole heart homogenate in the presence of endogenous scavenging systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative responses of cerebellar and cerebral arterioles to changes in PaCO2 in cats

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 PGF2 alpha on in vivo cerebral arteriolar diameter in cats and rats

We compared the effect of topical application of PGF2 alpha on cerebral arterioles in cats and rats equipped with an acutely implanted cranial window. Arterial diameter was measured using a microscope and image splitting device. PGF2 alpha in a concentration ranging from 10(-7) to 10(-5) M had no effect on large (greater than or equal to 100 microns) or small (less than 100 microns) cat pial arterioles, but induced a dose dependent constriction of rat pial arterioles with a maximum constriction to 76% of control diameter. Dilation of cat large cerebral arterioles by topically applied PGE2 was not affected by simultaneous application of PGF2 alpha and PGE2 induced dilation of small arterioles was decreased 3% by PGF2 alpha. While we and others have previously shown that both cat and rat brain can synthesize PGF2 alpha, it appears that PGF2 alpha is not likely to normally be a major modulator of cerebral arteriolar resistance in all species.

Mannitol causes compensatory cerebral vasoconstriction and vasodilation in response to blood viscosity changes

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, "rebound effect," and the absence of influence on CBF, all with one mechanism.

Effect of changes in magnesium ion concentration on cat cerebral arterioles

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.

Mediation of sarcoplasmic reticulum disruption in the ischemic myocardium: proposed mechanism by the interaction of hydrogen ions and oxygen free radicals

Acute myocardial ischemia results in a decrease in developed tension and an increase in resting tension. A breakdown of the excitation-contraction coupling system can explain the behavior of the ischemic muscle at a subcellular level. We have identified a specific defect in the sarcoplasmic reticulum (SR) from the ischemic myocardium; i.e., the uncoupling of calcium transport from ATP hydrolysis. The mediators of this excitation-contraction uncoupling process have not been identified. It is now established that the intracellular pH of the ischemic myocardium is in the range of 6.4 but the role of protons and potential role of free radicals have not been identified. We have hypothesized that protons and free radicals may interact to produce the excitation-contraction uncoupling of the ischemic myocardium. Cardiac SR was isolated from the wall of canine left ventricle and calcium uptake velocity and Ca2+ stimulated-Mg2+ dependent ATPase activity determined. Increasing proton concentration between pH 7.0 and 6.4 significantly reduced calcium uptake rates (pH 7.0 = 0.95 +/- 0.02; 6.4 = 0.50 +/- 0.02 mumoles Ca2+/mg-min; p less than 0.01) with no effect on ATPase activity. Calculated coupling ratios (mumoles Ca2+/mumoles Pi) decreased from 0.87 +/- 0.06 at pH 7.0 to 0.51 +/- 0.05 at pH 6.4. At pH 7.0, the generation of exogenous free radicals from the xanthine-xanthine oxidase system significantly depressed both calcium uptake rates (Control = 0.95 +/- 0.02; X+XO = 0.15 +/- 0.02) and ATPase activity (Control = 1.05 +/- 0.02; X+XO + 0.30 +/- 0.01 mumoles Pi/mg-min; p less than 0.01). The decreases in calcium uptake and in ATPase activity were completely reversible with superoxide dismutase (SOD). At pH 6.4 in the presence of xanthine and xanthine oxidase, there is a further depression of calcium uptake rates (Control = 0.50 +/- 0.02; X+XO = 0.11 +/- 0.01; p less than 0.05) but there is no SOD reversible component. The addition of SOD + 20mM mannitol normalized calcium transport at pH 6.4. The calculated coupling ratio at pH 6.4 in the presence of free radicals was 0.13. In contrast sarcoplasmic reticulum isolated from ischemic myocardium demonstrated a significant depression of calcium uptake rates at pH 7.1 which was further accentuated at pH 6.4. Ca2+-ATPase was significantly depressed at pH 7.1 but there was no accentuation at pH 6.4. It is concluded that no single species of free radical can explain the intracellular excitation-contraction uncoupling of the ischemic myocardium. The system can be explained by the interaction of hydrogen ions and superoxide anions producing both injury to the sarcoplasmic reticulum and the formation of lipid free radicals with hydroxyl-like activity.

