Regional,segmental, and temporal heterogeneity of cerebral vascular autoregulation

Autoregulation of cerebral blood flow is heterogeneous in several ways: regional, segmental, and temporal. We have found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial presure. Changes in blood flow are less in brain stem than in cerebrum during decreases and increases in cerebral perfusion pressure. Segmental heterogeneity of autoregulation has been demonstrated in two ways. Direct determination of segmental cerebral vascular resistance indicates that, while small cerebral vessels (<200 μm in diameter) make a major contribution to autoregulation during acute increases in pressure between 80 and 100 mm Hg, the role of large cerebral arteries (>200 μm) becomes increasingly important to the autoregulatory response at pressures above 100 mm Hg. Measurement of changes in diameter of pial vessels has shown that, during acute hypotension, autoregulation occurs predominantly in small resistance vessels (<100 μm). Finally, there is temporal heterogeneity of autoregulation. Sudden increases in arterial pressure produce transient increases in blood flow, which are not observed under steady-state conditions. In addition, the blood-brain barrier is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure. Thus, when investigating cerebral vascular autoregulation, regional, segmental, and temporal differences in the autoregulatory response must be taken into consideration.     

Sustained vessel dilation induced by increased pulsatile perfusion of porcine carotid arteries in vitro.

Arterial pulse pressure (PP) increases with exertional stress and ageing, and can modify vessel diameter in smaller vessels. To test if PP must exceed a certain range to influence vessel diameter, and determine if such effects are endothelium-dependent or intrinsic to vascular viscoelasticity, eight fresh excised porcine carotid artery segments were perfused with modified Krebs-Henseleit by a servo-controlled system generating physiological arterial pressure waveforms. In a separate group of vessels (n = 10), the endothelium was mechanically removed. Vessel external diameter was measured by video edge-detection. Vessels partially preconstricted with noradrenaline were perfused at 9 mL min(-1) mean flow, at mean pressure of 90 or 120 mmHg, and zero PP. PP alone was then increased to 40, 70, or 120 mmHg at 1 Hz cycling rate for 5 min, then returned to zero and vessel diameter measured immediately thereafter. The protocol was repeated after 10-20 min stabilization. Mean vessel diameter rose proportionally with PP only once PP exceeded 40 mmHg, with maximal increases of 6-9% at a PP of 120 mmHg. Similar responses were obtained in vessels with and without a functional endothelium, at both mean pressures. Thus, when exposed to higher than normal resting PP, conduit arteries dilate owing to the stress-relaxation response of their viscoelastic wall. This mechanism of PP-mediated vascular dilatation may contribute to enhanced organ perfusion when small resistance arteries are already dilated.     

Effect of sympathetic nerve stimulation and adrenoceptor blockade on pial arterial and venous calibre and on intracranial pressure in the cat

Pial arterial and venous calibre were continuously recorded through a closed cranial window preparation during cervical sympathetic nerve stimulation in 10 cats before and after alpha- and beta-adrenoceptor blockade. In addition, the intracranial pressure (ICP) was simultaneously recorded in 4 of the cats. Under resting conditions 33 arteries (mean diameter 130 micron) constricted by 11.7 +/- 0.8% and 80 venous portions (mean diameter 152 micron) constricted by 13.7 +/- 0.7% during sympathetic nerve stimulation. ICP decreased simultaneously by 16.5 +/- 6.2%. Administration of the alpha-adrenoceptor antagonist phenoxybenzamine i.v. (1.5 mg X kg-1) abolished the reduction of ICP and markedly reduced, but did not completely abolish, the constrictor response of arteries and veins. The beta-adrenoceptor antagonist propranolol (1.5 mg X kg-1) did not significantly alter the reduction of ICP or the response of pial veins and small arteries, whereas the response of arteries with a diameter greater than 150 micron was attenuated. It is concluded that the constriction of pial veins and arteries during sympathetic stimulation is mediated predominantly via alpha-adrenoceptors. The sympathetic nerves of cerebral blood vessels may have stronger influence on the cerebral capacitance than on resistance vessels under normotensive and normocapnic conditions.     

