Cerebral circulation and norepinephrine: relevance of the blood-brain barrier

Effects of dobutamine and prostacyclin on cerebral blood flow velocity in septic patients

PURPOSE

Both dobutamine and prostacyclin (PGI2) have been used to increase oxygen delivery in septic patients, but their effects on cerebral blood flow have not been well studied.

METHODS

In 10 septic patients with altered mental status, stable hemodynamic status, and normal lactatemia, we investigated the effects of successive infusions of dobutamine at 5 micrograms/kg/min and PGI2 at 5 ng/kg/min on mean blood flow velocity in a middle cerebral artery, using transcranial Doppler flowmetry.

RESULTS

Mean flow velocity increased with dobutamine (from 52 +/- 4 to 62 +/- 6 cm/s, P < .005) but not with PGI2 (from 55 +/- 5 to 57 +/- 5 cm/s, P = not significant). Each substance significantly increased cardiac index. Dobutamine increased arterial pressure from 85 +/- 6 to 91 +/- 5 mm Hg (P < .05), but PGI2 decreased it from 87 +/- 6 to 77 +/- 5 mm Hg (P < .005). With each agent, mean flow velocity was correlated with cardiac index (r = .51, P < .001) but not with arterial pressure. PGI2 reduced PaO2 from 103 +/- 10 to 82 +/- 6 mm Hg (P < .005). Cerebral oxygen delivery (estimated by the product of mean flow velocity and arterial oxygen content) increased by 19% with dobutamine but remained unchanged with PGI2.

CONCLUSIONS

Dobutamine and PGI2 at the administered doses exert different effects on arterial pressure and middle cerebral artery flow velocity in septic patients. According to these data, dobutamine increases cerebral oxygen delivery more than PGI2.

Effect of alloxan-induced diabetes on acetylcholinesterase activity from discrete areas of rat brain

The effect of experimental diabetes induced in rats was examined in brain areas like the septum, medial preoptic area, median eminence-arcuate region, amygdala, thalamus, hippocampus, pons and medulla. In all the areas studied, acute hyperglycemia caused an increase in the activity of acetylcholinesterase, the degradative enzyme of cholinergic system, whereas insulin administration reversed this effect. These findings suggest that the dysfunction of cholinergic system may also be involved in the diabetes associated CNS complications.

Effects of metabolic pH-alterations on cerebral blood flow and oxygen uptake following E. coli endotoxin in dogs

The aim of the present study was to investigate if metabolic pH-alterations have an influence on cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) after an injection of E. coli endotoxin. Following endotoxin in dogs with normal pH a decreased CBF and an increased CMRO2 have earlier been found. Thirteen anaesthetized dogs were subjected to metabolic pH-variations in blood by infusion of hydrochloric acid or sodium bicarbonate. Ten dogs received E. coli endotoxin in a dose of 1 mg.kg-1 bodyweight. CBF, CMRO2 and noradrenaline and adrenaline concentrations in blood and cerebrospinal fluid were measured repeatedly during normoxia and normocarbia. Measurements before endotoxin served as controls, together with three additional animals, where endotoxin was never given. In control measurements pH showed no influence on the variables studied. After endotoxin CBF, CMRO2 and noradrenaline in cerebrospinal fluid increased with decreasing arterial blood pH. The influence exerted by metabolic pH alterations in blood after endotoxin may be explained by hydrogen ions and monoamines passing over a blood-brain barrier (BBB), damaged by endotoxin, into the brain tissue causing vasodilation and neuronal activation.

Autonomic control of neuronal-astrocytic interactions, regulating metabolic activities, and ion fluxes in the CNS

It is generally assumed that the brain, in contrast to all other organs, is not equipped with an autonomic nervous system, regulating blood supply, and cellular activities. This may be because systemic administration of most drugs acting on monoaminergic or cholinergic receptors have little or no effect on cerebral blood flow and metabolism. However, intrathecal administration of noradrenaline does, indeed, influence both blood flow and energy metabolism in the brain. The present review focuses on effects of noradrenaline or serotonin on energy metabolism, turnover of amino acid transmitters and ion homeostasis, with special emphasis on the cellular localization. Noradrenergic agonists stimulate brain metabolism in vivo as well as many aspects of energy metabolism, Na+,K(+)-ATPase activity and uptake of transmitter amino acids in astrocytes in primary cultures, with little or no effect on corresponding preparations of neurons. Serotonin acts differently, decreasing potassium-induced release of glutamate from both neurons and astrocytes. Little is known about the effects of acetylcholine. The functional significance of these effects is discussed.

Beta-adrenergic receptor activity of cerebral microvessels in experimental diabetes mellitus

The effect of diabetes mellitus on beta-adrenergic receptor number (B(max)), receptor-cyclase coupling and adenylate cyclase (AC) activity was determined in cerebral microvessels isolated from control and streptozotocin induced diabetic rats after 5 weeks of induction of diabetes. Scatchard analysis of [125I]iodocyanopindolol (ICYP) binding indicated that the B(max) (fmol/mg) in diabetic rat cerebral microvessels (63.8 +/- 4.8) (mean +/- S.E.M.) was not significantly different from the B(max) in control rats (56.5 +/- 6.9). Isoproterenol competition of [125I]ICYP binding sites indicated that the percentage of beta-receptors expressing high affinity binding was 53.9 +/- 0.45% in control rats and 47.5 +/- 2.3% in diabetic rats. The total isoproterenol-stimulated AC activity (pmol cAMP/mg) in diabetic rats (76.7 +/- 6.1) was significantly lower than that in control rats (118.4 +/- 11.2) (P less than 0.01). However, the net isoproterenol-stimulated AC activity (i.e. total minus GTP-stimulated AC activity) was not altered in diabetes. The net sodium fluoride (NaF) stimulated AC activity in diabetic rats (109.5 +/- 11.4) was significantly lower than the control rats (154.3 +/- 16.3) (P less than 0.05). It is concluded that diabetes mellitus in rats is associated with reduced post receptor activation of adenylate cyclase in cerebral microvessels while the beta-adrenergic receptor density, affinity and receptor-cyclase coupling are not significantly altered.

Treatment of cerebral ischemia with Dextran-40 or Fluosol DA 20%

A cat stroke model was used to evaluate the efficacy of Dextran-40 (DEX) or Fluosol-DA 20% (FDA) in the treatment of focal cerebral ischemia. The animals were assigned randomly to one of three treatment groups: control, isovolemic hemodilution with DEX or isovolemic hemodilution with FDA. The oxidation state of cytochrome aa3 was measured in-vivo using near infrared reflectance spectrophotometry. The cerebral edema was measured by magnetic resonance imaging (MRI). The MRI edema indices for the three groups revealed that the FDA group had less edema (p less than 0.05), approaching that of non-stroke controls. The relative oxidation state of aa3 for the DEX group declined both during and after hemodilution. At the ninth hour post stroke the FDA group was better (aa3 more oxidized. p less than 0.025). Changes in blood and plasma components were reflective of the extent of hemodilution. Whole blood viscosity analysis revealed a difference (p less than 0.05) at the lower shear rates comparing DEX to FDA with FDA being higher than DEX. Two animals in each of the groups were allowed to awaken at the end of the procedure for functional assessment. These observations suggest that hemodilution with FDA following stroke significantly reduces early post-ischemic cerebral edema, improves oxidation in the peri-infarct area and appears to minimize functional deficits.

