Effect of chronic hypertension on acute hypertensive disruption of the blood-brain barrier in rats.

The effect of chronic hypertension on acute hypertensive disruption of the blood-brain barrier has been studied in only two models of hypertension, with inconsistent results. The purpose of this study was to reinvestigate whether chronic hypertension has a consistent effect on acute hypertensive disruption of the blood-brain barrier and to determine whether one of the previously studied models has an unusual response to chronic hypertension. We studied four rat models of chronic hypertension: spontaneously hypertensive rats (SHR), two-kidney, 1 clip Goldblatt rats (2K1C), rats treated with deoxycorticosterone acetate (DOCA) and NaCl, Dahl salt-sensitive rats fed a high salt diet, and two groups of normotensive controls: Wistar-Kyoto rats (WKY) and Dahl salt-sensitive rats fed a low salt diet. We caused acute hypertension in some rats with the use of bicuculline (1.2 mg/kg) and aortic occlusion. Rats without acute hypertension served as controls. Blood-brain barrier disruption was quantitated using the brain/blood ratio of 125I-labeled albumin. Acute hypertensive disruption was less in SHR, rats treated with DOCA-NaCl, and Dahl salt-sensitive rats fed a high salt diet, but not in 2K1C rats, as compared with normotensive controls. Acute hypertensive disruption was greater in Dahl salt-sensitive rats fed a low salt diet than in WKY. A series of control WKY, SHR, rats treated with DOCA-NaCl, 2K1C rats, and Dahl salt-sensitive rats fed low or high salt diets, but not subjected to acute hypertension, were also studied. Brain/blood 125I-albumin ratios were significantly less in these control rats not subjected to acute hypertension than in rats subjected to acute hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors that influence stroke in Dahl salt-sensitive rats.

Japanese rat chow and cerebral sympathetic denervation increase the incidence of stroke in stroke-prone spontaneously hypertensive rats. The purpose of this study was to determine if Japanese rat chow and sympathetic denervation would result in a high incidence of stroke in Dahl salt-sensitive (DS) rats, which have not been reported to be stroke prone. At 3 to 4 weeks of age, DS rats of both sexes began consumption of a high salt Japanese or American chow and underwent unilateral superior cervical sympathetic ganglionectomy. The rats fed American chow were found to have a high incidence of stroke (46%). Rats fed Japanese chow had shorter survival and a higher incidence of stroke (78%) than rats fed American chow (p less than 0.05). Blood pressure increased faster in DS rats fed Japanese chow (p less than 0.05). Metabolic studies indicated that increased sodium consumption accounted for only part of the acceleration of hypertension by Japanese rat chow. In DS rats grouped for equal levels of blood pressure, those fed Japanese chow had modestly reduced survival (p less than 0.05) compared with those fed American chow and had a greater incidence of stroke (85% vs 38%; p less than 0.05). Location of stroke was not influenced by removal of sympathetic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffusional support of the thoracic aorta in atherosclerotic monkeys.

We have demonstrated previously that there is increased blood flow through vasa vasorum in the aorta of atherosclerotic monkeys. In this study, a new method was used to examine diffusional support to the wall of blood vessels. Our goal was to determine whether proliferation of vasa vasorum in the outer media contributes to increased diffusional support of the atherosclerotic aorta and whether there are focal or homogeneous increases in diffusion to the aortic wall. Cynomolgus monkeys were fed normal or atherogenic diet for 1.5 or 4 years. Diffusional support of the thoracic aorta was determined by measuring the concentration of iodo[14C]antipyrine in the aortic media. The findings suggest that: 1) vasa vasorum provide only minimal nutritional support to the thoracic aorta of normal monkeys and monkeys with moderately severe atherosclerosis, and 2) in monkeys with severe atherosclerosis, diffusional support to the thoracic aorta is strikingly heterogeneous, ranging from minimal to large. We speculate that minimal diffusional support to the outer layers of the thickened atherosclerotic aorta may contribute to development of focal medial necrosis.

