Pressure-dependent Constriction Research Articles

Membrane depolarization is required for pressure-dependent pulmonary arterial tone but not enhanced vasoconstriction to endothelin-1 following chronic hypoxia.

Enhanced vasoconstriction is increasingly identified as an important contributor to the development of pulmonary hypertension. Chronic hypoxia results in enhanced Rho kinase mediated Ca2+ sensitization contributing to pressure-dependent pulmonary arterial tone as well as augmented vasoconstriction to endothelin-1 and depolarizing stimuli. We sought to investigate the interaction between these vasoconstrictor stimuli in isolated, pressurized, pulmonary arteries. We used the K+ ionophore, valinomycin, to clamp membrane potential (Vm) to investigate the role of membrane depolarization in endothelin-1 and pressure-dependent constriction, and endothelin-1 receptor inhibitors to determine whether membrane depolarization or stretch signal through endothelin-1 receptors. Clamping Vm prevented pressure-dependent tone, but not enhanced vasoconstriction to endothelin-1 following chronic hypoxia. Furthermore, endothelin-1 receptor inhibition had no effect on either pressure-dependent tone or vasoconstriction to KCl. As Src kinases contribute to both pressure-dependent tone and enhanced endothelin-1 vasoconstriction following chronic hypoxia, we further investigated their role in depolarization-induced vasoconstriction. Inhibition of Src kinases attenuated enhanced vasoconstriction to KCl. We conclude that membrane depolarization contributes to pressure-dependent tone but not enhanced vasoconstriction to ET-1, and that Src kinases serve as upstream mediators facilitating enhanced Rho kinase-dependent vasoconstriction following chronic hypoxia.

Partial loss of Ca2+‐Activated K+ channel (KCa) in Rat Cerebral Arterial Muscle in Insulin‐dependent diabetes mellitus

Cerebral arteries depolarize and constrict upon pressurization. This is one of the intrinsic mechanisms underlying autoregulation of cerebral blood flow (CBF). The Ca2+‐activated K+ channel (KCa) channel in arterial muscle is an important target through which cerebral arterial response to changes in endogenous or environmental stimulatory factors are sensed. The level of open state probability (NPo) of KCa channels determines the degree of myogenic tone and cytosolic Ca2+ concentration. In the present study we report impaired pressure‐dependent constriction of cerebral arteries and diminished density and NPo of KCa channel currents associated with reduced expression of KCa channel α‐ and β‐subunit proteins in cerebral arteries of genetically insulin‐dependent diabetic rats. In this study we also found that the expression of the CYP 4A ω‐hydroxylase protein and the production of 20‐HETE was significantly increased in the type 1 diabetic cerebral arteries compared to that in the non‐diabetic cerebral arteries. The observed changes in KCa channel function and expression of CYP 4A ω‐hydroxylase and production level of 20‐HETE could represent an important pathologic mechanisms that could underlie abnormalities in cerebral arterial reactivity in insulin‐dependent diabetes mellitus. It is concluded that the diminished KCa channel function, increased production of 20‐HETE and altered pressure‐dependent cerebral arterial constriction in insulin‐dependent diabetes mellitus may lead to impaired autoregulation of CBF that could contribute to increased risk of stroke in this disease condition.Support or Funding InformationNIH RO1 HL0338833This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Alterations to the middle cerebral artery of the hypertensive-arthritic rat model potentiates intracerebral hemorrhage.

AimsWe have recently created an age-dependent hypertensive-mono-arthritic animal model from the stroke-resistant spontaneously hypertensive rat to model populations with autoimmune disease who are hypertensive and are prone to stroke. The model exhibits signs of hemorrhagic stroke (HS) subsequent to chronic inflammation and hypertension. HS is also associated with the inability of middle cerebral arteries to undergo pressure dependent constriction (PDC). We investigated alterations in the cerebrovasculature of our hypertensive mono-arthritic animals that develop stroke.Main MethodsAnimals were fed either a high salt diet (HSD) (4% NaCl) or Purina chow (0.58% NaCl) from weaning. Complete Freund’s Adjuvant (CFA) was injected into the left hind paw at 21–28 weeks; controls received saline and histological and functional studies were performed.ResultsBrain damage was more prominent with the high salt, with inflammation exacerbating the damage. High salt alone significantly decreased middle cerebral artery’s (MCA’s) ability to undergo PDC. Inflammation significantly decreased the ability of cerebrovasculature to respond to pressure step in the regular salt diet. The responses to vasoactive peptides were also significantly attenuated in both inflamed groups regardless of diet.ConclusionInduction of chronic systemic inflammation increases brain damage, and affect the MCA’s vasogenic function, decreasing its ability to respond to intraluminal pressure. HSD further exacerbates organ damage associated with chronic inflammation, further compromising cerebrovascular function, and likely increasing the incidence of intracerebral hemorrhage and injury.