Effect of adrenergic drugs on cerebral blood flow, metabolism, and evoked potentials after delayed cardiopulmonary resuscitation in dogs

BACKGROUND AND PURPOSE

Epinephrine administration during cardiopulmonary resuscitation increases cerebral blood flow by increasing arterial pressure. We tested whether potential beta-adrenergic effects of epinephrine directly influence cerebral blood flow and oxygen consumption independently of raising perfusion pressure.

METHODS

Four groups of seven anesthetized dogs were subjected to 8 minutes of fibrillatory arrest followed by 6 minutes of chest compression, ventricular defibrillation, and 4 hours of spontaneous circulation. Cerebral perfusion pressure was increased to approximately equivalent ranges during resuscitation by either 1) epinephrine infusion, 2) epinephrine infusion after pretreatment with the lipophilic beta-adrenergic antagonist pindolol, 3) infusion of the alpha-adrenergic agonist phenylephrine, or 4) descending aortic balloon inflation without pressor agents.

RESULTS

We found no difference in cerebral blood flow, oxygen extraction, or oxygen consumption during chest compression among groups. After ventricular defibrillation, depressed levels of cerebral blood flow, cerebral oxygen consumption, and somatosensory evoked potential amplitude were not different among groups.

CONCLUSIONS

We detected no evidence that after 8 minutes of complete ischemia, epinephrine administration during resuscitation substantially influences cerebral blood flow or cerebral oxygen consumption independent of its action of raising arterial pressure or or that epinephrine has a negative impact on immediate metabolic or electrophysiological recovery attributable to its beta-adrenergic activity.

Prevention of endotoxin-induced increase of cerebral oxygen consumption in dogs by propranolol pretreatment

Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were studied in experimental endotoxic shock in dogs. Eight animals were pretreated with a beta-adrenoceptor blocking agent, propranolol (PPL), per os 12 mg/kg once a day for 7 days. Ten animals served as controls. After an intravenous injection of endotoxin, 1 mg/kg, CBF decreased in both groups, with no significant differences between the groups. CMRO2 increased in the control animals by about 18% from the baseline value both 1 and 2 h after the injection of endotoxin. CMRO2 in the PPL-pretreated animals was unchanged after endotoxin. The CMRO2-reactions to endotoxin in control and PPL animals were significantly different after both 1 and 2 h (P less than 0.05). The present results indicate that the increase in CMRO2 following intravenous endotoxin is mediated via beta-adrenoceptors.

Arterial oxygen consumption after hemorrhagic shock: the effect of beta-adrenergic agonist

Oxygen consumption of the rabbit femoral artery after hemorrhagic shock was studied. Hemorrhagic shock was initiated and maintained at 60 mm Hg of systolic blood pressure for 2 hours. A significant reduction in femoral artery oxygen consumption was observed after hemorrhagic shock (1.64 +/- 0.14 microliter/g/hr) when compared to oxygen consumption in the normal condition (2.52 +/- 0.22 microliter/g/hr). Application of the beta-agonist isoproterenol significantly increased oxygen consumption in the isolated femoral artery after hemorrhagic shock (2.66 +/- 0.20 microliter/g/hr), but did not exceed the normal values recorded without stimulation. Also, isoproterenol significantly increased oxygen consumption in the femoral artery of nonhemorrhagic condition (4.84 +/- 0.42 microliter/g/hr). The increase in oxygen consumption conditioned by isoproterenol was significantly lower after hemorrhagic shock compared with values in the nonhemorrhagic state. The data suggest that oxygen consumption is regulated by beta-adrenergic receptors, and the phenomenon of diminished oxygen consumption after hemorrhagic shock probably occurs because of changes in beta-adrenergic receptors, causing the appearance of change in the mechanism of oxygen consumption.

Effects of intracisternal endothelin-1 injection on blood flow to the lower brain stem

The central effects of endothelin-1 (Et-1, 10-30 pmol in 2.5 microliters injected intracisternally) have been investigated in the conscious rat. With 10 and 20 pmol Et-1, no significant change in blood pressure was observed. With 30 pmol Et-1, mean arterial blood pressure rose by 40 +/- 10 mm Hg with an accompanying modest, short-lived bradycardia at 2 min post-injection. Cerebral blood flow [( 14C]iodoantipyrine autoradiography), measured simultaneously with the hypertensive response, was markedly reduced throughout the caudal medulla and cerebellum (by up to 85%), while significant hyperaemia was evident in a number of forebrain structures (e.g. an increase of 78% in sensorimotor cortex). These observations have relevance to two distinct scientific areas. Concerning the significant effect of Et-1 in central cardiovascular control, these results caution against drawing conclusions from ventricular application with knowledge only of cardiovascular parameters. These results also illustrate the profound effects of Et-1 which is uniquely capable of overriding cerebral autoregulatory mechanisms.

Beneficial effects of isovolemic hemodilution using a perfluorocarbon emulsion in a stroke model

In a clinically applicable cat stroke model, 16 purpose-bred adult animals were used to evaluate the beneficial effects of two treatment regimens: isovolemic hemodilution with either a perfluorocarbon emulsion or dextran 40 (a glucose polymer). Animals that received these treatment regimens were then compared with a control group of untreated animals. Focal cerebral infarctions were produced by transorbital ligation of the left middle cerebral artery. The randomly allocated treatment arms of the study were instituted 3 hours after ligation of the middle cerebral artery, thereby simulating a human clinical situation. In vivo mitochondrial metabolic activity of the peri-infarct cerebral tissue was continually assessed by means of a multiwavelength near-infrared spectrophotometer. This allowed measurement of cellular oxygenation at the cytochrome aa3 level, the terminal member of the cytochrome chain. Sequential proton-based magnetic resonance imaging was used to measure intracerebral water in vivo. Cardiac output, oxygen consumption/delivery, chemical, histologic, and rheologic parameters were also assessed. The data collected were analyzed by group means and standard statistical analyses, which revealed that the group treated with the perfluorocarbon emulsion had both less brain edema in the early post-infarct period (p less than 0.05), as well as a higher level of oxidation of cytochrome aa3 (p less than or equal to 0.025). This evidence supports the premise that isovolemic hemodilution with an oxygen-carrying hemodiluent may be beneficial in the treatment of ischemic strokes.

Participation of type A monoamine oxidase in the activated deamination of brain monoamines shortly after reperfusion in rats

Changes in monoamine levels during and after ischemia and effects of RS-8359, a type A monoamine oxidase (MAO-A) inhibitor, were studied in the cerebral cortex, hippocampus, and striatum of rats killed by microwave irradiation. The patterns of the changes in norepinephrine (NE), dopamine (DA), and serotonin (5HT) levels were similar during ischemia: All these monoamines decreased in all three regions. After reperfusion, however, the three monoamines showed different patterns of changes: NE, except in the striatum, decreased further; DA increased over the controls; 5HT remained suppressed in all three regions. With regard to the metabolites of the monoamines, the changes during and after reperfusion were almost similar in all regions: O-methylated metabolites, normetanephrine and 3-methoxytyramine, markedly increased during ischemia; After reperfusion, the elevated levels of normetanephrine and 3-methoxytyramine returned to normal, while deaminated metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid, homovanillic acid (HVA), and 3-methoxy-4-hydroxy-phenylethyleneglycol clearly increased. RS-8359 pretreatment (30 mg/kg, p.o.) at an hour prior to ischemia elevated the levels of NE in the cortex and hippocampus during ischemia and inhibited the increases in DOPAC and HVA levels and the decrease in 3MT levels at 30 min after reperfusion. These results suggest that deamination of NE, DA, and 5HT is activated by the increases in the substrates for MAO in all three regions, except the noradrenergic system in the striatum, and that MAO-A participates in the activated deamination after reperfusion.

