Mechanism of cerebral arteriolar abnormalities after acute hypertension

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.

Pathophysiology of vascular consequences of experimental concussive brain injury

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Inhibition by free radical scavengers and by cyclooxygenase inhibitors of pial arteriolar abnormalities from concussive brain injury in cats

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.

Mechanism of action of vasoactive intestinal polypeptide on cerebral arterioles

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.

Cerebral arteriolar damage by arachidonic acid and prostaglandin G2

Application of arachidonic acid or prostaglandin G(2) to the brain surface of anesthetized cats induced cerebral arteriolar damage. Scavengers of free oxygen radicals inhibited this damage. Prostaglandin H(2), prostaglandin E(2), and 11,14,17-eicosatrienoic acid did not produce arteriolar damage. It appears that increased prostaglandin synthesis produces cerebral vascular damage by generating free oxygen radicals.

Dependence of pial arteriolar response to hypercapnia on vessel size

The responses of pial arterioles to changes in PaCO2 were studied in anesthetized cats equipped with cranial windows for the observation of the pial microcirculation. The percentile increase in vessel caliber induced by hypercapnia was size dependent, with the smaller vessels responding more vigorously than the larger ones. In contrast, the vasoconstrictor responses to arterial hypocapnia were size independent. Following topical application of phentolamine (10(-5) M), or after ipsilateral extirpation of the superior cervical ganglion, the increase in arteriolar caliber caused by moderate hypercapnia (5% CO2 inhalation) in the larger arterioles (greater than 100 micron in diameter) increased and became equal to that of the smaller arterioles (less than 100 micron in diameter). The response to hypocapnia was not affected by phentolamine. The results indicate that a major reason for the greater responsiveness of smaller vessels to moderate hypercapnia is the fact that the response of the larger vessels is limited by neurogenic vasoconstriction resulting from increased activity of adrenergic nerves originating from the cervical sympathetic chain. The absence of an influence of the sympathetic nervous system on the responses to hypocapnia is explained by the lack of significant resting neurogenic vasoconstrictor tone in pial arterioles of anesthetized cats.

Method for morphophysiological study of specific pial microvessels

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.

Role of prostaglandins in pial arteriolar response to CO2 and hypoxia

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.

Vasodilation of cat cerebral arterioles by prostaglandins D2, E2, G2, and I2

To determine the possible role that endogenously produced prostaglandins may play in the regulation of cerebral blood flow, the responses of cerebral precapillary vessels to prostaglandins (PG) D2, E2, G2, and I2 (8.1 X 10(-8) to 2.7 X 10(-5) M) were studied in cats equipped with cranial windows for direct observation of the microvasculature. Local application of PGs induced a dose-dependent dilation of large (greater than or equal to 100 microns) and small (less than 100 microns) arterioles with no effect on arterial blood pressure. The relative vasodilator potency was PGG2 greater than PGE2 greater than PGI2 greater than PGD2. With all PGs, except D2, the percent dilation of small arterioles was greater than the dilation of large arterioles. After application of prostaglandins in a concentration of 2.7 X 10(-5) M, the mean +/- standard error of the percent dilation of large and small arterioles was, respectively, 47.6 +/- 2.7 and 65.3 +/- 6.1 for G2, 34.1 +/- 2.0, and 53.6 +/- 5.5 for E2, 25.4 +/- 1.8, and 40.2 +/- 4.6 for I2, and 20.3 +/- 2.5 and 11.0 +/- 2.2 for D2. Because brain arterioles are strongly responsive to prostaglandins and the brain can synthesize prostaglandins from its large endogenous pool of prostaglandin precursor, prostaglandins may be important mediators of changes in cerebral blood flow under normal and abnormal conditions.

Responses of pial arterioles after prolonged hypercapnia and hypoxia in the awake rabbit

The changes in the responsiveness of pial arterioles to CO2 and in the composition of cortical cerebrospinal fluid bathing these vessels were studied in the awake rabbit before and after 6 days exposure to hypercapnia (7% CO2) or hypoxia (10% O2). The vasodilator response of pial arterioles to inhalation of 3--10% CO2 was diminished after prolonged hypercapnia and enhanced after prolonged hypoxia. After both hypoxia and hypercapnia, pial arteriolar responsiveness to CO2 was immediately returned toward control levels by washing the brain surface with normal artificial cerebrospinal fluid. The bicarbonate concentration of cerebrospinal fluid bathing the pial vasculature showed a significant decrease after hypoxia and a significant increase after hypercapnia, whereas CSF pH remained unaltered. We conclude that the alteration in responsiveness of pial arterioles to CO2 is due to a change in the chemical composition of the CSF bathing these vessels, involving an adjustment in the concentration of bicarbonate ions.