Organ culture as an in vitro model for the study of dual‐specificity phosphatase‐5 (DUSP‐5) and myogenic response in rat cerebral arterioles

Increase in intravascular pressure causes depolarization and constriction of cerebral arteries. The molecular mechanism of pressure‐induced myogenic response remains obscure. Our goal was to provide an optimal time window and preserved vascular function for an organ culture of cerebral arteries for the study of mechanisms involved in the regulation of cerebral arterial function. We initiated a comparative study of cultured and freshly isolated cerebral arteries in response to elevated intravascular pressure, vasodilator and vasoconstrictor stimuli, activity of Ca2+‐activated K+ (KCa) single‐channel current and protein expression of CYP 4 and DUSP‐5 enzymes. The results indicate that 72 h time frame of organ culture of cerebral arteries maintains cellular integrity, pressure dependent constriction, CYP4A2 and 4F protein expression, endothelium‐dependent and independent dilations, KCa channel current activity and vasoconstrictor response to serotonin. Furthermore, using a permeabilization, transfection technique the 72 h cultured arteries elicited a gene specific siRNA induced knockdown of DUSP‐5 protein expression. Subsequent pressure myograph studies and inhibitor studies revealed role of DUSP‐5 in regulation of myogenic tone. We propose a possible mechanism by which DUSP‐5 and ERK regulate myogenic response in cerebral arterioles. RO1 HL033833, RO1 HL092105, T32 HL007792.

Corrigendum

CorrigendumCorrigendumPublished Online:01 May 2011https://doi.org/10.1152/ajpheart.zh4-9876-corr.2011Original articleMoreSectionsPDF (316 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Volume 300, March 2011Volume 69, March 2011Pages H803–H812: Nystoriak MA, O'Connor KP, Sonkusare SK, Brayden JE, Nelson MT, Wellman GC. Fundamental increase in pressure-dependent constriction of brain parenchymal arterioles from subarachnoid hemorrhage model rats due to membrane depolarization. Am J Physiol Heart Circ Physiol 300: , 2011. First Published December 10, 2010; doi:10.1152/ajpheart.0760.2010 (http://ajpheart.physiology.org/cgi/content/full/300/3/H803).—There was an error in Fig. 1D, in the scale of the x-axis. The corrected figure appears below.Download figureDownload PowerPointThis article has no references to display. Download PDF Previous Back to Top FiguresReferencesRelatedInformation Related ArticlesFundamental increase in pressure-dependent constriction of brain parenchymal arterioles from subarachnoid hemorrhage model rats due to membrane depolarization 01 Mar 2011American Journal of Physiology-Heart and Circulatory Physiology More from this issue > Volume 300Issue 5May 2011Pages H1971-H1971 Copyright & PermissionsCopyright © 2011 the American Physiological Societyhttps://doi.org/10.1152/ajpheart.zh4-9876-corr.2011History Published online 1 May 2011 Published in print 1 May 2011 Metrics

Fundamental increase in pressure-dependent constriction of brain parenchymal arterioles from subarachnoid hemorrhage model rats due to membrane depolarization.