Effect of diabetes mellitus on disruption of the blood-brain barrier during acute hypertension

The goal of this study was to determine whether the susceptibility of the blood-brain barrier to disruption during acute hypertension is altered during diabetes mellitus. Intravital fluorescent microscopy and fluorescein-labelled albumin were used to evaluate disruption of the blood-brain barrier under control conditions and during acute arterial hypertension in non-diabetic and diabetic rats. Permeability of the blood-brain barrier was quantitated by calculating clearance of fluorescent-labelled albumin and by counting the number of microvascular leaky sites under control conditions and during acute hypertension. In non-diabetic rats, when systemic arterial pressure was increased from 109 +/- 5 (mean +/- S.E.) to 185 +/- 8 mmHg, clearance of albumin increased from 0.16 +/- 0.04 to 1.51 +/- 0.34 ml/s x 10(-6). In diabetic rats, when systemic arterial pressure was increased from 117 +/- 3 to 184 +/- 3 mmHg, clearance of albumin increased from 0.22 +/- 0.10 to 1.30 +/- 0.32 ml/s x 10(-6). These findings suggest that the susceptibility of the blood-brain barrier to disruption during acute hypertension is not altered during diabetes mellitus.

Neuropeptide Y and the cerebral circulation

The significance of neuropeptide Y (NPY) in the cerebral circulation has been examined in the rat using immunocytochemistry, isolated cerebral artery preparations, and quantitative autoradiographic techniques for determining local CBF and glucose utilisation. In the rat the middle cerebral artery and the lenticulostriate artery from which blood is supplied to the caudate nucleus were found to be invested with numerous perivascular NPY immunoreactive nerve fibres. NPY (3-300 nM) contracted rat middle cerebral artery segments in a concentration-dependent manner. Intracerebral microinjections of NPY (200 pmol) or vehicle (1 microliter) were performed in rats after full recovery from anaesthesia via previously implanted guide cannulae. Following injection of NPY into the striatum, local blood flow was markedly decreased by 36% throughout the ipsilateral caudate nucleus (e.g., from 104 +/- 25 to 67 +/- 15 ml 100 g-1 min-1; mean +/- SD), whereas glucose use in this region was not altered significantly (e.g., 73 +/- 8 and 74 +/- 10 mumol 100 g-1 min-1 with vehicle and NPY, respectively). Intrastriatal NPY did not alter CBF or glucose use in the majority of other brain areas, including all of the 40 contralateral regions examined and almost all regions within the ipsilateral hemisphere. In a small number of highly discrete brain areas remote from the injection site (e.g., amygdala), there were significant reductions in blood flow with minimal changes in glucose use. Since NPY is present around rat cerebral blood vessels, is capable of evoking their contraction, and has the ability to produce reductions in blood flow independently of oxidative metabolism, this neuropeptide may be of major importance in cerebrovascular regulation.

Continuous monitoring of cerebrocortical blood flow during stimulation of the cerebellar fastigial nucleus: a study by laser-Doppler flowmetry

Laser-Doppler flowmetry was used to continuously monitor cortical CBF during electrical stimulation of the fastigial nucleus (FN). Rats were anesthetized with isoflurane (0.75-5%), paralyzed, and artificially ventilated. The LDF probe was placed over a target region of the parietal cortex through a burr hole. The hypertension associated with FN stimulation was prevented by spinal cord transection at C1 with arterial pressure maintained by i.v. infusion of phenylephrine. After cord transection, CBF changed linearly with changes in arterial pco2 (r = 0.93; n = 23). FN stimulation (50-100 microA, 50 Hz, 1 s on/1 s off) produced sustained increases in CBF that developed slowly, reaching 50% of maximum within 24 +/- 1 s of stimulation (n = 17). After stimulation, CBF returned to baseline gradually within a time period (84-540 s) proportional to the duration of the stimulation (r = 0.93; n = 15). The CBF response was stimulus frequency and intensity dependent, was elicited only from restricted sites in FN, and was abolished by atropine (1 mg/kg, i.v.) or pentobarbital (30 mg/kg, i.v.). The slow temporal profile of the cerebrovasodilation is compatible with the hypothesis that in cerebral cortex local neurons mediate the vasodilation by interstitial release of vasoactive agents rather than by a direct action of neural processes on blood vessels. LDF is an effective technique for monitoring phasic change in CBF and may be useful in studies of the intrinsic neurogenic control of the cerebral circulation.

Pial venous pressure and acute hypertensive disruption of the blood-brain barrier in spontaneous and renal hypertension

The purpose of this study was to determine whether changes in pial venous pressure during acute hypertension account for altered acute hypertensive disruption of the blood-brain barrier in chronic hypertension. We studied 13 normotensive WKY rats, 7 spontaneously hypertensive rats (SHR), and 9 two-kidney, one-clip renal hypertensive rats of the same age. Pial venous pressure (servonull technique) and clearance of fluorescein-labeled dextran from pial vessels (as an estimate of permeability of the blood-brain barrier) were measured before and during acute hypertension produced by i.v. infusion of phenylephrine. Experiments were performed in anesthetized rats (50 mg/kg sodium pentobarbital i.p.). Blood and artificial cerebrospinal fluid pO2, pCO2 and pH were within normal ranges throughout the experiment. The change, time to peak and peak pial venous pressures were the same in all groups. The peak arterial pressure after phenylephrine was greater in the hypertensive rats compared to WKY rats. The time to peak mean arterial pressure was the same in all groups of rats. Clearance of FITC dextran was the same in WKY versus renal hypertensive rats, but less in SHR versus WKY rats (P less than 0.05 by analysis of variance). We conclude that something other than an attenuation of the increase in pial venous pressure protects the blood-brain barrier of SHR against acute hypertensive disruption.

Autoregulation and the CO2 responsiveness of cerebral blood flow after cardiopulmonary bypass

Cerebral blood flow (CBF) was measured by 133Xe clearance to determine whether there were any residual effects of cardiopulmonary bypass (CPB) on the CBF response to changes in arterial PCO2 or blood pressure in the early (3-8 hr) post-CPB period. During CPB, the nine patients studied were managed according to alpha-stat, temperature uncorrected, pH management. The mean +/- SD increase in CBF resulting from an increase in PaCO2 (1.35 +/- 0.5 ml.100 g-1.min-1.mmHg-1 PaCO2) was within the normal range, indicating appropriate CBF response to a change in PaCO2. There were no significant differences in CBF, being 25.7 ml.100 g-1.min-1 at a mean arterial blood pressure of 70 mmHg and 26.5 ml.100 g-1.min-1 at 110 mmHg, demonstrating intact cerebral autoregulation over this pressure range. We conclude that cerebral autoregulation and CO2 responsiveness are preserved in the immediate postoperative period after CPB using alpha-stat pH management.