Sustained vessel dilation induced by increased pulsatile perfusion of porcine carotid arteries in vitro

Arterial pulse pressure (PP) increases with exertional stress and ageing, and can modify vessel diameter in smaller vessels. To test if PP must exceed a certain range to influence vessel diameter, and determine if such effects are endothelium-dependent or intrinsic to vascular viscoelasticity, eight fresh excised porcine carotid artery segments were perfused with modified Krebs–Henseleit by a servo-controlled system generating physiological arterial pressure waveforms. In a separate group of vessels (n = 10), the endothelium was mechanically removed. Vessel external diameter was measured by video edge-detection. Vessels partially preconstricted with noradrenaline were perfused at 9 mL min^–1 mean flow, at mean pressure of 90 or 120 mmHg, and zero PP. PP alone was then increased to 40, 70, or 120 mmHg at 1 Hz cycling rate for 5 min, then returned to zero and vessel diameter measured immediately thereafter. The protocol was repeated after 10–20 min stabilization. Mean vessel diameter rose proportionally with PP only once PP exceeded 40 mmHg, with maximal increases of 6–9% at a PP of 120 mmHg. Similar responses were obtained in vessels with and without a functional endothelium, at both mean pressures. Thus, when exposed to higher than normal resting PP, conduit arteries dilate owing to the stress-relaxation response of their viscoelastic wall. This mechanism of PP-mediated vascular dilatation may contribute to enhanced organ perfusion when small resistance arteries are already dilated.     

Localization of the m5 muscarinic cholinergic receptor in rat circle of Willis and pial arteries

The expression and microanatomical localization of the muscarinic cholinergic m5 receptor subtype was investigated in rat circle of Willis and pial arteries by in situ hybridization, immunoblotting and immunohistochemistry. In situ hybridization histochemistry revealed a strong signal in the endothelium of circle of Willis and pial arteries and a moderate signal in the tunica media of the same arteries, within smooth muscle. Exposure of membranes of arteries to anti-m5 receptor protein antibodies caused the development of a band of approximately 81 kDa. Immunohistochemistry revealed the accumulation of m5 receptor protein immunoreactivity primarily within endothelium of circle of Willis and cerebral arteries and to a lesser extent in the tunica media, within smooth muscle. Medium (external diameter 200-100 microm) and small-sized (external diameter smaller than 100 microm) pial arteries displayed a significantly higher immune staining than large-sized pial arteries or circle of Willis arteries. The above data that are consistent with recent functional studies reporting cholinergic dilation of cerebral blood vessels mediated via a m5 receptor, have shown that both endothelial and muscular components of cerebral arteries synthesize and express a muscarinic m5 receptor. In view of the peculiar localization in cerebral vessels, handling of the muscarinic m5 receptor may be considered as an approach in the treatment of cerebrovascular disease.     

Vascular resistance of intestine, muscle and skin during blood pressure oscillation.

After the blood supply to the head was surgically limited to one common carotid artery in rabbits, the artery was compressed by exertion of stepwise elevated side pressure on a segment of the artery. Systemic arterial pressure rose and began to oscillate when the side pressure was increased to 50 to 60 mmHg. The oscillations were regular and occurred at a rate of 3 to 4 per minute with a wave height of 15 to 45 mmHg. Concomitant with systemic pressure oscillations, and in near synchrony were undulations in the perfusion pressures recorded during constant flow from the vascularly isolated abdominal skin, skeletal muscle and small intestine. With the graded elevation of side pressure on the common carotid artery, mean systemic pressure and perfusion pressure showed a nearly exponential relation. The ratio between the two variables was slightly less than one below the systemic pressure of 120 mmHg and rapidly increased above this level. At ratios greater than one, the peripheral resistance undulations in the isolated areas gave a larger minimum and maximum, a sharper contour and, particularly in the skin, a phase lag in the period of the wave form when compared to the systemic pressure oscillations. These observations highlight the differences in response characteristics of peripheral resistance vessels and those of cardiac output and different consecutive parts of the systemic arteries during periods of intense sympathetic activation.     

Renal autoregulation: evidence for the transmural pressure hypothesis.

