Effects of long-acting angiotensin-converting enzyme inhibitor, imidapril, on the lower limit of cerebral blood flow autoregulation in hypertensive rats

High salt diet impairs cerebral blood flow regulation via salt-induced angiotensin II suppression

OBJECTIVES

This study sought to determine whether salt-induced ANG II suppression contributes to impaired CBF autoregulation.

METHODS

Cerebral autoregulation was evaluated with LDF during graded reductions of blood pressure. Autoregulatory responses in rats fed HS (4% NaCl) diet vs LS (0.4% NaCl) diet were analyzed using linear regression analysis, model-free analysis, and a mechanistic theoretical model of blood flow through cerebral arterioles.

RESULTS

Autoregulation was intact in LS-fed animals as MAP was reduced via graded hemorrhage to approximately 50 mm Hg. Short-term (3 days) and chronic (4 weeks) HS diet impaired CBF autoregulation, as evidenced by progressive reductions of laser Doppler flux with arterial pressure reduction. Chronic low dose ANG II infusion (5 mg/kg/min, i.v.) restored CBF autoregulation between the pre-hemorrhage MAP and 50 mm Hg in rats fed short-term HS diet. Mechanistic-based model analysis showed a reduced myogenic response and reduced baseline VSM tone with short-term HS diet, which was restored by ANG II infusion.

CONCLUSIONS

Short-term and chronic HS diet lead to impaired autoregulation in the cerebral circulation, with salt-induced ANG II suppression as a major factor in the initiation of impaired CBF regulation.

Management of Brain Edema Complicating Stroke

Ischemic brain edema, the accumulation of fluid within the brain parenchyma following stroke, is a predictable consequence of both ischemic and hemorrhagic strokes. Its development is the result of injury to both brain parenchyma and the blood vessels supplying the parenchyma. Ischemic stroke produces both cytotoxic (intracellular) edema, which develops when cells are damaged, and vasogenic (extracellular) edema, which arises from injury to structures essential to blood-brain barrier integrity. An understanding of the distinction between cytotoxic and vasogenic edema is essential in preventing secondary brain injury, since the treatments for the two entities differ. The development of new brain imaging technologies has advanced our understanding of brain edema. Both computed tomography (CT) and magnetic resonance imaging (MRI) can detect edema. Specific MRI sequences such as diffusion-weighted imaging can distinguish cytotoxic and vasogenic subtypes, and thereby detect ischemic cell injury within minutes of the onset of symptoms.

Brain edema causes neurologic deterioration predominantly through its mass effect, which leads to distortion of the intracranial contents and impairment of both regional and global cerebral blood flow (CBF). Edema may also cause local tissue dysfunction. Management of the intracranial hypertension and tissue shifts caused by ischemic brain swelling is based on the fundamental relationship between pressure, flow, and resistance. Interventions are directed at preserving CBF and preventing secondary brain injury. Strategies include reducing intracranial blood volume with hypocapnia, reducing brain volume with osmotic agents, reducing cerebral metabolism with hypothermia and barbiturates, reducing resistance with rheologic agents, increasing blood pressure with vasoconstrictors, and expanding the cranial vault with decompressive surgery. All individual therapies must be used as part of a structured approach that involves frequent serial neurologic assessments, quantitative measures of pressure, flow, and resistance, and prespecified protocols for intervention.

Excess salt increases infarct size produced by photothrombotic distal middle cerebral artery occlusion in spontaneously hypertensive rats

Cerebral circulation is known to be vulnerable to high salt loading. However, no study has investigated the effects of excess salt on focal ischemic brain injury. After 14 days of salt loading (0.9% saline) or water, spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were subjected to photothrombotic middle cerebral artery occlusion (MCAO), and infarct volume was determined at 48 h after MCAO: albumin and hemoglobin contents in discrete brain regions were also determined in SHR. Salt loading did not affect blood pressure levels in SHR and WKY. After MCAO, regional cerebral blood flow (CBF), determined with two ways of laser-Doppler flowmetry (one-point measurement or manual scanning), was more steeply decreased in the salt-loaded group than in the control group. In SHR/Izm, infarct volume in the salt-loaded group was 112±27 mm3, which was significantly larger than 77±12 mm3 in the control group (p = 0.002), while the extents of blood-brain barrier disruption (brain albumin and hemoglobin levels) were not affected by excess salt. In WKY, salt loading did not significantly increase infarct size. These results show the detrimental effects of salt loading on intra-ischemic CBF and subsequent brain infarction produced by phototrhombotic MCAO in hypertensive rats.