Plasma epinephrine modulates the cerebrovasodilation evoked by electrical stimulation of dorsal medulla

We examined whether plasma epinephrine contributes to the increase in regional cerebral blood flow (rCBF) evoked by electrical stimulation of the dorsal medullary reticular formation (DMRF). Rats were anesthetized (alpha-chloralose, 30 mg/kg, s.c.), paralyzed and artificially ventilated. The DMRF was electrically stimulated through microelectrodes stereotaxically implanted. During stimulation, blood gases and arterial pressure were monitored and maintained within normal range. rCBF was determined in 11 dissected brain regions using the [14C]iodoantipyrine technique. Plasma epinephrine and norepinephrine were measured radioenzymatically in rats with intact adrenals or adrenalectomy, and with or without infusion of epinephrine. DMRF stimulation induced widespread increases in rCBF associated with a 50-fold increase in plasma epinephrine and a 20-fold increase in norepinephrine without changes in the electroencephalogram. In contrast, stimulation of the adjacent medial longitudinal fasciculus had no effect upon rCBF or plasma catecholamines. Acute bilateral adrenalectomy produced regionally selective reductions in the stimulation-coupled increases in rCBF throughout brain (P less than 0.05). Infusion of epinephrine in adrenalectomized rats to levels comparable to those observed in intact animals during DMRF stimulation did not by itself modify rCBF. However, when infused in conjunction with stimulation of the DMRF, but not medial longitudinal fasciculus, epinephrine fully restored the stimulus-related increases in rCBF in all brain regions to levels comparable to those observed in intact rats. We conclude that stimulation of the DMRF elevates rCBF through two mechanisms; by a neurally-mediated increase in local metabolism and thereby flow (adrenal independent secondary vasodilation) and by releasing epinephrine from adrenal medulla which secondarily acts to increase rCBF by an action on brain.

Regional cerebral blood flow in acute hypertension induced by adrenaline, noradrenaline and phenylephrine in the conscious rat

Hypertension was induced in conscious rats by intravenous infusion of phenylephrine (3, 6 or 12 micrograms kg-1 min-1), noradrenaline (3 micrograms min-1) or adrenaline (3 micrograms kg-1 min-1). Local cerebral blood flow was measured autoradiographically in 24 defined brain structures using [14C]iodoantipyrine as the diffusible tracer. The mean arterial pressure induced by adrenaline, noradrenaline and the two higher doses of phenylephrine was 158-168 mmHg with no significant differences between the groups. Only adrenaline significantly increased local cerebral blood flow in nine of the 24 structures studied. The smaller capacity for autoregulation after adrenaline compared with other drugs might be related to a beta-adrenoreceptor-stimulating effect.

Organ blood flow and somatosensory-evoked potentials during and after cardiopulmonary resuscitation with epinephrine or phenylephrine

Pure alpha-adrenergic agonists, such as phenylephrine, and mixed alpha- and beta-adrenergic agonists, such as epinephrine, raise perfusion pressure for heart and brain during cardiopulmonary resuscitation (CPR). However, with the high doses used during CPR, these drugs may directly affect vascular smooth muscle and metabolism in brain and heart. We determined whether at equivalent perfusion pressure, continuous infusion of phenylephrine (20 micrograms/kg/min) or epinephrine (4 micrograms/kg/min) leads to equal organ blood flow, cerebral O2 uptake, and cerebral electrophysiologic function. During 20 minutes of CPR initiated immediately upon ventricular fibrillation in anesthetized dogs, left ventricular blood flow was similar with epinephrine (45 +/- 9 ml/min/100 g) or phenylephrine (47 +/- 8 ml/min/100 g) infusion. The ratio of subendocardial to subepicardial blood flow fell equivalently during CPR with either epinephrine (1.23 +/- 0.06 to 0.70 +/- 0.05) or phenylephrine (1.32 +/- 0.07 to 0.77 +/- 0.05) administration. At similar levels of cerebral perfusion pressure (44 +/- 3 mm Hg), similar levels of cerebral blood flow were measured in both groups (27 +/- 3 ml/min/100 g). Cerebral O2 uptake was maintained at prearrest levels in both groups. Somatosensory-evoked potential amplitude was modestly reduced during CPR, but it promptly recovered after defibrillation. During CPR and at 2 hours after resuscitation, there were no differences between drug groups in the level of regional cerebral or coronary blood flow, cerebral O2 uptake, or evoked potentials. Therefore, with minimal delay in the onset of CPR and with equipotent pressor doses of phenylephrine and epinephrine, we found no evidence that one agent provides superior coronary or cerebral blood flow or that epinephrine by virtue of its beta-adrenergic properties adversely stimulates cerebral metabolism at a critical time that would impair brain electrophysiologic function. Moreover, epinephrine did not preferentially impair subendocardial blood flow as might be expected if it enhanced the strength of fibrillatory contractions.

Role of molecular charge in disruption of the blood-brain barrier during acute hypertension

Acute hypertension disrupts the blood-brain barrier and may neutralize the negative charge on cerebral endothelium. The goal of this study was to determine the effects of molecular charge on permeability of the blood-brain barrier during acute hypertension. Intravital fluorescent microscopy and fluorescein-labeled dextrans were used to evaluate disruption of the blood-brain barrier during acute hypertension in rats. Disruption of the blood-brain barrier was quantitated by calculating clearance of neutral dextran and of anionic dextran sulfate in two groups of rats. Pressure in pial venules, which are the primary site of disruption of the blood-brain barrier during acute hypertension, was measured using a servo-null device. When systemic arterial pressure was increased from 87 +/- 5 (mean +/- SEM) to 188 +/- 5 mm Hg, clearance of neutral dextran increased from 0.04 +/- 0.01 to 4.38 +/- 0.72 ml/sec x 10(-6). When systemic arterial pressure was increased from 91 +/- 4 to 181 +/- 3 mm Hg, clearance of anionic dextran sulfate increased from 0.02 +/- 0.01 to only 0.70 +/- 0.23 ml/sec x 10(-6). Increases in pial venular pressure were similar in the two groups. Thus, similar increases in systemic arterial pressure and pial venular pressure during acute hypertension produce less disruption of the blood-brain barrier to anionic dextran sulfate than neutral dextran. The findings suggest that 1) the net negative charge of cerebral vessels may be preserved during acute hypertension, and 2) molecular charge is an important determinant of the severity of disruption of the blood-brain barrier during acute hypertension.

Selective enhancement of intratumoural blood flow in malignant gliomas: experimental study in rats by intracarotid administration of adenosine or adenosine triphosphate

We studied the effect of intravenous and intracarotid infusion of adenosine and adenosine triphosphate (ATP) on the regional blood flow of intracerebrally transplanted RG-C6 tumours in rats, using the hydrogen clearance method. The intracarotid administration of adenosine or ATP selectively increased blood flow in the tumour, but did not produce any significant change either in the regional cerebral blood flow of the extratumoural ipsilateral hemisphere or in the ipsilateral hemisphere without tumour. The intracarotid administration of ATP at a dose of 10 micrograms/kg/min produced the most effective increase in the tumour blood flow (+51.5 +/- 16.8%). In contrast, both the intravenous administration of adenosine and that of ATP failed to increase tumour blood flow. These results may possibly indicate that intracarotid administration of the adenosine or ATP might contribute in selectively enhancing the delivery of anti-cancer agents to malignant brain tumours.