Autoregulation of glomerular filtration rate (GFR) was examined during uteral orarterial constriction in anesthetized dogs after renal denervation. GFR was sustaineduntil ureteral pressure greater than 80 mmHg, provided renal arterial pressure exceeded 180 mmHg, but fell at ureteral pressure less than 54 mmHg when arterial pressure averaged 127 plus or minus 5 mmHg; renal blood rose as GFR declined. Ethacrynic acid, saline, or mannitol infusion increased tubular pressure without reducing GFR,but during subsequent ureteral constriction GFR fell at uteral pressure less than 40mmHg. During arterial constriction GFR was maintained at lower arterial pressures in hydropenic than in diuretic dogs. Because of thisdifference in the range of autoregulation, saline infusion increased GFR more in hydropenic than in diuretic dogs except at high arterial pressure. This response to reduced plasma oncotic pressure and the constancy of GFR over a wide range of proximal tubular and arterial pressure indicate constancy of thehydrostatic transmural pressure of glomerular capillaries. Afferent arteriolar resistance is, in addition to a regulation by transmural pressure, perhaps controlled by vascular stretch receptors in the glomeruli.     

Determinants of systemic zero-flow arterial pressure

Thirteen pentobarbital-anesthetized dogs whose carotid sinuses were isolated and perfused at a constant pressure were placed on total cardiac bypass. With systemic venous pressure held at 0 mmHg (condition 1), arterial inflow was stopped for 20 s at intrasinus pressures of 50, 125, and 200 mmHg. Zero-flow arterial pressures under condition 1 were 16.2 +/- 1.3 (SE), 13.8 +/- 1.1, and 12.5 +/- 0.8 mmHg, respectively. In condition 2, the venous outflow tube was clamped at the instant of stopping the inflow, causing venous pressure to rise. The zero-flow arterial pressures were 19.7 +/- 1.3, 18.5 +/- 1.4, and 16.4 +/- 1.2 mmHg for intrasinus pressures of 50, 125, and 200 mmHg, respectively. At all levels of intrasinus pressure, the zero-flow arterial pressure in condition 2 was higher (P less than 0.005) than in condition 1. In seven dogs, at an intrasinus pressure of 125 mmHg, epinephrine increased the zero-flow arterial pressure by 3.0 mmHg, whereas hexamethonium and papaverine decreased the zero-flow arterial pressure by 2 mmHg. Reductions in the hematocrit from 52 to 11% resulted in statistically significant changes (P less than 0.01) in zero-flow arterial pressures. Thus zero-flow arterial pressure was found to be affected by changes in venous pressure, hematocrit, and vasomotor tone. The evidence does not support the literally interpreted concept of the vascular waterfall as the model for the finite arteriovenous pressure difference at zero flow.     

Effect of induced hypertension on experimentally-induced cerebral arterial spasm

Ten adult cats were anesthetized and ventilated by respirator. After the basilar artery was exposed transclivally and visualized with an operative microscope, mean arterial blood pressure (MABP) was raised gradually by intravenous drip infusion of norepinephrine (5-20 micrograms/kg) or angiotensin-II-amide (0.3-1.0 micrograms/kg). At various blood pressures, microphotographs were taken. There was no appreciable change in vessel diameter at a MABP ranging from 78 to 191 mmHg. The blood pressure was allowed to return to the initial baseline level. Arterial spasm was produced by the topical application of 0.2 M calcium gluconate, which decreased the arterial diameter by 13 to 58 percent for more than 60 min. Blood pressure was increased again after the production of the arterial spasm. Significant increases in the diameter of the arteries were produced by the drug-induced hypertension at levels of MABP ranging from 82 to 192 mmHg. The maximum arterial dilations ranged from 123 to 208 percent of the untreated control. The degree of dilation of the arteries almost paralleled the rise in MABP. Norepinephrine and angiotensin-II had a similar effect on both the blood pressure and the arterial diameter. Induced hypertension would be expected to improve blood flow parameters in the case of spastic cerebral arteries.     

Vasoconstrictor reactions in spontaneously hypertensive rats versus Wistar Kyoto can be increased or decreased depending on the conditions of perfusion.

The reactions of resistance vessels in SHR and WKY hindquarters were compared during saline or blood perfusion. During saline constant-flow perfusion at all initial pressures (80-200 mmHg) sympathetic vasoconstrictor effects were greater in SHR than those in WKY. During perfusion at constant and equal pressure vasoconstrictor responses were greater in SHR vs. WKY only at high pressure--200 mmHg. On the other hand, under constant pressure conditions at lower pressures (80 and 120 mmHg) sympathetic stimulation induced weaker responses in SHR than in WKY, which at, for example, 80 mmHg was the case at every frequency of sympathetic stimulation used (2-20 Hz). Also, the responses to exogenous noradrenaline and vasopressin occurred during perfusion at low (80 mmHg) and for both equal constant-pressure conditions lower in SHR than in WKY. Comparison of sympathetic effects in SHR and WKY during blood hindquarter perfusion revealed similar results. Also, when SHR and WKY responses were compared at their ordinary levels of constant-pressure, sympathetic vasoconstrictor effects in SHR were lower than those in WKY.     