Oxygen radicals and vascular damage

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.

Responses of cerebral arterioles to increased venous pressure

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.

Increased phospholipase C activity after experimental brain injury

Phospholipase C activity was measured in 1000 X G centrifuged cellular fractions isolated from cerebral cortical homogenates obtained from either control cats or cats subjected to experimental fluid-percussion brain injury. Phospholipase C activity was determined directly by measuring the Ca++-dependent conversion of membrane-bound, labeled phosphatidate to diacylglycerol or indirectly by measuring the diacylglycerol-dependent (brain diacylglycerol content) formation of phosphatidylcholine in the presence of labeled cytidine diphosphate (CDP) choline. Phospholipase C activity determined by either method was about two time greater in cell fractions isolated from animals subjected to brain injury than in controls (p less than 0.01). The brain injury-induced rise in phospholipase C activity may be responsible, at least in part, for generating diacylglycerol that may be a source of free arachidonic acid that stimulates prostaglandin synthesis. These changes may account for the rise in brain prostaglandin levels that has been demonstrated earlier to occur after this type of brain injury.

Cyclooxygenase products of arachidonic acid metabolism in cat cerebral cortex after experimental concussive brain injury

Previous studies have suggested that following experimental fluid percussion brain injury, increased prostaglandin (PG) synthesis, with its concomitant production of oxygen free radicals, causes functional and morphological abnormalities of the cerebral arterioles. The purpose of this study was to chemically determine if PGs are altered following this injury. To facilitate interpretation of neurochemical measurements the cats were ventilated, blood pressure was measured, and a cranial window, for microscopic observation of pial arteriolar diameter was inserted. PG levels were determined in quick-frozen cortical tissue removed from control and 3 groups of injured cats at 1.5, 8,0, and 60 min after injury. Analysis of PGE2, PGF2 alpha, and 6-keto-PGF1 alpha was performed by HPLC and GC/MS. The control levels of PGE2, PGF2 alpha, and 6-keto-PGF1 alpha were 216 +/- 44, 210 +/- 48, and 48 +/- 12 ng/g wet weight, respectively. Following injury, produced by a 22 ms increase in intracranial pressure, the pial arterioles dilated irreversibly and a transient hypertensive response occurred, thereby producing hyperemia. During the maximum hyperemic response, the total PGs were 75% of control. At 8 min after injury, when blood pressure returned to control level, the PGs were 158% of control and PGs fell to 111% of control at 60 min. These experiments supported our previous studies implicating increased PG synthesis in te genesis of the physiologic and morphologic sequelae of experimental concussive brain injury.

Prostaglandins in physiological and in certain pathological responses of the cerebral circulation

The most abundant prostaglandin produced by brain tissue varies from species to species. The most abundant prostaglandin produced by brain microvessels is PGI2, PGG2, PGH2, PGI2, PGE2, PGD2, and arachidonic acid dilated cerebral arterioles. Cyclooxygenase inhibitors (indomethacin, AHR-5850), in doses that reduced prostaglandin synthesis substantially, did not affect resting vascular caliber and did not influence the responses of cerebral arterioles to arterial hypoxia, arterial hypercapnia, or arterial hypocapnia, suggesting that prostaglandins are not involved in the mediation of these responses. The vasodilator action of vasoactive intestinal peptide on cerebral arterioles was blocked by these cyclooxygenase inhibitors. The cerebral arteriolar damage induced by fluid-percussion brain injury was inhibited by pretreatment with cyclooxygenase inhibitors, or with free radical scavengers. Topical application of arachidonic acid or PGG2, reproduced the damage seen with brain injury. These findings show that prostaglandins are mediators of the cerebral arteriolar damage due to brain injury and that their mechanism of action is dependent on the generation of free oxygen radicals.