Noradrenergic function in generalized anxiety disorder: effects of yohimbine in healthy subjects and patients with generalized anxiety disorder

There is extensive preclinical and clinical support for the hypothesis that hyperactivity of noradrenergic neuronal systems is related to the pathophysiology of some forms of human anxiety. In the present investigation, the behavioral, biochemical, and cardiovascular responses to the alpha 2-adrenergic receptor antagonist, yohimbine, was determined in 20 patients with generalized anxiety disorder (GAD) and 20 healthy subjects. The responses to yohimbine were generally similar in the two groups except there was a trend for the yohimbine-induced increase in plasma 3-methoxy-4-hydroxyphenylglycol to be less in the GAD patients. These findings contrast with previous studies of the effects of yohimbine in panic disorder patients and, thereby, support a neurobiological distinction between these two disorders.

Influence of norepinephrine administration upon pituitary hormone secretion in normal men

This study has investigated the effects of 6.2, 12.5, 25, 50 and 100 ng/kg/min/60 min of NE infused to normal men. Blood samples were obtained every 10 min, before, during and after drug administration for 3 consecutive h. Plasma levels on NE, LH, FSH, PRL, and GH were measured in all samples. The administration of 12.5 ng/kg/min over 60 min of NE induced a significant increase (p less than 0.001) in plasma NE levels (n = 5) from a mean (+/- SE) baseline of 239 +/- 14 ng/L to 706 +/- 54 ng/L which peaked and plateaued at 40 min. The calculated area under the curve was 18562 +/- 3537 ng/L/h of NE and significantly higher (p less than 0.001) than during the h before the infusion (2358 +/- 780 ng/L/h). This increase in plasma NE correlated well with the rise in plasma LH which showed a steady increase from baseline of 7.4 +/- 1.3 mIU/ml to a significant (p less than 0.05) peak of 11 +/- 1.9 mIU/ml at the end of the infusion. Furthermore, analysis of the area under the curve revealed a greater (p less than 0.05) LH release during the NE infusion (180 +/- 18 mIU/ml/h) than before the infusion (92 +/- 17 mIU/ml/h). With the exception of the studies utilizing 12.5 ng/kg/min/60 min, all other doses of NE resulted in no significant and/or consistent changes in plasma concentration of LH, FSH, GH and PRL. Thus, the direct participation of NE in the control of LH secretion in humans seems to occur in a very narrow window.

Cerebrovascular and cerebrometabolic effects of intracarotid infused platelet-activating factor in rats

Platelet-activating factor has been implicated in a variety of disease processes including ischemic brain injury and endotoxic shock, but its effects on cerebral blood flow (CBF) and metabolism in normal brain have not been described. The effects of platelet-activating factor on global CBF (hydrogen clearance) and the global cerebral metabolic rate for oxygen (CMRO2) were studied in halothane-N2O anesthetized Wistar rats. Hexadecyl-platelet-activating factor infused into the right carotid artery (67 pmol/min) for 60 min decreased mean arterial pressure (MAP) from 122 +/- 4 (x +/- SEM) to 77 +/- 6 mm Hg and CBF from 159 +/- 12 to 116 +/- 14 ml/100 g/min (p less than 0.002). In contrast, CMRO2 increased from 9.7 +/- 0.9 to 11.7 +/- 1.1 ml/100 g/min after 15 min (p less than 0.05). In controls rendered similarly hypotensive by blood withdrawal and infused with the platelet-activating factor vehicle, CMRO2 was unchanged, whereas CBF transiently decreased then returned to baseline at 60 min. These cerebrovascular and cerebrometabolic effects of PAF are reminiscent of and may be relevant to hypoperfusion and hypermetabolism observed after global brain ischemia and in endotoxic shock.

Effect of monoamine oxidase inhibition on the regional cerebral blood flow response to circulating noradrenaline

The effect of an acute i.v. infusion of noradrenaline (NA) on regional cerebral blood flow (rCBF) was investigated in the awake rat using [14C]iodoantipyrine as diffusible tracer. The contribution of vascular monoamine oxidase (MAO) to the efficiency of the enzymatic blood-brain barrier (BBB) to catecholamines was assessed by measuring the multiregional cerebrovascular response to circulating NA given alone or after i.v. administration of the monoamine oxidase inhibitor, clorgyline. Since i.v. infusion of NA elevates blood pressure, the influence of NA on the cerebrovascular bed was first studied by determining the relationship between rCBF and the mean arterial pressure (MAP). When the MAP was only slightly increased (to approximately 130 mm Hg), a trend to flow decrease under NA infusion was observed. Secondly, we compared the effects of NA on rCBF in animals treated or not treated with clorgyline. This was performed under moderate hypertension (within the 'autoregulated' range of MAP) to avoid any risk of mechanical damage to the BBB. Clorgyline administration alone did not significantly modify rCBF, but the subsequent i.v. infusion of NA induced an increase in rCBF (weighted mean 14%) in all structures investigated. The differences being statistically significant (P less than 0.05) in 5 out of 13 structures by up to 20%. Compared to studies involving disruption of the morphological BBB in which plasma NA elicits a widespread important increase in blood flow, the weak cerebrovascular effects we observed provide indirect evidence for the efficiency of the BBB to catecholamines in the conscious rat within the autoregulated range of arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of basal forebrain in the primary cholinergic vasodilation in rat neocortex produced by systemic administration of cismethrin

The pyrethroid insecticide cismethrin (9 mumol/kg) causes a large blood flow increase in cerebral cortex, without a parallel increase in metabolism. A unilateral lesion of the basal forebrain attenuated the blood flow increase in the cortex ipsilateral to the lesion but augmented that in the contralateral cortex. Cortical choline acetyltransferase was similarly affected. Atropine sulphate substantially reduced the flow increase and was additive to the lesion effects. Systemic cismethrin is thus capable of activating a cholinergic vasodilation in the cortex and, in the parietal cortex at least, a substantial proportion of the flow increase is mediated by extrinsic projections from the basal forebrain.

Adrenaline-induced hypertension: morphological consequences of the blood-brain barrier disturbance

Acute hypertension may transiently open the blood-brain barrier (BBB). To determine whether such temporary exposure of the brain parenchyma to plasma constituents may lead to permanent morphological alterations, acute hypertension was induced by i.v. adrenaline in conscious rates given Evan's blue and horseradish peroxidase as tracers. The brain were perfused in situ 24 h later: 17 of 21 brains showed multifocal sites of extravasation of the tracers and of endogenous plasma albumin, fibrinogen and fibronectin identified by immunohistochemistry. The proteins spread locally in the parenchyma and were taken up by neurons. Within the leaking sites in the cortex, hippocampus, thalamus and basal ganglia some shrunken and grossly distorted acidophilic neurons were present. Focal areas of sponginess were observed in the subpial and subependymal zones. Thus, a transient opening of the BBB may lead to neuronal damage.