Role of substantia innominata in cerebral blood flow autoregulation

Ascending projections from the substantia innominata (SI) may have an important role in the regulation of cerebral blood flow (CBF). However, several reports have suggested that unilateral lesion of the SI does not affect CBF autoregulation. On the other hand, it is also reported that several cortical and subcortical functions may be regulated not only by ipsilateral SI, but also by contralateral SI. Thus, the objective of this study is to test the hypothesis that bilateral lesions of the SI affect CBF autoregulation. Experiments were performed on anesthetized male Sprague-Dawley rats. Ibotenic acid or physiological saline was microinjected into bilateral SI. Rats were classified into four groups as follows: bilateral SI lesion rats (ibotenic acid was injected bilaterally), left or right SI lesion rats (ibotenic acid was injected into the unilateral SI and saline into the contralateral SI), and control rats (saline was injected bilaterally). Ten days after injection, CBF in the left frontal cortex was measured by laser-Doppler flowmetry during stepwise controlled hemorrhagic hypotension. In bilateral SI lesion rats, CBF was started to decrease significantly at 80 mm Hg (p<0.01). In the other three groups, CBF was well maintained until 50 mm Hg. Changes in CBF through stepwise hypotension in bilateral SI lesion rats were significantly different from the other groups (p<0.01). These results suggest that bilateral SI regulates cortical vasodilator mechanisms during hemorrhagic hypotension. Under unilateral SI lesion, some compensatory effects from the contralateral SI may maintain CBF autoregulation.

Valsartan improves the lower limit of cerebral autoregulation in rats

The effects of angiotensin II type 1 receptor blockers (ARBs) on cerebral blood flow (CBF) autoregulation have not been fully clarified. Thus, we examined the acute effect of valsartan, the most selective ARB, on CBF autoregulation in spontaneously hypertensive rats. Intravenous administration of valsartan (0.3 mg/kg) reduced the mean arterial pressure (MAP) from 184+/-5 (mean+/-SEM) to 174+/-5 mmHg (p<0.001) without affecting CBF as measured by laser-Doppler flowmetry. The lower limit of CBF autoregulation (the MAP at which the CBF was 80% of the baseline value) in the valsartan-treated group (122+/-3 mmHg) was significantly lower than that in the control group (135+/-4 mmHg, p<0.05). Reverse transcribed-polymerase chain reaction and immunohistochemical staining demonstrated that both angiotensin II type 2 receptors and angiotensin II type 1 receptors (AT1Rs) were expressed in endothelial and smooth muscle cells of the rat cerebral arteries. These results suggest that specific inhibition of AT1Rs in the cerebral circulation causes the leftward shift of the lower limit of autoregulation.

Nocturnal blood pressure decrease is associated with increased regional cerebral blood flow in patients with a history of ischemic stroke

BACKGROUND

It remains controversial whether there is a J-shaped relationship between blood pressure and recurrent stroke among patients with a recent history of ischemic stroke.

OBJECTIVE

To investigate the relationship between regional cerebral blood flow (rCBF) and the dipping of nocturnal blood pressure in patients receiving antihypertensive treatment after ischemic stroke.

METHODS AND RESULTS

Forty-seven patients with a previous history of ischemic stroke and 37 patients with a history of non-ischemic stroke underwent 24-h ambulatory blood pressure monitoring and rCBF measurement with single photon emission computed tomography. Of the 47 patients with ischemic stroke, 30 were diagnosed as having suffered atheromatous or embolic stroke, and 37 had an ischemic lesion in the territory of the carotid artery. Systolic and diastolic blood pressures during daytime and night-time were controlled at less than 140/90 mmHg by a low-salt diet or long-acting antihypertensive agents, or both. In patients with ischemic stroke, there were significant negative correlations between the percentage change in nocturnal blood pressure and rCBFs in the thalamus (r = -0.33, P = 0.02), putamen (r = -0.34, P = 0.02) and cerebral cortex (r = -0.31, P = 0.03). Multivariate analysis revealed that only the percentage change in nocturnal blood pressure was related to rCBF. There was a significant positive correlation between rCBFs in the thalamus and the cerebral cortex (r = 0.74, P < 0.05). In patients with non-ischemic stroke, there was no significant correlation between the percentage change in nocturnal blood pressure and rCBFs.