Intracerebral (parenchymal) arterioles are morphologically and physiologically unique compared with pial arteries and arterioles. The ability of subarachnoid hemorrhage (SAH) to induce vasospasm in large-diameter pial arteries has been extensively studied, although the contribution of this phenomenon to patient outcome is controversial. Currently, little is known regarding the impact of SAH on parenchymal arterioles, which are critical for regulation of local and global cerebral blood flow. Here diameter, smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)), and membrane potential measurements were used to assess the function of intact brain parenchymal arterioles isolated from unoperated (control), sham-operated, and SAH model rats. At low intravascular pressure (5 mmHg), membrane potential and [Ca(2+)](i) were not different in arterioles from control, sham-operated, and SAH animals. However, raising intravascular pressure caused significantly greater membrane potential depolarization, elevation in [Ca(2+)](i), and constriction in SAH arterioles. This SAH-induced increase in [Ca(2+)](i) and tone occurred in the absence of the vascular endothelium and was abolished by the L-type voltage-dependent calcium channel (VDCC) inhibitor nimodipine. Arteriolar [Ca(2+)](i) and tone were not different between groups when smooth muscle membrane potential was adjusted to the same value. Protein and mRNA levels of the L-type VDCC Ca(V)1.2 were similar in parenchymal arterioles isolated from control and SAH animals, suggesting that SAH did not cause VDCC upregulation. We conclude that enhanced parenchymal arteriolar tone after SAH is driven by smooth muscle membrane potential depolarization, leading to increased L-type VDCC-mediated Ca(2+) influx.

The Effects of Poststroke Captopril and Losartan Treatment on Cerebral Blood Flow Autoregulation in SHRsp with Hemorrhagic Stroke

The ability of captopril and losartan treatment to restore cerebral blood flow (CBF) autoregulation after intracerebral hemorrhagic stroke (HS) was assessed in Kyoto-Wistar stroke-prone hypertensive rats (SHRsp). Laser Doppler techniques assessed CBF autoregulation in the middle cerebral artery (MCA) perfusion domain and a pressure myograph was used to measure pressure-dependent constriction (PDC) in isolated MCAs before and after stroke and after 13, 33, and 63 days of poststroke captopril or losartan treatment. The treatments did not lower blood pressure (BP) and equally suppressed plasma aldosterone after HS. The HS development was associated with the loss of CBF autoregulation, high CBF, increased CBF conductance to elevations in BP, and the loss of PDC in the MCAs. Both treatments restored these functions to prestroke levels within 13 days. The PDC and CBF autoregulation subsequently deteriorated after 63 days of captopril treatment while being maintained at prestroke levels over all durations of losartan treatment. The SHRsp subjected to 35 days of poststroke losartan treatment exhibited less blood-brain barrier (BBB) disruption and brain herniation than captopril-treated SHRsp. The superior ability of losartan to restore CBF autoregulation and myogenic function may have contributed to the more effective attenuation of cerebral damage after HS.

Alterations in the modulation of cerebrovascular tone and blood flow by nitric oxide synthases in SHRsp with stroke

The modulation of myogenic function and cerebral blood flow (CBF) by nitric oxide (NO) synthases (NOS) was assessed in the middle cerebral arteries (MCAs) of Kyoto Wistar stroke prone hypertensive rats (SHRsp) in relation to haemorrhagic stroke development. MCAs were studied with a pressure myograph. CBF in MCA perfusion domain was measured using laser Doppler techniques. NOS isozymes were identified using immunohistochemistry. MCAs expressed endothelial, neuronal, and inducible NOS (eNOS, nNOS, and iNOS, respectively) in the endothelium, nNOS and traces of iNOS in smooth muscle and adventitial cells. Before stroke, MCA pressure-dependent constriction (PDC) was superimposed over basal non-pressure-dependent tone (BNPDT). Endothelial NO generation and non-endothelial nNOS but not iNOS reduced BNPDT and increased the lumen diameter at which PDC initiated without altering the amplitude of PDC. NOS inhibition decreased CBF and increased the upper blood pressure limit of autoregulation. PDC, CBF autoregulation, and NOS dilatory influence were lost, and BNPDT was increased in MCAs from SHRsp with stroke. The expression of NOS isozymes and MCA reactivity to NO donors was not altered. NOS activity was not recovered by in vitro l-arginine or tetrahydrobiopterin supplementation, l-arginase inhibition or superoxide scavengers. The loss of PDC and CBF autoregulation during hypertension may facilitate over-perfusion and cerebral haemorrhage formation in SHRsp. NOS dysfunction in MCAs preceded stroke and involved the inactivation of eNOS and nNOS in areas not subjected to hyper-distension. The elevation in BNPDT due to NOS inactivation may oppose over-perfusion in the absence of CBF autoregulation.