Adrenergic receptors in bovine retinal microvessels: presence of alpha 2- and beta- but not alpha 1-receptors

In bovine retinal microvessels, alpha 1, alpha 2- and beta-adrenergic receptors were characterized by binding assay, using [3H]prazosin, [3H]para-aminoclonidine and [125I]iodocyanopindolol as radioligands, respectively. The microvessels were purified from bovine eyes by differential centrifugation through a high concentration of bovine serum albumin followed by use of a glass bead filtration technique. In the preparation, specific binding sites for [3H]para-aminoclonidine and [125I]iodocyanopindolol were observed, whereas [3H]prazosin binding was not detected. The [3H]para-aminoclonidine binding sites localized to the microvessels were characterized by high affinity and saturability (KD: 173 +/- 9 pM; Bmax: 394 +/- 11 fmol/mg protein) as well as the [125I]iodocyanopindolol binding sites (KD: 20 +/- 3 pM; Bmax: 43 +/- 4 fmol/mg protein). Furthermore, the specificity of both binding sites was pharmacologically evaluated by measuring the inhibitory effects of various adrenergic reagents on binding. The existence of alpha 2- and beta-adrenergic receptors which were characterized by high affinity, saturability and stereospecificity, leads to the hypothesis that the retinal microcirculation is under neuronal control.

Experimental traumatic brain injury elevates brain prostaglandin E2 and thromboxane B2 levels in rats

Prostaglandin E2 (PGE2) and thromboxane B2 (TxB2) levels were measured in rats following experimental traumatic brain injury. Rats (n = 36) were prepared for fluid percussion brain injury under pentobarbital anesthesia. Twenty-four hours later, rats were lightly anesthetized using methoxyflurane, injured (2.3 atm), and killed 5 or 15 min later. Twelve of the rats died before and are not included in the analyses. The following groups were used for data analysis: group I (n = 6) were sham-injured rats prepared for injury but not injured: group II (n = 6) were injured and killed 5 min later; group III (n = 12) were injured and killed 15 min posttrauma. Thirty seconds prior to sacrifice by decapitation into liquid nitrogen, all rats were injected with indomethacin (3 mg/kg, intravenously [IV]) to prevent postmortem PG synthesis. After sacrifice, brains were removed, weighed, and homogenized in a small quantity of phosphate buffer with indomethacin (50 micrograms/ml). PGE2 and TxB2 levels were determined using double-label radioimmunoassays. Posttraumatic convulsions were observed in 5 of 12 rats in group III and these rats were analyzed separately. PGE2 and TxB2 levels increased significantly (p less than 0.05) in both hemisphere and brainstem 5 min posttrauma. Fifteen minutes after injury, both PGE2 and TxB2 levels remained elevated but the levels were lower than at 5 min in the rats that did not exhibit posttraumatic seizures. This decrease in PG levels at 15 min was not observed in the rats that had seizures after injury and both PGE2 and TxB2 levels remained high in hemispheres and brainstem. Thus, fluid percussion brain injury results in substantial elevations in PGE2 and TxB2 levels and posttraumatic seizures exacerbate the observed increases.

Aging modifies the asymmetry in brain microvascular regulation

Cerebral ischemia induced by unilateral carotid occlusion in rats decreases in an asymmetric manner the number of beta-adrenergic receptors in microvessels prepared from cerebral cortexes ipsilateral and contralateral to the side of the ligature. In particular, the reduction is more pronounced in the left hemisphere in case of both right and left carotid ligature. The greater receptor decrease in the left side of the brain was shown to depend on the integrity of interhemispheric connections. We show that the changes in capillary beta-adrenergic receptors in response to unilateral carotid occlusion are qualitatively modified during aging. In particular, the asymmetry in the response pattern observed in young rats is lost. The mechanisms underlying this phenomenon may be based on an age-related impairment in the transfer of neuronal information between the two sides of the brain.

A transient hypertensive opening of the blood-brain barrier can lead to brain damage. Extravasation of serum proteins and cellular changes in rats subjected to aortic compression

A transient increase in blood pressure was induced in 15 male Sprague Dawley rats by clamping the upper abdominal aorta for 8-10 min. Three rats served as controls. The brains were fixed by perfusion 2 h or 7 days later. Evan's blue-albumin (EBA) was used for macroscopic evaluation of the blood-brain barrier (BBB) integrity. Extravasated plasma albumin, fibrinogen and fibronectin were demonstrated by immunohistochemistry on paraffin sections. Glial fibrillary acidic protein (GFAP) was visualized in the same way. Parallel sections were analyzed for possible parenchymal changes associated with the BBB breakdown. Multiple focal areas of BBB opening were seen in the brains of the three rats killed 2 h after the hypertensive episode. The plasma proteins were present in the vascular wall, extracellular space and within certain neurons. Shrunken acid fuchsin positive neurons were seen in some areas of extravasation. After 7 days, in 5 out of 12 rats a few local lesions with EBA leakage and positive immunostaining for plasma proteins were seen. Structurally these lesions were characterized by shrinkage, fuchsinophilia and disintegration of neurons and proliferation of astrocytes. Thus, a transient opening of the BBB by acute hypertension may lead to permanent tissue damage.

Mechanisms of protection of the blood-brain barrier during acute hypertension in chronically hypertensive rats

Spontaneously hypertensive rats are less susceptible than normotensive rats to disruption of the blood-brain barrier during acute hypertension. The purpose of this study was to examine mechanisms that protect the blood-brain barrier from disruption in chronically hypertensive rats during acute hypertension. Normotensive Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were studied using intravital fluorescent microscopy and fluorescein-labeled dextran. Disruption of the blood-brain barrier was characterized by the appearance of microvascular leaky sites and quantitated by the clearance of fluorescein-labeled dextran. We measured pressure (servo null) in pial arterioles and venules 40 to 60 micron in diameter. In WKY, acute, phenylephrine-induced hypertension increased pial arteriolar pressure by 47 +/- 7 mm Hg (mean +/- SE) and pial venous pressure by 20 +/- 2 mm Hg. Leaky sites increased from 0 to 28 +/- 2. In SHRSP, acute hypertension increased pial arteriolar pressure 44 +/- 8 mm Hg, but pial venous pressure increased only 6 +/- 1 mm Hg and leaky sites increased from 0 to only 6 +/- 1. All leaky sites were venular. In another group of WKY and SHRSP, we increased pial venous pressure passively with a neck cuff. In WKY, venous pressure increased by 22 +/- 2 mm Hg, and leaky sites increased from 0 to 23 +/- 2. In SHRSP, venous pressure increased by 19 +/- 1 mm Hg, and leaky sites increased from 0 to 24 +/- 2. Thus, when venous pressure is increased to the same level in WKY and SHRSP, disruption of the blood-brain barrier is similar. We conclude that protection of the blood-brain barrier during acute hypertension in SHRSP is related to attenuation of increases in pial venous pressure, not pial arteriolar pressure, and the blood-brain barrier in venules of SHRSP probably is not inherently resistant to disruption.