Oxygen consumption of pial arteries

Pial arterioles and arteries, normally responsive to alterations in PaCO2, were obtained by microdissection from the parietal cortex of 53 anesthetized cats. The oxygen consumption of these vessels, determined with the Cartesian diver microrespirometer, was found to be size dependent. It increased progressively from vessels 60 mum in diameter to vessels 250 mum in diameter. In vessels 250--375 mum in diameter there was a progressive decrease in oxygen consumption with increasing vessel size. The oxygen consumption was not altered by increasing the oxygen concentration in the diver from 20% to 95%, or that of glucose from 1.1 to 2.0 mM. Planimetry of histological sections showed that the percentage of endothelium and smooth muscle increased as vessel diameter increased from 60 to 200 mum, and thereafter decreased with increasing vessel size. When these differences in the composition of the vessel wall were taken into account, there was still residual dependence of the oxygen consumption on vessel diameter. This may represent intrinsic differences in the metabolic rate of vascular smooth muscle in vessels of different size.

Vasoconstrictor effect of angiotensin on pial arteries

The effect of topical application of angiotensin on pial arterioles was examined in anesthetized cats equipped with a cranial window for the direct observation of the pial microcirculation of the parietal cortex. Angiotensin in a dose of 0.01 to 1 microgram/ml constricted pial arterioles and arteries strongly. The response of the smaller vessels was greater than that of the larger ones. Intravenous administration of angiotensin in a dose of 0.04--3.8 microgram/min raised arterial blood pressure and constricted the larger pial arteries. While the infusion of angiotensin was continued at the same dose, the blood pressure was then reduced to the control level via bleeding into a reservoir. This abolished the vasoconstriction of the larger pial arteries, showing that this effect was due to autoregulatory adjustments to the rise in blood pressure and not due to a direct effect of angiotensin. We conclude that, despite the strong constrictor effect of angiotensin on pial arteries, intravenous angiotensin can be used to study the effects of arterial hypertension on the cerebral circulation.

Role of tissue hypoxia in local regulation of cerebral microcirculation

The mechanism of action of hypoxia on cerebral blood vessels and its role in the regulation of the cerebral circulation were investigated in anesthetized cats. Arterial hypoxia produced marked cerebral arteriolar vasodilation, which was partially reversed by perfusing the space under the cranial window with artificial cerebrospinal fluid (CSF) containing 6-94% oxygen. More marked increase in the local supply of oxygen, via perfusion of the space under the cranial window with fluorocarbon FC-80 equilibrated with 100% oxygen, completely eliminated the vasodilation induced by arterial hypoxia. Fluorocarbon equilibrated with 100% N2 had no effect on the vasodilation. The vasodilation associated with hypotension was completely reversed by perfusion with fluorocarbon equilibrated with 100% oxygen and was unaffected by perfusion with fluorocarbon or CSF equilibrated with gas not containing oxygen. The vasodilation associated with Metrazole-induced seizures was partially reversed by perfusion with fluorocarbon containing oxygen. The results show that hypoxia dilated cerebral blood vessels entirely via a local mechanism, that hypoxia is the dominant mechanism involved in the vasodilation associated with hypotension, and that it is, at least partially, responsible for the vasodilation associated with seizures.

Responses of cerebral arteries and arterioles to acute hypotension and hypertension

The responses of cerebral precapillary vessels to changes in arterial blood pressure were studied in anesthetized cats equipped with cranial windows for the direct observation of the pial microcirculation of the parietal cortex. Vessel responses were found to be size dependent. Between mean arterial pressures of 110 and 160 mmHg autoregulatory adjustments in caliber, e.g., constriction when the pressure rose and dilation when the pressure decreased, occurred only in vessels larger than 200 micron in diameter. Small arterioles, less than 100 micron in diameter, dilated only at pressures equal to or less than 90 mmHg; below 70 mmHg their dilation exceeded that of the larger vessels. When pressure rose to 170- 200 mmHg, small vessels dilated while the larger vessels remained constricted. At very high pressures (greater than 200 mmHg) forced dilation was frequently irreversible and was accompanied by loss of responsiveness to hypocapnia. Measurement of the pressure differences across various segments of the cerebral vascular bed showed that the larger surface cerebral vessels, extending from the circle of Willis to pial arteries 200 micron in diameter, were primarily responsible for the adjustments in flow over most of the pressure range.