Stroke related alterations in the modulation of cerebrovascular pressure dependent constriction (PDC) by nitric oxide synthases (NOS) in stroke prone spontaneously hypertensive rats (SHRsp)

We studied the modulation of PDC by NOS in middle cerebral arteries (MCAs) of SHRsp in relation to hemorrhagic stroke (HS) using a pressure myograph. Confocal microscopy indicated endothelial and neuronal NOS (eNOS, nNOS) in the endotheliumn, nNOS and inducible NOS (iNOS) in the smooth muscle and nNOS in the adventitia of MCAs. In MCAs from prestroke SHRsp, endothelial removal, or the nonspecific NOS inhibitor Nω‐nitro‐L‐arginine methyl ester (LNAME,100µM) shifted PDC to a 100 mmHg pressure step to constricted levels. Endothelial removal did not alter PDC in the presence of LNAME. In de‐endothelialized MCAs, the selective nNOS inhibitor AAAN (10μM, n‐[(4S)‐4‐amino‐5‐[(2‐aminoetheyl)amino]pentyl]‐N‐nitroguanidine) also shifted PDC to more constricted levels. The effects of AAAN could be duplicated but not amplified by LNAME or the NO scavenger hydroxocobalamine (200μM). The iNOS inhibitor 1400W (1.13μM) did not alter tone or PDC. We believe NO generated by endothelial eNOS (and perhaps nNOS) and nonendothelial nNOS (but not iNOS) shifts the operating range of PDC to more dilated diameters in MCAs from prestroke SHRsp. After HS, MCAs from SHRsp could not elicit PDC and were unreactive to LNAME. Reactivity to the NO donor Na‐nitroprusside was unaltered. The MCAs exhibited the presence of eNOS and nNOS, however the latter enzymes appeared to lack the ability to generate NO.

The restoration of cerebrovascular myogenic function after hemorrhagic stroke development in stroke prone hypertensive rats

Hemorrhagic stroke (HS) development in Kyoto Wistar stroke prone hypertensive rats (SHRsp) is associated with the loss of pressure dependent constriction (PDC) in middle cerebral arteries (MCAs). We attempted to restore PDC by administering oral Captopril (CAP, 50 mg/kg/day) or Losartan (LOS, 35 mg/kg/day) treatment after stroke. MCAs were studied using a pressure myograph. MCAs from SHRsp with HS did not elicit constriction to a 100 mmHg pressure step or in response to PKC activation (phorbol dibutyrate 1μM, with 3μM nifedipine). Elevations in [K+]o (80mM) produced depolarization with attenuated constriction. The MCAs from post stroke SHRsp also could not utilize the sarcoplasmic release of Ca2+ to produce constraction (vasopressin 0.12 μM, with 3μM nifedipine). Post stroke CAP or LOS treatment restored these functions to pre‐stroke levels within 7 days without altering blood pressure. These functions deteriorated after 30 days of CAP treatment but were maintained past 100 days of LOS treatment. We believe that the loss of PDC after HS is produced by a dysfunctional PKC system and the presence of voltage gated Ca2+ channel that exhibits an attenuated response to depolarization. The enhanced ability of LOS to restore these functions after HS may be related to the physiological benefits of maintaining high plasma angiotensin II in combination with AT‐1 receptor blockade.

The importance of aldosterone suppression in the restoration of cerebrovascular myogenic function in SHRsp during post stroke Captopril and Losartan treatment