Rat brain interstitial fluid pressure measured with micropipettes

Interstitial fluid pressure (IFP) in the rat brain has been measured with micropipettes, 2-4 microns in diameter, after trepanation and incision of dura. IFP was compared with cerebrospinal fluid pressure (CSFP) measured by a 0.3-mm (OD) cannula in the lateral ventricle. Control IFP averaged 3.43 +/- 0.65 (SD) mmHg (n = 83) with a corresponding CSFP of 3.53 +/- 0.71 (n = 83). Pretreatment with indomethacin to reduce inflammation during preparation did not affect IFP and CSFP. Infusion of a 20% mannitol solution amounting to 5% of body weight resulted in an average IFP of -5.66 mmHg and CSFP of -4.56 mmHg 20 min after start of the infusion, after which a gradual rise in both pressures was observed. Similar results were obtained after infusion of hypertonic urea. Inflation of a cuff around the rat neck resulted in an immediate and similar rise in IFP and CSFP. Increasing the arterial CO2 partial pressure to 80 mmHg increased IFP to 12-14 mmHg and pial venular pressure to 13-15 mmHg, while CSFP rose to about 10 mmHg. The micropuncture technique for measurement of brain IFP is rapid and practically atraumatic and may be used in a variety of animals.     

Pressure-flow relations in coronary circulation

The blood vessels that run on the surface of the heart and through its muscle are compliant tubes that can be affected by the pressures external to them in at least two ways. If the pressure outside these vessels is higher than the pressure at their downstream ends, the vessels may collapse and become Starling resistors or vascular waterfalls. If this happens, the flow through these vessels depends on their resistance and the pressure drop from their inflow to the pressure around them and is independent of the actual downstream pressure. In the first part of this review, the physics of collapsible tubes is described, and the possible occurrences of vascular waterfalls in the body is evaluated. There is good evidence that waterfall behavior is seen in collateral coronary arteries and in extramural coronary veins, but the evidence that intramural coronary vessels act like vascular waterfalls is inconclusive. There is no doubt that in systole there are high tissue pressures around the intramyocardial vessels, particularly in the subendocardial muscle of the left ventricle. The exact nature and values of the forces that act at the surface of the small intramural vessels, however, are still not known. We are not certain whether radial (compressive) or circumferential and longitudinal (tensile) stresses are the major causes of vascular compression; the role of collagen struts in modifying the reaction of vessel walls to external pressures is unknown but possibly important; direct examination of small subepicardial vessels has failed to show vascular collapse. One of the arguments in favor of intramyocardial vascular waterfalls has been that during a long diastole the flow in the left coronary artery decreases and reaches zero when coronary arterial pressure is still high: it can be as much as 50 mmHg in the autoregulating left coronary arterial bed and approximately 15-20 mmHg even when the vessels have been maximally dilated. These high zero flow pressures, especially during maximal vasodilatation, have been regarded as indicating a high back pressure to flow that is due to waterfall behavior of vessels that are exposed to tissue pressures.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

A comparative study of middle cerebral pressure in dogs and macaques

1. A comparison has been made of the pressures recorded from pial branches of the middle cerebral artery in dogs and macaques. This pressure has been shown to be between 88 and 95% of femoral arterial pressure in dogs under chloralose anaesthesia, and between 80 and 90% of femoral arterial pressure in macaques similarly anaesthetized.2. The effect of occlusion of the main vessels in the neck is shown to differ considerably in the two species. Blood pressure within the forebrain of the dog is shown to be largely dependent upon the integrity of the external carotid artery, whereas in the monkey the external carotid artery is without effect in the maintenance of forebrain blood pressure. Occlusion of the four major arteries in the neck is shown to produce a greater effect in the macaque and to be accompanied by signs of medullary ischaemia in this species.3. After occlusion of the main middle cerebral artery, arterial pressure measured distal to the occlusion depends upon the integrity of collateral vessels from the other cerebral arteries. When only a branch of the middle cerebral artery is occluded, the greater part of the residual blood pressure depends upon anastomoses from other branches of the middle cerebral artery itself.     

