Cerebrovascular Contractility Research Articles

Role of TRP ion channels in cerebral circulation and neurovascular communication

The dynamic regulation of blood flow is essential for meeting the high metabolic demands of the brain and maintaining brain function. Cerebral blood flow is regulated primarily by 1) the intrinsic mechanisms that determine vascular contractility and 2) signals from neurons and astrocytes that alter vascular contractility. Stimuli from neurons and astrocytes can also initiate a signaling cascade in the brain capillary endothelium to increase regional blood flow. Recent studies provide evidence that TRP channels in endothelial cells, smooth muscle cells, neurons, astrocytes, and perivascular nerves control cerebrovascular contractility and cerebral blood flow. TRP channels exert their functional effects either through cell membrane depolarization or by serving as a Ca2+ influx pathway. Endothelial cells and astrocytes also maintain the integrity of the blood-brain barrier. Both endothelial cells and astrocytes express TRP channels, and an increase in endothelial TRP channel activity has been linked with a disrupted endothelial barrier function. Therefore, TRP channels can play a potentially important role in regulating blood-brain barrier integrity. Here, we review the regulation of cerebrovascular contractility by TRP channels under healthy and disease conditions and their potential roles in maintaining blood-brain barrier function.

BMAL1 Disrupted Intrinsic Diurnal Oscillation in Rat Cerebrovascular Contractility of Simulated Microgravity Rats by Altering Circadian Regulation of miR-103/CaV1.2 Signal Pathway.

The functional and structural adaptations in cerebral arteries could be one of the fundamental causes in the occurrence of orthostatic intolerance after space flight. In addition, emerging studies have found that many cardiovascular functions exhibit circadian rhythm. Several lines of evidence suggest that space flight might increase an astronaut’s cardiovascular risks by disrupting circadian rhythm. However, it remains unknown whether microgravity disrupts the diurnal variation in vascular contractility and whether microgravity impacts on circadian clock system. Sprague-Dawley rats were subjected to 28-day hindlimb-unweighting to simulate the effects of microgravity on vasculature. Cerebrovascular contractility was estimated by investigating vasoconstrictor responsiveness and myogenic tone. The circadian regulation of CaV1.2 channel was determined by recording whole-cell currents, evaluating protein and mRNA expressions. Then the candidate miRNA in relation with Ca2+ signal was screened. Lastly, the underlying pathway involved in circadian regulation of cerebrovascular contractility was determined. The major findings of this study are: (1) The clock gene BMAL1 could induce the expression of miR-103, and in turn modulate the circadian regulation of CaV1.2 channel in rat cerebral arteries at post-transcriptional level; and (2) simulated microgravity disrupted intrinsic diurnal oscillation in rat cerebrovascular contractility by altering circadian regulation of BMAL1/miR-103/CaV1.2 signal pathway.

Berberine alleviates the cerebrovascular contractility in streptozotocin-induced diabetic rats through modulation of intracellular Ca²⁺ handling in smooth muscle cells.