Treatment of Kyoto Wistar stroke prone spontaneously hypertensive rats (SHRsp) with Captopril (CAP) or Losartan (LOS) after hemorrhagic stroke (HS) restores pressure dependent constriction (PDC) in middle cerebral arteries (MCAs) . We assessed the importance of aldosterone (ALD) suppression during both treatments. Plasma ALD was elevated in SHRsp with HS over prestroke levels (3.9 ± 0.8 nM vs 0.15 ± 0.09 nM). SHRsp with HS were treated orally with 50mg/kg/day CAP or 35 mg/kg/day LOS for 28 days. During the last 10 days of treatment ALD (0.66 μg/hr) or vehicle was infused via an osmotic pump . CAP and LOS treatment suppressed plasma ALD (0.23 ± 0.07 and 0.075 ± 0.044 nM respectively), whereas supplementation elevated ALD (6.27±1.75 and 4.49 ± 0.80 nM respectively). ALD supplementation did not alter BP. After 28 days of CAP or LOS treatment, MCAs from SHRsp with HS (studied using a pressure myograph) regained PDC and the ability to constrict in response to protein kinase C activation (1μM phorbol dibutyrate) and elevated [K+]o depolarization. ALD supplementation prevented these changes in CAP but not LOS treated SHRsp. Elevation in plasma ALD enhance the deterioration of myogenic function. We believe that poststroke treatment with LOS (vs CAP) promotes systemic beneficial effects that permit the restoration of cerebrovascular function even under conditions of where plasma ALD is elevated.

Effects of Poststroke Losartan Versus Captopril Treatment on Myogenic and Endothelial Function in the Cerebrovasculature of SHRsp

We assessed the ability of poststroke captopril and losartan treatment to reverse myogenic and endothelial dysfunction in the middle cerebral arteries of Kyoto-Wistar stroke-prone spontaneously hypertensive rats (SHRsp) that developed intracerebral hemorrhagic stroke. SHRsp were sampled before and after stroke development and after up to 37 days of captopril (50 mg/kg per day) or losartan (35 mg/kg per day) treatment initiated after stroke. Pressure-dependent constriction to a 100-mm Hg pressure step, constriction to nitric oxide synthase inhibition (100 micromol/L N(omega)-nitro-l-arginine methyl ester), and endothelium-dependent vasodilation to bradykinin (1.6 micromol/L), 2-f-LIGRLO-NH(2) (1 micromol/L, a protease-activated receptor-2 agonist), and A23187 (2 micromol/L) were evaluated in middle cerebral arteries at 100 mm Hg with a pressure myograph. Middle cerebral arteries from SHRsp with stroke could not constrict to pressure or nitric oxide synthase inhibition, lacked the ability to vasodilate to bradykinin, and exhibited attenuated dilation and vasomotion in response to A23187. Vasodilation to 2-f-LIGRLO-NH(2) was unaltered. The aforementioned cerebrovascular alterations were reversed after 31 days of poststroke losartan but not of captopril treatment in the absence of an antihypertensive effect. Captopril treatment restored middle cerebral artery constriction to pressure, NOS inhibition, and bradykinin vasodilation temporarily after 7 to 18 days of treatment, after which function deteriorated to a level observed in SHRsp at stroke. Aspects of poststroke cerebrovascular dysfunction, which likely play an important role in altering and modulating cerebral blood flow autoregulation, can be reversed in SHRsp more effectively after stroke development by blocking angiotensin II type 1 receptors as opposed to lowering angiotensin II levels.

Stroke development in stroke-prone spontaneously hypertensive rats alters the ability of cerebrovascular muscle to utilize internal Ca2+ to elicit constriction.

The ability of middle cerebral arteries (MCAs) to utilize intracellular smooth muscle (SM) Ca2+ to produce constriction in response to pressure and agonists was assessed in relation to hemorrhagic stroke development in Wistar-Kyoto stroke-prone (SHRSP) and stroke-resistant (srSHR) spontaneously hypertensive rats. MCAs were studied with the use of a pressure myograph at 100 mm Hg. MCAs from srSHR and prestroke SHRSP exhibited pressure-dependent constriction and constricted in response to vasopressin or serotonin in the presence of nifedipine or the absence of [Ca2+]o. MCAs from poststroke SHRSP lost the latter functions and could only constrict in response to vasopressin/serotonin in Krebs' solution containing Ca2+ in the absence of nifedipine. This indicated that the SM could not utilize internal Ca2+ for constriction and maintained constriction by Ca2+ entry through L-type channels. The MCAs of poststroke SHRSP could not constrict to [K+]o-induced depolarization, suggesting that the agonist-induced opening of the L-type channels occurred by mechanisms other than SM depolarization. Depletion of the sarcoplasmic SM Ca2+ stores of MCAs from srSHR with cyclopiazonic acid did not prevent pressure-dependent constriction. Stroke in SHRSP produced a defect in the ability of MCAs to constrict in response to vasopressin or serotonin via the use of an intracellular source of Ca2+. This could be promoted by an inability of the SM to release intracellular Ca2+, by the depletion of internal Ca2+ stores, or by a decrease in the contractile sensitivity to Ca2+ released from the internal stores.