Mechanisms of cerebrovascular dilation by ether in monkeys

We hypothesized that when the depth of ether anesthesia is increased from 2 to 5%, cerebral vessels dilate secondary to circulating catecholamine stimulation of cerebral metabolism. Cerebral blood flow (CBF) by 133Xe clearance and cerebral metabolic rate for oxygen (CMRO2) were measured on 2% and then 5% ether in air in two groups of seven monkeys each during mechanical ventilation. Propranolol, 0.5 mg/kg i.v., was infused over 5 min in one group, and the other received saline. All measurements were repeated on 5% and 2% ether. Cerebrovascular resistance (CVR) fell by 30%, from 2.28 +/- 0.61 (mean +/- SD) to 1.51 +/- 0.28 mm Hg ml-1 100 g-1 min-1 (p less than 0.01), with the increase in ether from 2 to 5%. CBF and CMRO2 were unaltered from values of about 45 ml 100 g-1 min-1 and 2.3 ml 100 g-1 min-1, respectively. During 5% ether anesthesia, propranolol had no effect on CBF, CMRO2, or CVR. On 2% ether, it increased CVR twofold, from 1.5 +/- 0.30 to 3.0 +/- 1.0 mm Hg ml-1 100 g-1 min-1, and decreased CBF by 33%, from 48 +/- 8 to 32 +/- 10 ml 100 g-1 min-1. Plasma epinephrine was two-fold higher on 2% compared to 5% ether, both before and after saline or propranolol infusion. In monkeys, cerebrovascular dilation by ether at 5% compared to 2% is not secondary to catecholamine stimulation of CMRO2. It may result from a direct effect of either plasma catecholamines or ether on the cerebrovasculature.

A brief episode of severe arterial hypertension induces delayed deterioration of brain function and worsens blood flow after transient multifocal cerebral ischemia

Transient arterial hypertension occurs sporadically following cerebral air embolism and may occur during the acute phase of stroke. This study used an animal model of multifocal cerebral ischemia induced by air embolism and reversed by recompression to assess the effect of induced hypertension on the evoked response recovery, local cerebral blood flow, intracranial pressure, and brain water in 19 anesthetized dogs (Canis familiaris). Six received 0.4 ml of air via the internal carotid artery, 8 received intracarotid air and 10 micrograms/kg norepinephrine to produce transient hypertension, and 5 received intracarotid saline and norepinephrine. The average evoked response recovery in the air-only group was 58.3 +/- 7.7% (mean +/- SEM) of control after 4 hours of recompression; the air plus hypertension group recovery was 15.4 +/- 2.7% (p less than 0.01). The final evoked response in the dogs receiving hypertension alone did not differ from control values. Seven of 8 dogs in the air plus hypertension group had very low blood flows; only 1 of 4 in the air-only group had very low flows. The amount of brain water and the intracranial pressure were not detectably different at the end of treatment among all 3 groups. These results support a role for endothelial damage produced by air and hypertension in potentiating the process of postischemic hypoperfusion.

Time-related asymmetric changes of brain microvessel beta-adrenergic receptors in the two hemispheres after carotid occlusion

The effect of short term and long term ischemia induced by right carotid occlusion was studied on beta-adrenergic receptor function in rat cerebral microvessels. The results show a different time-dependent responsiveness of the two hemispheres to ischemia, with a pronounced and more persistent decrease in the number of capillary beta-receptors in the left side of the brain. The data suggest the existence of asymmetries in the control of brain microvasculature which may mediate the different time-course of beta-receptor changes in response to ischemia.

Regional cerebral perfusion during hypertension depends on the hypertensive agent

Cerebral blood flow (CBF) was measured in 40 regions of the rat central nervous system by the [14C]iodoantipyrine autoradiographic technique during a moderate elevation in mean arterial blood pressure (to ca. 150 mmHg), induced by i.v. infusion of either dopamine (DA) or noradrenaline (NA). Hypertension induced by DA resulted in significant increases (median = 44%) in local CBF in 38 of the 40 brain regions investigated. In contrast, during NA infusion, CBF was elevated only slightly (median = 15%) in a few (8 of 40) brain regions (P less than 0.05). The cerebrovascular response to hypertension appears to be dependent upon the catecholamine which is employed to elicit the elevation in arterial blood pressure.

Effect of serotonin and its antagonist ketanserin on pial vessels

The effect of serotonin (5-HT) and its antagonist ketanserin on the cerebral circulation was investigated in two series totaling 24 cats using the cranial window technique. 5-HT elicited a marked dilatation of small pial arteries, whereas large arteries tended to constrict. Intravenous administration of ketanserin reversed the constriction of large arteries, causing dilatation, and reduced the extent of small arteries' dilatation. In a randomized study, i.v. administration of ketanserin in its solvent versus the solvent alone revealed a strong dilatatory effect of the solvent on pial arteries (17 +/- 1.8%), which partly jeopardized a possible constrictory effect of ketanserin, as ketanserin plus solvent induced less dilatation of small pial arteries than the solvent alone. The present data support the view that serotonin exerts a dual effect on cerebral arteries, namely, dilatation of small and constriction of large vessels. The antagonist ketanserin reverses this effect, but the strong dilatatory effect of the solvent alone masks the antiserotoninergic effect.

Vasoactive drugs produce selective changes in flow to experimental brain tumors

Most vasoactive drugs do not readily penetrate the blood-brain barrier and do not affect cerebral blood flow. We tested the hypothesis that vasoactive drugs may alter blood flow to brain tumors in which the blood-brain barrier is abnormal. Blood flow was measured with microspheres in dogs with brain tumors induced by avian sarcoma virus. Intravenously administered adenosine increased blood flow to tumor more than twofold but did not alter flow to normal brain. Intravenously administered norepinephrine decreased blood flow to tumor but not to normal brain. Thus, vasoactive drugs, which have little effect on blood flow to normal cerebrum, produce large changes in blood flow to brain tumors. We also examined responses to systemic hypercapnia. Hypercapnia increased blood flow to normal cerebrum more than twofold but failed to increase flow to tumors. Impaired vasodilator responses to hypercapnia in brain tumors, which cannot be explained primarily by an abnormality of the blood-brain barrier, probably reflect another fundamental difference between vessels in normal brain and brain tumors. The finding that vasoactive drugs have selective effects on blood flow to brain tumors has important implications for delivery of lipid-soluble chemotherapeutic drugs to the tumors.

In vivo modulation of norepinephrine-induced cerebral oxygenation states by hypoxia and hyperoxia

The effect of intravenous norepinephrine (NE) administration on three O2-dependent parameters of cerebral oxygenation was studied in the parietal cortex of skull intact anesthetized rats. Reflectance spectrophotometry was used to measure in vivo changes in cortical hemoglobin saturation (Hb/HbO2), blood volume (BV), and cytochrome c oxidase (cyt. a,a3) oxidation-reduction state. The influence of arterial pressure of oxygen (paO2) on norepinephrine-induced changes in cortical microcirculatory O2 delivery and cyt. a,a3 redox state was tested under conditions of normoxia, hypoxia, and hyperoxia. Norepinephrine produced cyt. a,a3 redox changes which were independent of compensatory alterations in cortical blood volume and changes in systemic blood pressure at the tested physiological extremes. During normoxia, NE caused dose-dependent systemic pressure-related increases in the oxidation level of cyt. a,a3. Conversely, in hypoxia NE caused a reduction. Microcirculatory and cyt. a,a3 redox responses to low doses of NE during hyperoxia were similar to those obtained at high doses during normoxia. The kinetic pattern of changes in hemoglobin saturation, cyt. a,a3 redox state, and cortical blood volume during normoxia and hypoxia was consistent with direct alteration in oxygen delivery to the respiratory chain and possible modification of cerebral oxidative metabolism. Blood-brain barrier alterations and vascular smooth muscle resistance changes to NE under tested conditions of oxygenation are postulated to be responsible for the observed results.