Mass transfer properties of gases in fluorocarbons

The solubility and diffusion coefficient of oxygen and carbon dioxide were measured in both the pure and emulsified of two fluorocarbons, perfluorotributylamine and perfluorobutyl perfluorotetrahydrofuran. The solubility coefficient and the diffusivity of oxygen in the pure form of the fluorocarbons decreased with decreasing partial pressure of Oxygen (PO2), but the solubility coefficient and the diffusivity of carbon dioxide were independent of its partial pressure. The solubility and diffusion of oxygen in the emulsified form of the fluorocarbons followed the pattern expected from the behavior of oxygen in the pure fluorocarbon. The experimental results suggest that there exists a physical and/or chemical interaction between oxygen and the fluorocarbons, and this interaction is more pronounced at partial pressures below 150 mm Hg. Comparisons of the oxygen content in the fluorocarbon emulsions with that in whole blood show that at PO2 of 760 mm Hg, the oxygen content in the fluorocarbon emulsions is approximately one-half that in whole blood, but at PO2 of 50-150 mm Hg, the oxygen content in the emulsions decreases to values which are less than one-tenth that of whole blood.

Analysis of vasoactivity of local pH, PCO2 and bicarbonate on pial vessels

The mechanism by which the local effect of CO2ON pial arterioles is exerted was examined in anesthetized cats equipped with a cranial window for the direct observation of the microcirculation of the parietal cortex. The dilation of pial arterioles in response to application of artificial cerebrospinal fluid with low pH was the same whether or not the PCO2 of the solution was maintained in the normal range or markedly increased. The constriction of pial arterioles in response to application of artificial cerebrospinal fluid with high pH was the same whether or not the PCO2 of the solution was maintained in the normal range or markedly decreased. Finally, pial arterioles did not change their caliber in response to application of cerebrospinal fluid with unchanged pH but markedly increased or decreased Pco, or bicarbonate ion concentration. These results show that the action of CO2 on cerebral vessels is exerted via changes in extracellular fluid pH and that molecular CO2 and bicarbonate ions do not have independent vasoactivity on these vessels.

Local mechanism of CO2 action of cat pial arterioles

The effect of local hypercapnic acidosis or local hypocapnic alkalosis on pial arterioles were studied in anesthetized cats equipped with a cranial window for the direct observation of the pial microcirculation of the parietal cortex. Changes in PCO2 and pH of the extracellular fluid were induced by perfusing the space under the cranial window with artificial cerebrospinal fluid equilibrated with different concentrations of CO2, while PaCO2 was maintained constant. Hypercapnic acidosis dilated and hypocapnic alkalosis constricted pial arteioles markedly. The results indicate that a basis exists for considering CO2 as a mediator for local regulation of brain blood flow. The vasodilation associated with arterial hypercapnia was abolished by a reduction in CSF PCO2 equal in magnitude to the rise in arterial blood PCO2, suggesting that the action of CO2 is entirely local.

Effects of respiratory gases on cytochrome A in intact cerebral cortex: is there a critical Po2?

Changes in the redox level of cytochrome a and in the amount of oxygenated hemoglobin were measured by dual wavelength reflectance spectrophotometry in the intact cerebral cortex of cats (cerveau isolé preparation) and in unanesthetized rabbits with chronically implanted cranial windows. Increases in inspired oxygen were accompanied by an increase in the oxidation level of cytochrome a and an increase in the amount of oxygenated hemoglobin in the optical field. These changes were larger in the presence of 5% CO2. Reduction of the inspired oxygen concentration produced a decrease in the oxidation/reduction ratio of cytochrome a and a disoxygenation of hemoglobin. The presence of CO2 at these lower oxygen levels diminished the reduction of cytochrome a and the disoxygenation of hemoglobin. These data indicate that, in the resting subject, the reduction levels of cytochrome a are well above the low values seen in isolated mitochondria. They also indicate that the blood supply to the cerebral cortex is regulated at a level of slight hypoxia.

Circulatory effects of prolonged hypoxia before and during antihistamine

Five chronically instrumented healthy dogs were exposed to a 5-day period of breathing 10% oxygen in a chamber. The response to hypoxia was found to be time dependent. During the first 24 h of hypoxia the circulatory response was characterized by increases in cardiac output, heart rate, pulmonary and systemic arterial blood pressures, and pulmonary vascular resistance. Systemic vascular resistance increased; left atrial pressure decreased. During the early part of hypoxia the animals became hypocapnic; the arterial blood pH rose significantly. During the rest of the hypoxic period cardiac output, heart rate, and arterial blood pH returned to the control values; pulmonary and systemic arterial pressures and pulmonary vascular resistance remained significantly elevated. Systemic vascular resistance rose; left atrial pressure remained below control. This response to hypoxia was not substantially modified when the experiment was repeated during the administration of the antihistamine promethazine, an H1-receptor blocking agent, in a dose which blocked the pulmonary vasoconstrictor response to small doses of exogenous histamine. The circulatory response to acute hypoxia in five anesthetized dogs was not modified by intravenous administration of metiamide, an H2-receptor blocking agent.