Alterations in autoregulatory and myogenic function in the cerebrovasculature of Dahl salt-sensitive rats.

Dahl salt-sensitive rats fed an 8.7% NaCl diet exhibited hypertensive encephalopathy and developed seizures associated with areas of blood-brain barrier (BBB) disruption without brain ischemia. The incidence of hemorrhagic stroke was low (7/47). We tested the hypothesis that a defect in cerebral blood flow (CBF) autoregulation under hypertensive conditions preceded hypertensive encephalopathy. Brain ischemia and BBB disruption were assessed with the use of tetrazolium red staining and Evans blue dye extravasation, respectively. Myogenic constriction to pressure was measured in isolated middle cerebral arteries (MCAs) with a pressure myograph. CBF autoregulation was assessed with the use of laser-Doppler techniques. Asymptomatic rats fed 8.7% NaCl had MCAs that developed an age-related attenuation in their ability to constrict to pressure, which was amplified in rats exhibiting hypertensive encephalopathy. The MCAs of rats with hemorrhagic stroke lost this function and developed large degrees of basal tone. The majority (4/6) of asymptomatic rats fed high salt for longer than 3 weeks exhibited a linear relationship between CBF and blood pressure. The characteristics of CBF regulation were consistent with the possible absence of autoregulation coupled with cerebrovascular vasoconstriction. Both MCA pressure-dependent constriction and CBF autoregulation in the MCA perfusion domain were lost before the development of hypertensive encephalopathy or hemorrhagic stroke. These defects could contribute to the development of BBB disruption during hypertension. Cerebrovascular vasoconstriction in the absence of CBF autoregulation may protect the brain from excessive overperfusion during hypertension and could account for the low incidence of cerebral hemorrhage in this model.

Electromechanical alterations in the cerebrovasculature of stroke-prone rats.

Cerebrovascular pressure-dependent constriction (PDC) is associated with smooth muscle (SM) depolarization and Ca(2+) influx through voltage-gated channels. We studied the alterations in electromechanical contraction in the middle cerebral arteries (MCAs) of stroke-prone Wistar-Kyoto spontaneously hypertensive rats (SHRsp) in relation to the stroke-related loss of PDC. Constriction to pressure, elevated [K(+)](o) and/or [Ca(2+)](o), and SM membrane potentials (E(m)) were measured in isolated pressurized MCAs of SHRsp and stroke-resistant SHR. MCAs of SHRsp exhibited an age-related decrease in PDC before hemorrhagic stroke and a loss of PDC after stroke. At 100 mm Hg, the MCAs of poststroke SHRsp maintained partial constriction that was not altered with pressure but was inhibited by nifedipine (1 micromol/L). The MCAs of poststroke SHRsp constricted to vasopressin (0.17 micromol/L) but not to elevated [K(+)](o). When pressure was reduced from 100 to 0 mm Hg, the MCAs from young prestroke SHRsp exhibited SM hyperpolarization (-38 to -46 mV), whereas those of poststroke SHRsp maintained a constant, depolarized E(m) (-34 mV). Alterations in E(m) with varying [K(+)](o) suggested that there was a decrease in SM K(+) conductance in the MCAs of poststroke SHRsp. The observation that the MCAs of poststroke SHRsp depolarize but do not constrict to elevated [K(+)](o) suggests the presence of dysfunctional voltage-gated Ca(2+) channels. The inability to alter E(m) with pressure or to constrict to depolarization could partially contribute to the loss of PDC in the MCAs of poststroke SHRsp.

Stroke-prone spontaneously hypertensive rats lose their ability to auto-regulate cerebral blood flow prior to stroke.