BackgroundVascular dysfunction is a distinctive phenotype in diabetes mellitus. Current treatments mostly focus on the tight glycemic control and few of these treatments have been designed to directly recover the vascular dysfunction in diabetes. As a classical natural medicine, berberine has been explored as a possible therapy for DM. In addition, it is reported that berberine has an extra-protective effect in diabetic vascular dysfunction. However, little is known whether the berberine treatment could ameliorate the smooth muscle contractility independent of a functional endothelium under hyperglycemia. Furthermore, it remains unknown whether berberine affects the arterial contractility by regulating the intracellular Ca2+ handling in vascular smooth cells (VSMCs) under hyperglycemia.MethodsSprague–Dawley rats were used to establish the diabetic model with a high-fat diet plus injections of streptozotocin (STZ). Berberine (50, 100, and 200 mg/kg/day) were intragastrically administered to control and diabetic rats for 8 weeks since the injection of STZ. The intracellular Ca2+ handling of isolated cerebral VSMCs was investigated by recording the whole-cell L-type Ca2+ channel (CaL) currents, assessing the protein expressions of CaL channel, and measuring the intracellular Ca2+ in response to caffeine. Our results showed that chronic administration of 100 mg/kg/day berberine not only reduced glucose levels, but also inhibited the augmented contractile function of cerebral artery to KCl and 5-hydroxytryptamine (5-HT) in diabetic rats. Furthermore, chronic administration of 100 mg/kg/day berberine significantly inhibited the CaL channel current densities, reduced the α1C-subunit expressions of CaL channel, decreased the resting intracellular Ca2+ ([Ca2+]i) level, and suppressed the Ca2+ releases from RyRs in cerebral VSMCs isolated from diabetic rats. Correspondingly, acute application of 10 μM berberine could directly inhibit the hyperglycemia-induced CaL currents and suppress the hyperglycemia-induced Ca2+ releases from RyRs in cerebral VSMCs isolated from normal control rats.ConclusionsOur study indicated that berberine alleviated the cerebral arterial contractility in the rat model of streptozotocin-induced diabetes via regulating the intracellular Ca2+ handling of smooth muscle cells.

Regulatory mechanism of endothelin receptor B in the cerebral arteries after focal cerebral ischemia.

Background and PurposeIncreased expression of endothelin receptor type B (ETBR), a vasoactive receptor, has recently been implied in the reduced cerebral blood flow and exacerbated neuronal damage after ischemia-reperfusion (I/R). The study explores the regulatory mechanisms of ETBR to identify drug targets to restore normal cerebral artery contractile function as part of successful neuroprotective therapy.MethodsWe have employed in vitro methods on human and rat cerebral arteries to study the regulatory mechanisms and the efficacy of target selective inhibitor, Mithramycin A (MitA), to block the ETBR mediated contractile properties. Later, middle cerebral artery occluded (MCAO) rats were used to substantiate the observations. Quantative PCR, immunohistochemistry, western blot and wire myograph methods were employed to study the expression and contractile properties of cerebral arteries.ResultsIncreased expression of specificity protein (Sp1) was observed in human and rat cerebral arteries after organ culture, strongly correlating with the ETBR upregulation. Similar observations were made in MCAO rats. Treatment with MitA, a Sp1 specific inhibitor, significantly downregulated the ETBR mRNA and protein levels. It also significantly reduced the ETBR mediated cerebrovascular contractility. Detailed analysis indicated that ERK1/2 mediated phosphorylation of Sp1 might be essential for ETBR transcription.ConclusionTranscription factor Sp1 regulates the ETBR mediated vasoconstriction in focal cerebral ischemia via MEK-ERK signaling, which is also conserved in humans. The results show that MitA can effectively be used to block ETBR mediated vasoconstriction as a supplement to an existing ischemic stroke therapy.

Early MEK1/2 Inhibition after Global Cerebral Ischemia in Rats Reduces Brain Damage and Improves Outcome by Preventing Delayed Vasoconstrictor Receptor Upregulation

BackgroundGlobal cerebral ischemia following cardiac arrest is associated with increased cerebral vasoconstriction and decreased cerebral blood flow, contributing to delayed neuronal cell death and neurological detriments in affected patients. We hypothesize that upregulation of contractile ETB and 5-HT1B receptors, previously demonstrated in cerebral arteries after experimental global ischemia, are a key mechanism behind insufficient perfusion of the post-ischemic brain, proposing blockade of this receptor upregulation as a novel target for prevention of cerebral hypoperfusion and delayed neuronal cell death after global cerebral ischemia. The aim was to characterize the time-course of receptor upregulation and associated neuronal damage after global ischemia and investigate whether treatment with the MEK1/2 inhibitor U0126 can prevent cerebrovascular receptor upregulation and thereby improve functional outcome after global cerebral ischemia. Incomplete global cerebral ischemia was induced in Wistar rats and the time-course of enhanced contractile responses and the effect of U0126 in cerebral arteries were studied by wire myography and the neuronal cell death by TUNEL. The expression of ETB and 5-HT1B receptors was determined by immunofluorescence.ResultsEnhanced vasoconstriction peaked in fore- and midbrain arteries 3 days after ischemia. Neuronal cell death appeared initially in the hippocampus 3 days after ischemia and gradually increased until 7 days post-ischemia. Treatment with U0126 normalised cerebrovascular ETB and 5-HT1B receptor expression and contractile function, reduced hippocampal cell death and improved survival rate compared to vehicle treated animals.ConclusionsExcessive cerebrovascular expression of contractile ETB and 5-HT1B receptors is a delayed response to global cerebral ischemia peaking 3 days after the insult, which likely contributes to the development of delayed neuronal damage. The enhanced cerebrovascular contractility can be prevented by treatment with the MEK1/2 inhibitor U0126, diminishes neuronal damage and improves survival rate, suggesting MEK1/2 inhibition as a novel strategy for early treatment of neurological consequences following global cerebral ischemia.