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.

Cimetidine-sensitive analgesia: investigation of the importance of stress-induced changes in arterial pressure.

Because previous studies have suggested that activation of baroreceptors could mediate stress-induced analgesia, the effect of acute exposure to footshock on mean arterial pressure (MAP) and pain sensitivity was simultaneously determined in conscious rats receiving the histamine H2 receptor antagonist cimetidine or vehicle. Continuous exposure to 3 min of inescapable footshock (3.5 mA) dramatically decreased pain sensitivity, with no increase in post-stress MAP, when compared to no shock controls. The histamine H2-antagonist cimetidine (100 mg/kg, IP) had no significant effect on MAP in resting or stressed animals, but inhibited the stress-induced analgesia, showing that the antagonism of the analgesia is not mediated by modulation of post-stress MAP. Although footshock failed to elicit a significant increase in MAP, a highly significant correlation was found between individual analgesic scores and shock-induced pressure changes in animals treated with cimetidine; in animals receiving vehicle, no such correlation was observed, although the use of a cutoff in analgesic testing may explain this. These results suggest the existence of a stress-induced analgesic mechanism resistant to cimetidine, but associated with elevated MAP.

Intracellular mechanisms of constriction of rat aorta by ethanol.

The intracellular mechanisms mediating vasoconstriction by ethanol are poorly understood. This investigation was designed to provide evidence on the role of protein kinase C (PKC) and calmodulin in vasoconstriction by ethanol. We studied helically cut strips of rat aorta that were exposed to ethanol before and in the presence of the PKC inhibitors calphostin C (79, 239, and 798 nM) or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7, 10 microM), and the calmodulin inhibitor, trifluoperazine (TFP, 10 microM). To test for the specificity of the PKC inhibitors, we measured the responses of aortas to potassium and phorbol 12-myristate 13-acetate (PMA) in the absence and presence of calphostin C and H7. To test for the specificity of TFP, we measured the responses of aortas to serotonin, potassium, PMA, and the thromboxane A2 mimic. 9,11-dideoxy-11 alpha, 9 alpha-epoxy-methanoprostaglandin F2a (U46619), in the absence and presence of TFP. We also studied the effect of the combination of calphostin C and TFP on constriction of the aorta by ethanol. We also measured the importance of intracellular and extracellular calcium in constriction of the aorta by ethanol. Force generation was measured before, and then during exposure of the strips to calcium-free buffer with EGTA, or calcium-free buffer with EGTA plus caffeine. We found that both PKC inhibitors antagonized vasoconstriction by ethanol and PMA. However, H7 antagonized contractions by potassium, but calphostin C did not. We found that TFP caused 99 +/- 1% inhibition of maximum contraction to serotonin, 90 +/- 4% inhibition of maximum contraction to potassium, 63 +/- 6% inhibition of maximum contraction to PMA, and 8 +/- 5% inhibition of maximum contraction to U46619. TFP caused a 22 +/- 8% inhibition of contraction to ethanol. The combination of TFP and calphostin C antagonized vasoconstriction by ethanol to a degree similar to that of calphostin C alone. We also found that contractions to ethanol were only 16 +/- 7% of control values in a calcium-free plus EGTA buffer. Contractions to ethanol were 0 +/- 1% of control values in calcium-free buffer with EGTA plus caffeine. We conclude that: 1-vasoconstriction by ethanol is, at least in part, mediated by PKC; 2-constriction by ethanol is mediated to a minimal extent by calmodulin, and 3-part of the constriction by ethanol of the aorta is mediated by a caffeine-sensitive pool of intracellular calcium.

Continuous measurement of hindquarter resistance changes to nerve stimulation and intraarterial drug administration in rats.