We hypothesized that the loss of cerebral blood flow (CBF) auto-regulation under hypertensive conditions could promote cerebrovascular over-perfusion and haemorrhage formation. The possibility that CBF auto-regulation becomes defective prior to haemorrhagic stroke development was assessed in Wistar- Kyoto stroke-prone spontaneously hypertensive rats (SHRsp) and related to the myogenic responsiveness of the cerebrovasculature to pressure. Laser Doppler techniques were used to measure relative CBF in relation to mean arterial pressure (MAP 130-260 mmHg) within the perfusion domains of the middle (MCA) and posterior (PCA) cerebral arteries. The ability of isolated MCAs and PCAs to constrict to a 120 mmHg pressure step (pressure-dependent constriction) was measured using a pressure myograph. Two weeks prior to stroke, 10-week-old pre-stroke SHRsp exhibited near-constant CBF regulation to a 200 mmHg MAP. Thirteen-week-old pre-stroke SHRsp and age-matched post-stroke SHRsp lost their ability to auto-regulate CBF in the MCA and PCA perfusion domains. CBF increased at a high rate and in a linear manner with MAP. A distinct upper limit to CBF auto-regulation was absent. Pressure-dependent constriction was attenuated prior to stroke, and lost after stroke in isolated MCAs, but not the PCAs, of SHRsp. The loss of CBF auto-regulation prior to stroke in SHRsp could enhance cerebral perfusion and facilitate the initiation of haemorrhage. Such dysfunction after stroke could produce secondary haemorrhages. Defects in pressure-dependent constriction cannot fully account for the pattern of CBF auto-regulation loss observed in post-stroke SHRsp.

Cerebrovascular alterations in protein kinase C-mediated constriction in stroke-prone rats.

Cerebrovascular pressure-dependent constriction may involve the smooth muscle production of diacylglycerol, which could facilitate constriction by activating protein kinase C (PKC). A dysfunctional PKC system could promote the loss of pressure-dependent constriction. We attempted to determine whether the alterations in pressure-dependent constriction in the middle cerebral arteries (MCAs) observed in relation to stroke development in Wistar-Kyoto stroke-prone spontaneously hypertensive rats (SHRsp) were associated with defects in the ability of the arteries to constrict in response to PKC activation. MCAs were sampled from SHRsp before and after stroke development and in stroke-resistant Wistar-Kyoto spontaneously hypertensive rats. A pressure myograph was used to test the ability of the arteries to constrict in response to a 100 mm Hg pressure step and subsequently to contract in response to phorbol 12,13-dibutyrate in the presence of nifedipine (3 micromol/L). Pressure-dependent constriction and constriction in response to phorbol dibutyrate in the MCAs were inhibited by PKC inhibitors (staurosporine [40 nmol/L], chelerythrine [12 micromol/L], bisindolylmaleimide [5 micromol/L]), declined with age before stroke development in SHRsp, and were absent after stroke. There was a significant relationship between pressure- and phorbol dibutyrate-induced constriction (r=0.815, P<0. 05). Phorbol esters interact with the same activation site as diacylglycerol to stimulate PKC. An inability to constrict in response to phorbol dibutyrate may reflect unresponsiveness to diacylglycerol and may contribute to the loss of pressure-dependent constriction associated with stroke in the MCAs of SHRsp. The loss of this autoregulatory function before stroke could increase the risk of cerebral hemorrhage.

Electrical conduction within the cerebrovasculature of stroke-prone spontaneously hypertensive rats

Alterations in electrical conductivity between smooth muscle cells (SMCs) can alter the spread and effectiveness of electromechanical SMC contraction. We attempted to determine whether alterations in pressure-dependent constriction (PDC) occurring in relation to stroke development within the middle cerebral arteries (MCAs) of Wistar-Kyoto stroke-prone hypertensive rats (SHRsp) were associated with changes in electrical conductivity between the SMCs. Current was injected into nonpressurized MCAs, using a suction electrode. The conducting distance along the length of the MCA where the amplitude of the membrane potential deflection (electronic potential) produced by current injection declined to 1/e (length constant) was used to measure conductivity. PDC to a 100 mmHg pressure step was measured with a pressure myograph. A loss of PDC in the MCAs of SHRsp preceded stroke development. Heptanol (4 mM), a gap junction communication inhibitor, reversibly inhibited conductivity and PDC in the MCA of prestroke SHRsp. The ability of heptanol to reversibly inhibit PDC was likely not related to it's ability to alter electrical conduction. The length constant of electrical conduction in the MCAs was about 0.75 mm and didn't differ between MCA sampled from pre-versus post-stroke SHRsp or Sprague-Dawley rats. It was concluded that alterations in electrical conductivity along the MCA could modify the spread of PDC, but such changes do not contribute to the loss of PDC within the MCA of poststroke SHRsp.