Maternal food restriction modulates cerebrovascular structure and contractility in adult rat offspring: effects of metyrapone

Although the effects of prenatal undernutrition on adult cardiovascular health have been well studied, its effects on the cerebrovascular structure and function remain unknown. We used a pair-fed rat model of 50% caloric restriction from day 11 of gestation to term, with ad libitum feeding after birth. We validated that maternal food restriction (MFR) stress is mediated by glucocorticoids by administering metyrapone, a corticosterone synthesis inhibitor, to MFR mothers at day 11 of gestation. At age 8 mo, offspring from Control, MFR, and MFR + Metyrapone groups were killed, and middle cerebral artery (MCA) segments were studied using vessel-bath myography and confocal microscopy. Colocalization of smooth muscle α-actin (SMαA) with nonmuscle (NM), SM1 and SM2 myosin heavy-chain (MHC) isoforms was used to assess smooth muscle phenotype. Our results indicate that artery stiffness and wall thickness were increased, pressure-evoked myogenic reactivity was depressed, and myofilament Ca(2+) sensitivity was decreased in offspring of MFR compared with Control rats. MCA from MFR offspring exhibited a significantly greater SMαA/NM colocalization, suggesting that the smooth muscle cells had been altered toward a noncontractile phenotype. MET significantly reversed the effects of MFR on stiffness but not myogenic reactivity, lowered SMαA/NM colocalization, and increased SMαA/SM2 colocalization. Together, our data suggest that MFR alters cerebrovascular contractility via both glucocorticoid-dependent and glucocorticoid-independent mechanisms.

Crosstalk Between the Angiotensin and Endothelin-System in the Cerebrovasculature

Investigations have shown a multifactorial process as cause for the poor outcome after subarachnoid hemorrhage (SAH), including inflammation, early brain injury, cortical spreading depression, lack of cerebral autoregulation and the cerebral vasospasm (CVS) itself. Losartan may have a beneficial effect after SAH - preventing CVS, restoring cerebral autoregulation, reducing inflammation and early brain injury. Also some data is available for an AT1-receptor-upregulation and upregulated gene expression after subarachnoid hemorrhage, but the functional role of angiotensin on the cerebrovascular contractility is still not completely understood. Therefore, the aim of the present investigation was to detect functional interactions between the AT1-receptor blockade (by losartan) and the endothelin-1 (ET-1) dependent vasoconstriction and vasorelaxation in the basilar artery. To investigate the functional role of losartan on rat's basilar artery, changes of the vasoreactivity in an organ bath were determined. Under losartan the ET-1 induced contraction is decreased. After incubation with BQ-788, an ET(B)-receptor antagonist, the lowered contraction is abolished. In precontracted vessels under losartan and BQ-123, an ET(A)-receptor antagonist, ET-1 induced a higher relaxation. AT1-receptor antagonism causes a modulatory effect in ET(B)-receptor-dependent vasorelaxation in the basilar artery. AT1-receptor antagonism due to losartan induces the upregulation of the NO-pathway with a significantly increased relaxation accompanied with enhanced sensitivity of the ET(B)-receptor. Losartan has a dose-dependent antagonistic effect to the ET-1 induced contraction, which seems to ET(B)-receptor dependent. This antagonistic effect could be another beneficial effect after subarachnoid hemorrhage, additionally to the known effects after stroke: preventing CVS, restoring cerebral autoregulation, reducing inflammation and early brain injury.