Mechanical and dimensional adaptation of rabbit carotid artery culturedin vitro

The effects of the mechanical environment on arterial walls were investigated in rabbit common carotid arteries, cultured for six days under three different intraluminal pressures (0, 80 and 160 mmHg) in a perfusion culture system. The mechanical responses following the culture were examined using a quasi-static pressure-diameter test. Specimen viability was determined by smooth muscle contraction induced with KCl. Eighteen out of 21 cultured segments showed a peak reduction in diameter of more than 10% and were used for the analysis. The arterial segments cultured at 0 mmHg had a significantly smaller diameter than those cultured at other pressures. The segments cultured at higher pressure had lower incremental elastic moduli at 20 and 80 mmHg and higher moduli at 160 mmHg. The walls of the cultured segments were thicker in groups with higher pressure. These results indicate that, even in culture, the mechanical environment is a major determinant for the mechanical property and dimensions of the arterial wall. Arterial walls may respond to their mechanical environment even if other factors, such as hormonal environment and nervous stimuli, are kept unchanged.     

Reflex responses from the main pulmonary artery and bifurcation in anaesthetised dogs

This study was undertaken to determine the reflex cardiovascular and respiratory responses to discrete stimulation of pulmonary arterial baroreceptors using a preparation in which secondary modulation of responses from other reflexes was prevented. Dogs were anaesthetised with -chloralose, artificially ventilated, the chests widely opened and a cardiopulmonary bypass established. The main pulmonary arterial trunk, bifurcation and extrapulmonary arteries as far as the first lobar arteries on each side were vascularly isolated and perfused through the left pulmonary artery and drained via the right artery through a Starling resistance which controlled pulmonary arterial pressure. Pressures distending systemic baroreceptors and reflexogenic regions in the heart were controlled. Reflex vascular responses were assessed from changes in perfusion pressures to a vascularly isolated hind limb and to the remainder of the subdiaphragmatic systemic circulation, both of which were perfused at constant flows. Respiratory responses were assessed from recordings of efferent phrenic nerve activity. Increases in pulmonary arterial pressure consistently evoked increases in both perfusion pressures and in phrenic nerve activity. Both vascular and respiratory responses were obtained when pulmonary arterial pressure was increased to above about 30 mmHg. Responses increased at higher levels of pulmonary arterial pressures. In 13 dogs increases in pulmonary arterial pressure to 45 mmHg increased systemic perfusion pressure by 24 +/- 7 mmHg (mean +/- S.E.M.) from 162 +/- 11 mmHg. Setting carotid sinus pressure at different levels did not influence the vascular response to changes in pulmonary arterial pressure. The presence of a negative intrathoracic pressure of -20 mmHg resulted in larger vascular responses being obtained at lower levels of pulmonary arterial pressure. This indicates that the reflex may be more effective in the intact closed-chest animal. These results demonstrate that stimulation of pulmonary arterial baroreceptors evokes a pressor reflex and augments respiratory drive. This reflex is likely to be elicited in circumstances where pulmonary arterial pressure increases and the negative excursions of intrathoracic pressure become greater. They are likely, therefore, to be involved in the cardio-respiratory response to exercise as well as in pathological states such as pulmonary hypertension or restrictive or obstructive lung disease.     

Mechanical and dimensional adaptation of rabbit carotid artery culturedin vitro

The effects of the mechanical environment on arterial walls were investigated in rabbit common carotid arteries, cultured for six days under three different intraluminal pressures (0, 80 and 160 mmHg) in a perfusion culture system. The mechanical responses following the culture were examined using a quasi-static pressure-diameter test. Specimen viability was determined by smooth muscle contraction induced with KCl. Eighteen out of 21 cultured segments showed a peak reduction in diameter of more than 10% and were used for the analysis. The arterial segments cultured at 0 mmHg had a significantly smaller diameter than those cultured at other pressures. The segments cultured at higher pressure had lower incremental elastic moduli at 20 and 80 mmHg and higher moduli at 160 mmHg. The walls of the cultured segments were thicker in groups with higher pressure. These results indicate that, even in culture, the mechanical environment is a major determinant for the mechanical property and dimensions of the arterial wall. Arterial walls may respond to their mechanical environment even if other factors, such as hormonal environment and nervous stimuli, are kept unchanged.     