Changes in vascular reactivity are thought to play an important role in the maintenance and/or pathogenesis of various disorders of the cardiovascular system including hypertension. In some animal models, results of studies examining reactivity of the hindquarters vasculature using different techniques are conflicting. The purpose of this study was to validate a new method for measuring vascular reactivity of the hindquarters: a method based on a modification of a technique used previously for renal autoperfusion. We tested the method in spontaneously hypertensive rats (SHR) in order to allow comparison of our results to those obtained previously using other techniques. We found that reactivity of the SHR hindquarters to norepinephrine was normal (compared to that of normotensive Wistar-Kyoto control rats, WKY), while reactivity to nerve stimulation was increased (when results were expressed as an absolute change in resistance). Dilator reactivity to acetylcholine and sympathetic nerve crush was greater in SHR. Resistance of the hindquarters at maximal dilation was 25% higher in SHR. Except for the resistance at maximal dilation, these results contrast with those obtained in earlier studies using an artificial pump to perfuse SHR hindquarters with blood or artificial media. However, the results agree with those reported in conscious, adult SHR. We conclude that: 1) our autoperfusion technique allows comparatively simple vascular reactivity testing to be performed in blood-perfused hindquarters of the rat, and 2) reactivity of the SHR hindquarters to dilator stimuli may be greater than in WKY rats.

Effects of chronic hypertension and sympathetic nerves on the cerebral microvasculature of stroke-prone spontaneously hypertensive rats.

The purpose of this study was to examine hemodynamic mechanisms which protect cerebral vessels against chronic hypertension, and contribute to protective effects of sympathetic nerves in the cerebral circulation. We studied stroke-prone spontaneously hypertensive rats and normotensive Wistar-Kyoto rats. At 3-4 weeks of age, all rats underwent removal of one superior cervical sympathetic ganglion. Approximately 1 year later, we cut the superior cervical sympathetic nerve contralateral to the chronic ganglionectomy and exposed pial arterioles on the cerebral cortex ipsilateral or contralateral to the chronic ganglionectomy. We measured aortic, pial arteriolar, and venous pressures with a servo-null technique, and cerebral blood flow with microspheres. Large artery resistance and small vessel resistance were calculated. During control conditions, pressure in pial arterioles was higher in stroke-prone spontaneously hypertensive rats (83 +/- 6 mm Hg) (mean +/- SE) than in Wistar-Kyoto rats (60 +/- 3 mm Hg, P less than 0.05), even though large artery resistance was almost two-fold greater in stroke-prone spontaneously hypertensive rats than in Wistar-Kyoto rats (P less than 0.05). During maximal dilation produced by seizures, large artery resistance was almost three-fold higher in stroke-prone spontaneously hypertensive rats than in Wistar-Kyoto rats (P less than 0.05). Small vessel resistance also was increased in stroke-prone spontaneously hypertensive rats. During seizures in stroke-prone spontaneously hypertensive rats, large artery resistance was 29% lower in chronically denervated vessels than in acutely denervated vessels (P less than 0.05). Three stroke-prone spontaneously hypertensive rats had pial vessels with a "sausage string" appearance.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffusional support of arteries.

The relative role of the arterial lumen, adventitial vasa vasorum, and medial vasa vasorum in nutritional support of arteries is unclear. We have used a newly developed autoradiographic method to study diffusion of metabolically inert [14C]antipyrine into arteries to determine the relative importance of different pathways in nutritional support of arteries. [14C]antipyrine was homogenously distributed across the media of small arteries within 15 s, which indicates that diffusion into the central media was rapid. In the thoracic and abdominal aorta, levels of antipyrine were higher in the inner media (P less than 0.05) than in the middle of the media. Levels of antipyrine in outer media of the thoracic aorta (which has medial and adventitial vasa) were comparable to those observed in the inner media, but antipyrine levels were lower in outer than in inner media of the abdominal aorta (which has adventitial vasa only). Ligation of intercostal arteries, which are the source of medial vasa vasorum in the thoracic aorta, decreased diffusional support to the outer media of the thoracic aorta. We conclude that 1) diffusional support is more effective in thinner muscular arteries than in the aorta, 2) both luminal and abluminal sources of nutrition are important, particularly for the aorta, 3) vasa vasorum appear to be important for adequate diffusional support of the thoracic aortic media, and 4) medial vasa vasorum may be more effective than adventitial vasa vasorum in nourishing the aorta.