Male-Female Differences in Upregulation of Vasoconstrictor Responses in Human Cerebral Arteries

Background and purposeMale-female differences may significantly impact stroke prevention and treatment in men and women, however underlying mechanisms for sexual dimorphism in stroke are not understood. We previously found in males that cerebral ischemia upregulates contractile receptors in cerebral arteries, which is associated with lower blood flow. The present study investigates if cerebral arteries from men and women differ in cerebrovascular receptor upregulation.Experimental approachFreshly obtained human cerebral arteries were placed in organ culture, an established model for studying receptor upregulation. 5-hydroxtryptamine type 1B (5-HT1B), angiotensin II type 1 (AT1) and endothelin-1 type A and B (ETA and ETB) receptors were evaluated using wire myograph for contractile responses, real-time PCR for mRNA and immunohistochemistry for receptor expression.Key resultsVascular sensitivity to angiotensin II and endothelin-1 was markedly lower in cultured cerebral arteries from women as compared to men. ETB receptor-mediated contraction occurred in male but not female arteries. Interestingly, there were similar upregulation in mRNA and expression of 5-HT1B, AT1, and ETB receptors and in local expression of Ang II after organ culture.Conclusions and ImplicationsIn spite of receptor upregulation after organ culture in both sexes, cerebral arteries from women were significantly less responsive to vasoconstrictors angiotensin II and endothelin-1 as compared to arteries from men. This suggests receptor coupling and/or signal transduction mechanisms involved in cerebrovascular contractility may be suppressed in females. This is the first study to demonstrate sex differences in the vascular function of human brain arteries.

Functional Effects of Levosimendan in Rat Basilar Arteries In Vitro

Levosimendan is a novel calcium sensitizer that is an established treatment for congestive heart failure. In coronary vessels, levosimendan has a vasorelaxant, endothelium-independent effect and an antagonistic effect on endothelin-1 (ET-1). There is also some data for a neuroprotective effect in a traumatic brain injury model, and levosimendan can prevent the reduction of the luminal area of the basilar artery. We considered that patients who suffer heart attack after subarachnoid hemorrhage (SAH) might respond well to levosimendan, which might also be useful to induce hypertension in patients with cerebral vasospasm.However, the functional effects of levosimendan in the cerebrovasculature are unknown. Here, we investigated the functional role of levosimendan on rat basilar artery by assessing vasocontractile reactivity in response to ET-1, sarafotoxin S6c, acetylcholine, sodium nitroprusside, cGMP, and prostaglandin F2α (PGF).Contrary to observations in coronary vessels, levosimendan did not affect the ET-1 system in cerebral arteries; neither ET(A)-receptor-induced contraction nor ET(B)-receptor-dependent relaxation were changed. For the nitric oxide (NO) pathway, only a slight increase was detected. Rather, levosimendan caused significant and dose-dependent relaxation after PGF precontraction.To our knowledge, this is the first report that describes levosimendan-induced functional changes of cerebrovascular contractility and relaxation. Under physiological conditions, levosimendan did not influence ET(A)/ET(B)-receptor signaling or the NO pathway. Interestingly, levosimendan seemed to affect the prostaglandin system and dosedependently reversed PGF- induced contraction. We did not detect a vasospastic potential for levosimedan in cerebral arteries, suggesting that it would be safe for use in SAH patients.

Early events triggering delayed vasoconstrictor receptor upregulation and cerebral ischemia after subarachnoid hemorrhage.