The carotid sinus baroreceptor reflex in the pregnant rabbit

1. The reflex responses to baroreceptor stimulation have been compared in eight pregnant and eight non-pregnant anaesthetized female rabbits.2. The vascularly isolated, innervated carotid sinus was exposed for 30 sec to a series of non-pulsatile pressures ranging from 30 to 230 mmHg. The contralateral sinus nerve and both aortic nerves were cut. Systemic arterial pressure and heart rate were measured at each sinus pressure.3. The range of arterial pressure change which could be evoked from the isolated innervated sinus was less in the pregnant than in the non-pregnant rabbits. Mean changes were 98 and 61 mmHg respectively. On the other hand changes in heart rate were similar in the two groups (45 and 43 beats/min respectively).4. The smaller blood pressure response in the pregnant animals resulted from a lesser rise in systemic arterial pressure at low levels of sinus pressure. At high sinus pressures the blood pressure fell to a similar level in both groups of animals.5. Pressure on the great vessels by the gravid uterus was not a factor since there was no consistent difference between the responses obtained with the rabbit lying on its back or on its side.6. Mechanisms which might be responsible for the difference found are discussed.     

Prostacyclin and thromboxane in cerebral vasospasm II: Effects of thromboxane synthetase inhibitor (OKY-1581) on experimentally-induced cerebral vasospasm

OKY-1581, a thromboxane A2 (TXA2) synthetase inhibitor, was administered to cats with normal and constricted basilar arteries. At a dose of 60mg/kg (i.v.), both normal and constricted vessels dilated, and the mean arterial blood pressure (MABP) fell from 55 to 75 mmHg. If MABP remained constant, vessel diameter did not change. Subarachnoid hemorrhage (SAH) was simulated by intracisternal injection of autologous arterial blood. Regional cerebral blood flow (rCBF) was assessed by the heat clearance and H2 clearance methods. The two methods presented similar response profiles. rCBF responses to intravenous OKY-1581 fell into 3 categories: A) no change in rCBF, B) decrease in rCBF related to MABP and C) increase in rCBF in the presence of hypotension. Types A and B were observed in 3 out of 10 control cats and 4 out of 14 SAH-induced cats, with Type C responses in the remainder. There was no significant difference between the groups. While the results do not support a major role for TXA2 in cerebral vasospasm pathogenesis, OKY-1581 may still be useful in the treatment of cerebral vasospasm, as it improves distal and deep circulation and inhibits platelet aggregation.     

Nitric oxide synthase inhibitors unmask acetylcholine-mediated constriction of cerebral vessels in the in vitro isolated guinea-pig brain

The control of arterial vascular tone by acetylcholine contributes to the regulation of cerebral blood flow. We analysed the effects of intraluminal application of acetylcholine (1microM) on the cerebral vascular tone by measuring changes in resistance to perfusion pressure in an isolated guinea-pig brain preparation maintained in vitro by arterial perfusion under constant flow. Acetylcholine induced a reproducible, fast-onset dilation that was prevented by the nitric oxide scavenger Methylene Blue (10microM) and by the muscarinic receptor antagonist atropine (0.1microM). Prolonged arterial perfusion with the nitric oxide synthase inhibitors N-nitro-L-arginine (1mM) and N-nitro-L-arginine methyl ester (30-100microM) induced a slowly developing increase of 25.9+/-13. 44mmHg in vascular tone and blocked the acetylcholine-induced vasodilation. In these experimental conditions, the dilation determined by the nitric oxide donor nitroprusside (0.1microM) was unaffected. In five experiments, the blockade of dilation unmasked a slow acetylcholine-mediated vasoconstriction (14.40+/-3.85mmHg) that was antagonized by atropine.The results demonstrate that acetylcholine exerts two simultaneous and opposite effects on guinea-pig cerebral vessels, characterized by a slow direct constriction concealed in physiological conditions by a fast vasodilation mediated through the release of nitric oxide by endothelial cells. Acetylcholine-mediated increase in vascular tone may play a role in aggravating cerebral perfusion when endothelial cell damage occurs during brain ischemia.