Effect of common carotid occlusion on beta-adrenergic receptor function in cerebral microvessels

beta-adrenergic receptors were measured in cerebral microvessels of gerbils and rats after ligature of the right or left common carotid artery. The results indicate a decrease in the number of beta-adrenergic receptors in brain microvessels of both ipsilateral and contralateral hemispheres. This event may reflect altered patterns of the neuronal regulation of brain microvasculature and may be related to cerebrovascular alterations which are concomitant with ischemia. Furthermore, the results show that the decrease in beta-receptor density is more pronounced in the left hemisphere, independently on the side of carotid occlusion. This finding suggests that microvessel function in the left side of the brain is more vulnerable to hypoxia effects.

alpha-Adrenergic receptors in cerebral microvessels of normotensive and spontaneously hypertensive rats

In rat cerebral microvessels, we characterized alpha 1- and alpha 2-adrenergic receptors, using [3H]prazosin and [3H]-p-amino-clonidine as radioligands. [3H]Prazosin binding to the cerebral microvessels was saturable and of high affinity (dissociation constant of 78 pM), with a maximum binding of 48 fmol/mg protein. [3H]Prazosin binding reached equilibrium within 15 minutes and was dissociated by the addition of 10 microM phentolamine. The inhibitory effects of isomers of norepinephrine and epinephrine on the binding showed that l-isomers were over 10 times more potent than d-isomers. [3H]-p-Amino-clonidine binding to the cerebral microvessels was saturable and of high affinity (KD = 0.61 nM) with a Bmax of 73 fmol/mg protein. The binding reached equilibrium within 30 minutes, and was dissociated by the addition of 100 microM l-norepinephrine. l-Isomers of norepinephrine and epinephrine were over 10 times more potent than d-isomers in displacing the binding. Thus, both [3H]prazosin and [3H]-p-amino-clonidine bindings to the cerebral microvessels were characterized by saturability, high affinity, reversibility, and stereo-specificity. Furthermore, the specificity of both binding sites was pharmacologically evaluated by the inhibitory effects of various adrenergic agonists and antagonists on the bindings. These data indicate the existence of alpha-adrenergic receptors in the cerebral microvessels and are consistent with the hypothesis that the cerebral microcirculation is regulated by adrenergic innervation. Furthermore, the receptors were measured in cerebral microvessels of spontaneously hypertensive rats and Wistar-Kyoto controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of asymmetric vertebral blood flow upon the vestibulo-ocular reflex of the rabbit

Per-rotational nystagmus was recorded in rabbits with unilaterally narrowed vertebral arteries or following unilateral cervical sympathectomies. Asymmetry of per-rotational nystagmus could not be observed when the animal's systemic arterial blood pressure was within the normal range (about 90-120 mm Hg). However, directional preponderance of nystagmus occurred when blood pressure either increased or decreased beyond this level. The reason for the occurrence of directional preponderance can be interpreted as a failure of autoregulation of cerebral and/or inner ear blood flow.

Resistance vessels supplying the median eminence of the rabbit, rat, and cat

The ultrastructure of arterioles supplying the median eminence of eight rats, eight rabbits, and two cats was studied after vascular perfusion with phosphate buffered aldehyde fixatives. There were terminal arterioles with a lumen diameter of 50-70 micron within the pars tuberalis. Smaller arterioles (precapillary sphincters and metarterioles) with a lumen diameter of 15-20 micron were present on the surface of the median eminence. Arterioles were not observed to penetrate the neuropil but were seen to supply the external capillary plexus of the median eminence. Direct innervation of arterioles supplying the median eminence was not present and hence regulation of median eminence blood flow by peripheral sympathetic mechanisms appears unlikely. Resistance vessels were found to be closely related to axon terminals on the surface of the median eminence and to fenestrated capillaries of the external plexus. In addition, the endothelial cells of arterioles were characterized by the presence of pits and vesicles which may play a role in transendothelial transport. These findings suggest two mechanisms by which blood flow into the median eminence can be regulated: (a) by central catecholaminergic systems terminating in the median eminence and (b) by catecholamine secretions from the adrenal medulla.

Increased susceptibility to osmotic disruption of the blood-brain barrier in chronic hypertension

We examined the effects of chronic hypertension and acute reduction of arterial pressure on the susceptibility of the blood-brain barrier (BBB) to disruption. The BBB was disrupted with an intracarotid injection of 1.6 M arabinose in spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP), and normotensive Wistar-Kyoto (WKY) rats. Permeability of the BBB was determined from the ratio of 125I-albumin in brain to 125I-albumin in blood. When the BBB was intact, permeability was less than 0.4%. After hypertonic arabinose, permeability of the BBB was greater (mean +/- SE) in SHRSP (17.6% +/- 1.6%) and in SHR (21.1% +/- 3.1%) than in WKY (10.3% +/- 2.4%) (p less than 0.05). When arterial pressure of SHRSP was reduced acutely with nitroprusside before arabinose, the BBB permeability to albumin was not reduced (21.5% +/- 1.5%). In other rats, we examined survival after osmotic disruption. In SHRSP, 14 of 15 rats died within 1 day after osmotic disruption with marked cerebral edema. In WKY, four of 15 rats died (p less than 0.05 vs SHRSP). When arterial pressure of SHRSP was reduced before arabinose, mortality was reduced to six of 15 (p less than 0.05 vs untreated SHRSP). We conclude that the BBB in SHRSP has enhanced vulnerability that is detrimental to survival. Reduction of arterial pressure improves survival in SHRSP without affecting BBB permeability to albumin.(ABSTRACT TRUNCATED AT 250 WORDS)

Significance of the cerebrovascular effects of immobilization stress in the rabbit

The question of the significance of the cerebrovascular effects of stressful situations in animals is still controversial. In the present article, an experimental model of immobilization stress in the rabbit is described, and its specificity in relation to arterial blood pressure and PaCO2 is investigated. CBF was measured with the multiregional tissue sampling technique using [14C]-ethanol as tracer. After dissipation of althesin anesthesia, the stress reaction was elicited by tactile abdominal stimuli. The response was evidenced by an instantaneous acute hypertension (+33.8% during the CBF measurement period). Within the first minute of the reaction, the CBF was significantly increased in all nine structures studied by 39% (caudate nucleus) to 82% (parieto-temporal cortex). The study of the influence of arterial blood pressure and the PaCO2 on CBF showed that cerebrovascular autoregulation and CO2 sensitivity were differently affected in the various structures during the stress reaction. However, the stress response of the brain circulation could not be entirely ascribed to one or both of these two systemic factors, thus suggesting the contribution of a local intrinsic activation. The model presented here could be useful for long-term studies of cerebrovascular repercussions of repeated acute hypertensions of a stressful nature.