Cardiovascular and body fluid changes after aortic baroreceptor deafferentation.

These experiments were performed to determine the role of fluid volume contraction in the compensatory cardiovascular response to an abrupt neurogenically mediated increase in systemic arterial blood pressure. Acute neurogenic hypertension was produced by selective aortic baroreceptor deafferentation (ABD) in the rat. Salt and water balance, fluid compartment volumes, and arterial pressure were measured before and for 5 days after the induction of hypertension by ABD. In rats with ABD, arterial pressure was increased approximately 30 mmHg 1 day after deafferentation and declined 15-20 mmHg over the next 4 days. Plasma volume and extracellular fluid volume were decreased 5 days after ABD. Urine output did not increase after ABD, and the fluid volume contraction was instead the result of a significant decrease in water intake. Sham-operated rats did not exhibit significant changes in any of these parameters. Sodium balance decreased slightly in both sham-operated and ABD rats postsurgery. These results confirm that fluid volume contraction is a prominent compensatory response to an abrupt rise in arterial pressure, but suggest that this compensation is not necessarily mediated through alterations in renal salt and water excretion.

Protection of cerebral vessels by sympathetic nerves and vascular hypertrophy.

Acute increases in arterial blood pressure, beyond the autoregulatory capacity of cerebral blood vessels, produce an increase in cerebral blood flow, passive dilatation of cerebral arterioles and venules, and disruption of the blood-brain barrier (BBB). Stimulation of sympathetic nerves during acute hypertension attenuates the increase in cerebral blood flow and protects the BBB. It has been assumed that the protective effect of sympathetic stimulation is accomplished by constriction of large cerebral arteries, protecting downstream arterioles and capillaries. We have found, however, that sympathetic stimulation does not attenuate increases in pial arteriolar pressure accompanying acute hypertension. We now suggest that the protective effect of sympathetic stimulation may be accomplished by attenuation of increases in pial venous pressure. In chronic hypertension, blood pressure is sufficiently elevated that one might expect an increase in cerebral blood flow and disruption of the BBB. However, cerebral blood flow is normal and the BBB is intact in chronic hypertension. Protection of the cerebral circulation during chronic hypertension is accomplished in part by a trophic effect of sympathetic nerves, which promotes hypertrophy of cerebral blood vessels. Vascular hypertrophy attenuates increases in microvascular pressure accompanying hypertension. Thus, stimulation of sympathetic nerves protects cerebral vessels during acute hypertension, perhaps by attenuation of increases in cerebral venous pressure, and a trophic effect of sympathetic nerves promotes vascular hypertrophy and protects cerebral vessels during chronic hypertension.

Hemodynamic and neural mechanisms of acute neurogenic hypertension in the rat.

Experiments were performed to test the hypothesis that acute hypertension caused by aortic baroreceptor deafferentation (ABD) is the result of sympathetic vasoconstriction. Cardiac output (CO), mean arterial pressure (MAP), and total peripheral resistance (TPR) were measured before and after ABD in anesthetized and conscious rats. The role of the sympathetic nervous system in acute ABD-induced hypertension was evaluated by examining the ability of adrenalectomy, adrenal demedullation, guanethidine or combined adrenal demedullation, and guanethidine pretreatment to prevent, and total autonomic blockade to reverse, ABD-induced hypertension. CO did not change significantly after ABD at any time, whereas MAP and TPR increased significantly (P less than 0.05). Only combined adrenal demedullation and guanethidine pretreatment prevented ABD-induced hypertension, and autonomic blockade normalized MAP in ABD rats. Normalization of blood pressure was the result of a decreased TPR. It is concluded that acute ABD-induced hypertension results from vasoconstriction caused by neurally released and/or circulating catecholamines.