BackgroundUpregulation of vasoconstrictor receptors in cerebral arteries, including endothelin B (ETB) and 5-hydroxytryptamine 1B (5-HT1B) receptors, has been suggested to contribute to delayed cerebral ischemia, a feared complication after subarachnoid hemorrhage (SAH). This receptor upregulation has been shown to be mediated by intracellular signalling via the mitogen activated protein kinase kinase (MEK1/2) - extracellular regulated kinase 1/2 (ERK1/2) pathway. However, it is not known what event(s) that trigger MEK-ERK1/2 activation and vasoconstrictor receptor upregulation after SAH.We hypothesise that the drop in cerebral blood flow (CBF) and wall tension experienced by cerebral arteries in acute SAH is a key triggering event. We here investigate the importance of the duration of this acute CBF drop in a rat SAH model in which a fixed amount of blood is injected into the prechiasmatic cistern either at a high rate resulting in a short acute CBF drop or at a slower rate resulting in a prolonged acute CBF drop.ResultsWe demonstrate that the duration of the acute CBF drop is determining for a) degree of early ERK1/2 activation in cerebral arteries, b) delayed upregulation of vasoconstrictor receptors in cerebral arteries and c) delayed CBF reduction, neurological deficits and mortality. Moreover, treatment with an inhibitor of MEK-ERK1/2 signalling during an early time window from 6 to 24 h after SAH was sufficient to completely prevent delayed vasoconstrictor receptor upregulation and improve neurological outcome several days after the SAH.ConclusionsOur findings suggest a series of events where 1) the acute CBF drop triggers early MEK-ERK1/2 activation, which 2) triggers the transcriptional upregulation of vasoconstrictor receptors in cerebral arteries during the following days, where 3) the resulting enhanced cerebrovascular contractility contribute to delayed cerebral ischemia.

Abstract 355: β-Adrenergic Stimulation Impact on Cerebrovascular Contractility Through Actin Cytoskeleton Disorganization in Rabbits

Beta-adrenoceptor stimulations have been well-known to initiate cardiac hypertrophy accompanied by extracellular remodeling that predisposes heart failure and cardiac dysarrhythmia. However, less is known concerning cerebrovascular dysfunction induced by beta-adrenergic stimulation. We investigated whether cerebrovascular functions and related biochemical signal pathways were altered during beta-adrenergic stimulations in rabbits. New Zealand white rabbits have been injected with isoproterenol (ISO, 0.3 mg/kg/day) for 7 days. We have studied the alteration of protein expression in cerebral arteries (CAs) using 2DE proteomics and western blot analysis and further analyzed those data using bioinformatics software. Reactive oxygen species generation and followed DNA damage were assessed deteriorative effect of ISO on CAs. Vasocontractility response to angiotensin II (Ang II) and related biochemical signal pathways were assessed to evaluate functional alteration of CAs. Proteomic analysis revealed remarkably decreased expression of cytoskeleton organizing proteins, (e.g. actin related protein 1A and 2, α-actin, capping protein Z beta, and vimentin) and anti-oxidative stress proteins (e.g. heat shock protein 9A and stress-induced-phosphoprotein 1) in ISO-stimulated CAs. As a cause of dysregulation of actin cytoskeleton organization, we found decreased level of RhoA/ROCK1, major regulators of actin cytoskeleton organization. As functional consequences of proteomic alteration, we found the decreased transient Ca 2+ efflux and contraction response to Ang II and high K + in ISO-stimulated CAs. We suggested that ISO stimulations reduced RhoA/ROCK activities and modified key actin based cytoskeletal and antioxidative proteins in CAs, which consequently lead to impair cerebrovascular contractility response to Ang II in rabbits.