CONCLUSIONS

These findings indicate that the decrease in nocturnal blood pressure is associated with the increase in rCBF in patients with a history of ischemic stroke in the territory of the carotid artery.

Protection of the cardiovascular system by imidapril, a versatile angiotensin-converting enzyme inhibitor

Imidapril hydrochloride (imidapril) is a long-acting, non-sulfhydryl angiotensin-converting enzyme (ACE) inhibitor, which has been used clinically in the treatment of hypertension, chronic congestive heart failure (CHF), acute myocardial infarction (AMI), and diabetic nephropathy. It has the unique advantage over other ACE inhibitors in causing a lower incidence of dry cough. After oral administration, imidapril is rapidly converted in the liver to its active metabolite imidaprilat. The plasma levels of imidaprilat gradually increase in proportion to the dose, and decline slowly. The time to reach the maximum plasma concentration (T(max)) is 2.0 h for imidapril and 9.3 h for imidaprilat. The elimination half-lives (t(1/2)) of imidapril and imidaprilat is 1.7 and 14.8 h, respectively. Imidapril and its metabolites are excreted chiefly in the urine. As an ACE inhibitor, imidaprilat is as potent as enalaprilat, an active metabolite of enalapril, and about twice as potent as captopril. In patients with hypertension, blood pressure was still decreased at 24 h after imidapril administration. The antihypertensive effect of imidapril was dose-dependent. The maximal reduction of blood pressure and plasma ACE was achieved with imidapril, 10 mg once daily, and the additional effect was not prominent with higher doses. When administered to patients with AMI, imidapril improved left ventricular ejection fraction and reduced plasma brain natriuretic peptide (BNP) levels. In patients with mild-to-moderate CHF [New York Heart Association (NYHA) functional class II-III], imidapril increased exercise time and physical working capacity and decreased plasma atrial natriuretic peptide (ANP) and BNP levels in a dose-related manner. In patients with diabetic nephropathy, imidapril decreased urinary albumin excretion. Interestingly, imidapril improved asymptomatic dysphagia in patients with a history of stroke. In the same patients it increased serum substance P levels, while the angiotensin II receptor antagonist losartan was ineffective. These studies indicate that imidapril is a versatile ACE inhibitor. In addition to its effectiveness in the treatment of hypertension, CHF, and AMI, imidapril has beneficial effects in the treatment of diabetic nephropathy and asymptomatic dysphagia. Good tissue penetration and inhibition of tissue ACE by imidapril contributes to its effectiveness in preventing cardiovascular complications of hypertension. The major advantages of imidapril are its activity in the treatment of various cardiovascular diseases and lower incidence of cough compared with some of the older ACE inhibitors.

Bradykinin mediates the acute effect of an angiotensin-converting enzyme inhibitor on cerebral autoregulation in rats

BACKGROUND AND PURPOSE

In patients with stroke and long-standing hypertension, the autoregulation curve of cerebral blood flow (CBF) shifts toward higher blood pressure levels. Angiotensin-converting enzyme (ACE) inhibitors reduce blood pressure and shift the autoregulation curve back to normal in hypertensive patients. ACE inhibitors have 2 major pharmacological properties: they inhibit both the production of angiotensin II and the breakdown of kinins. Hence, we investigated whether the effect of an ACE inhibitor on the lower limit of CBF autoregulation is mediated by the potentiation of bradykinin-mediated vasodilatation.

METHODS

In 28 male Sprague-Dawley rats, CBF was measured by laser-Doppler flowmetry during stepwise controlled hypotension. The lower limit of CBF autoregulation was defined as the mean arterial pressure at which CBF decreased by 20% of the baseline value. The rats were treated with an ACE inhibitor, captopril, in the captopril group; a bradykinin BK2-receptor antagonist, Hoe140, in the Hoe140 group; and both agents in the captopril+Hoe140 group. Other rats served as a control group. The lower limits of CBF autoregulation were compared among the 4 groups.

RESULTS

In the captopril group, the lower limit of CBF autoregulation was 43+/-8 mm Hg (mean+/-SD), which was significantly lower than that in the control group (57+/-14 mm Hg). Inhibition of bradykinin abolished the effect of captopril on the lower limit of CBF autoregulation. Hoe140 alone had no significant effect on the lower limit of CBF autoregulation.

CONCLUSIONS

These results suggest that the shift of the lower limit of CBF autoregulation by captopril is mediated, at least in part, by bradykinin.