Prevention of stroke and preservation of the functions of cerebral arteries by treatment with perindopril in stroke-prone spontaneously hypertensive rats

The aim of this study was to determine whether the prevention of stroke with perindopril treatment in stroke-prone spontaneously hypertensive rats (SHRSP) is associated with the preservation of the myogenic properties of the cerebral arteries. After weaning at 4 weeks of age, male SHRSP were fed a Japanese-style rat diet with high salt to induce stroke development. Treatment with perindopril was given by gavage every morning beginning at 6 weeks of age. There were three experimental groups: two groups treated with 4 mg.kg-1.day-1 perindopril for different durations (8 or 12 weeks) and one control group consisting of littermates given distilled water. All the control animals developed stroke and died within 14 weeks of age, and myogenic response of the middle cerebral arteries (MCA) to pressure increase was lost in these animals. In contrast, all the treated SHRSP survived during the treatment period, and myogenic response of the MCA was preserved. After withdrawal of the treatment, SHRSP treated for a longer period (12 weeks) also survived longer than those treated for a shorter period (8 weeks). The subsequent development of stroke and death following treatment withdrawal after 8 or 12 weeks of treatment was associated with the loss of pressure-dependent constriction in MCA. A longer treatment duration also increased the stiffness of the MCA, MCA from SHRSP after 12 weeks of treatment had smaller external and lumen diameters, and thicker walls than those from the 8-week treatment group. In a separate study, we found that treatment of SHRSP with 1 or 4 mg.kg-1.day-1 of perindopril for 24 weeks beginning at 6 weeks of age also protected them against death related to stroke, because these rats survived up to 43 weeks of age, when the experiment was terminated. We conclude that there is an association between the absence of myogenic response in cerebral arteries and stroke development in SHRSP. Perindopril treatment preserves the myogenic response of MCA in SHRSP and prevents the stroke development in these animals. A prolonged treatment could increase the survival of SHRSP through a remodelling of the MCA and increasing the stiffness of the cerebral arteries.

Nitric oxide modulates but does not impair myogenic vasoconstriction of the afferent arteriole in spontaneously hypertensive rats. Studies in the isolated perfused hydronephrotic kidney.

Renal autoregulation curves are reset toward higher renal arterial pressure in spontaneously hypertensive rats (SHR) compared with those in Wistar-Kyoto rats (WKY). We previously demonstrated that myogenic afferent arteriolar constriction is shifted to higher renal arterial pressure. To investigate whether nitric oxide participates in the regulation of myogenic tone, we examined the effect of nitro-L-arginine on myogenic afferent arteriolar constriction in kidneys from SHR and WKY, using the isolated perfused hydronephrotic kidney. Elevating pressures from 40 to 80 mm Hg caused increases in afferent arteriolar diameter in WKY (from 18.2 +/- 0.4 to 19.0 +/- 0.3 micron) and SHR (from 17.3 +/- 0.6 to 18.4 +/- 0.6 micron). Further pressure elevation elicited constriction at 100 mm Hg in WKY (17.9 +/- 0.3 micron), but significant constriction was observed at 120 mm Hg in SHR (17.3 +/- 0.6 micron), indicating a resetting in myogenic responses to higher pressures. In WKY, after treatment with 10 mumol/L nitro-L-arginine, afferent arterioles exhibited pressure-dependent constriction, with a threshold pressure for constriction at 80 mm Hg. The addition of 100 mumol/L nitro-L-arginine had no further effect on myogenic responsiveness in WKY. In contrast, in SHR, nitro-L-arginine dose-dependently shifted the myogenic responses toward lower renal arterial pressure, with threshold pressures for constriction observed at 100 mm Hg (10 mumol/L) and 80 mm Hg (100 mumol/L).(ABSTRACT TRUNCATED AT 250 WORDS)