Cerebral circulation: effects of sympathetic nerves and protective mechanisms during hypertension.

The first goal of this study was to examine bilateral effects of reflex activation of sympathetic nerves on the cerebral circulation. Seizures, which activate sympathetic nerves, were induced in animals with intact nerves and after bilateral cervical sympathetic denervation. Increases in cerebral blood flow (microspheres) and decreases in cerebral vascular resistance were similar in denervated and innervated animals. Thus, during intense metabolic stimulation, metabolic factors are the primary determinant of cerebral blood flow, and bilateral effects of sympathetic nerves are minimal. The second goal of this study was to examine the role of vascular hypertrophy in protection of the cerebral circulation. Cerebral perfusion pressure was decreased on one side by clipping one carotid artery in 4-week-old stroke-prone spontaneously hypertensive rats. Two to four months later, the clip was removed, and seizures were induced. Disruption of the blood-brain barrier in the cerebrum occurred predominantly on the clipped side. We suggest that reduction in perfusion pressure attenuates development of cerebral vascular hypertrophy and thereby increases susceptibility to disruption of the blood-brain barrier. Thus, hypertrophy of cerebral vessels during chronic hypertension may protect the cerebral circulation.

No rarefaction of cerebral arterioles in hypertensive rats.

A reduction in the density of small arterioles (rarefaction) has been reported in several vascular beds of the spontaneously hypertensive rat (SHR). There have been conflicting reports on the existence of rarefaction in the pial vasculature of SHR. In this study, we determined whether there was rarefaction of pial arterioles in several models of hypertension. We studied SHR; two-kidney, one-clip Goldblatt hypertensive rats; deoxycorticosterone-salt hypertensive rats; and Dahl salt-sensitive rats fed high salt diet. The two groups of normotensive controls were Wistar--Kyoto rats and Dahl salt-sensitive rats fed low salt diet. The duration of hypertension was about 2 months. Density of first-, second-, third-, and fourth-order arterioles was determined by counting the number of vessels from enlarge photographs. We also measured the lengths of segments of the arterioles. We did not observe any evidence of rarefaction of arterioles in the pial vasculature in any of the hypertensive groups of rats. We conclude that (i) rarefaction of arterioles does not occur in the pial microvasculature after approximately 2 months of hypertension and (ii) rarefaction of pial arterioles does not account for abnormalities in the cerebral circulation of hypertensive rats such as protection of the blood-brain barrier or changes in autoregulation of cerebral blood flow.

Hypertension and cerebral arteries: relationship to disruption of the blood-brain barrier.

This study was performed to determine whether structural changes in cerebral arterioles could account for differences in susceptibility of the blood-brain barrier to acute hypertensive disruption between hypertensive and normotensive animals. We studied spontaneously hypertensive rats (SHR), 3 other models of hypertension and their normotensive controls. The age and duration of hypertension of the rats were matched to an earlier study showing that protection of the blood-brain barrier was usually found in rats with chronic hypertension. We measured the dimensions of fixed pial arterioles and minimal cerebrovascular resistance produced by bicuculline. Minimal cerebrovascular resistance was not different between the groups of animals. There were no differences in the area of the media of pial arterioles between any of the groups. In addition, we examined the possibility that sympathetic nerves might affect cerebrovascular resistance during bicuculline in SHR. The presence of sympathetic nerves in SHR, but not WKY, reduced the degree of cerebral vascular dilation during bicuculline. From these data we conclude that 1) structural changes in cerebral vessels do not account for protection of the blood-brain barrier in rats with a moderate duration of hypertension and 2) sympathetic nerves may have an exaggerated effect on cerebral vessels of SHR.