Postnatal maturation attenuates pressure-evoked myogenic tone and stretch-induced increases in Ca2+in rat cerebral arteries

Although postnatal maturation potently modulates agonist-induced cerebrovascular contractility, its effects on the mechanisms mediating cerebrovascular myogenic tone remain poorly understood. Because the regulation of calcium influx and myofilament calcium sensitivity change markedly during early postnatal life, the present study tested the general hypothesis that early postnatal maturation increases the pressure sensitivity of cerebrovascular myogenic tone via age-dependent enhancement of pressure-induced calcium mobilization and myofilament calcium sensitivity. Pressure-induced myogenic tone and changes in artery wall intracellular calcium concentrations ([Ca(2+)](i)) were measured simultaneously in endothelium-denuded, fura-2-loaded middle cerebral arteries (MCA) from pup [postnatal day 14 (P14)] and adult (6-mo-old) Sprague-Dawley rats. Increases in pressure from 20 to 80 mmHg enhanced myogenic tone in MCA from both pups and adults although the normalized magnitudes of these increases were significantly greater in pup than adult MCA. At each pressure step, vascular wall [Ca(2+)](i) was also significantly greater in pup than in adult MCA. Nifedipine significantly attenuated pressure-evoked constrictions in pup MCA and essentially eliminated all responses to pressure in the adult MCA. Both pup and adult MCA exhibited pressure-dependent increases in calcium sensitivity, as estimated by changes in the ratio of pressure-induced myogenic tone to wall [Ca(2+)](i). However, there were no differences in the magnitudes of these increases between pup and adult MCA. The results support the view that regardless of postnatal age, changes in both calcium influx and myofilament calcium sensitivity contribute to the regulation of cerebral artery myogenic tone. The greater cerebral myogenic response in P14 compared with adult MCA appears to be due to greater pressure-induced increases in [Ca(2+)](i), rather than enhanced augmentation of myofilament calcium sensitivity.

Expression of several cytoskeletal proteins in ovine cerebral arteries: developmental and functional considerations.

Cytoskeleton proteins play important roles in regulating vascular smooth muscle (VSM) contraction and relaxation. We tested the hypotheses that the expression levels of several of these proteins change significantly during the course of development, and that these changes contribute to age-related changes in contractile responses. In cerebral arteries from 95-day (d) gestation and 140-d fetus, newborn lambs, and adult sheep, by Western immunoblot (n= 5 for each age) we quantified the relative expression of alpha-actin, alpha-tubulin, cyclophilin A, and proliferating cell nuclear antigen (PCNA). In addition, we examined middle cerebral artery tension responses to phenylephrine (PHE) stimulation in the absence or presence of cytochalasin D (3 x 10(-7)m) and nocodazole (3 x 10(-6)m), inhibitors of alpha-actin and alpha-tubulin polymerization, respectively. The expression levels of alpha-actin and cyclophilin A varied little during the course of development. In contrast, alpha-tubulin expression was approximately 2.5-fold greater in both fetal age groups as compared to adult. Also, as compared to adult and as expected, expression of PCNA was several-fold greater in cerebral arteries of the 95-d fetus (x8), 140-d fetus (x 5), and newborn (x 3). In both adult and fetal middle cerebral artery, cytochalasin D-induced inhibition of actin polymerization decreased PHE-induced contraction, to approximately 60 and approximately 40% of control, respectively (despite no significant change in expression level). In contrast, alpha-tubulin inhibition by nocodazole showed little effect on PHE-induced tension (in spite of the age-related decrease in expression). In conclusion, expression levels of alpha-actin, a thin filament protein involved in contraction, remained relatively constant during the course of development, as did the effects of inhibition of its polymerization on contractility. In contrast, alpha-tubulin, important in intracellular protein trafficking, showed a significant age-related decrease in expression and played a relatively minor role in contractility. The present studies suggest that other cytoskeletal structural proteins and/or elements of pharmaco-mechanical coupling are important to developmental differences in cerebrovascular contractility. In addition, the relatively constant expression levels of alpha-actin and cyclophilin A with development, suggest that these are useful internal standards for studies of cytosolic protein expression.

Ins(1,4,5)P3 receptors in cerebral arteries: changes with development and high-altitude hypoxia.

We and others have shown that adrenergic-mediated contractile responses in cerebral vessels in vitro differ with vessel segment, with developmental age, and with high-altitude, long-term hypoxia. This is associated with significant differences in alpha 1-adrenergic receptor density and norepinephrine (NE)-induced response of the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. To test the hypothesis that vessel-specific, developmental, and hypoxic-associated contractility changes are mediated, in part, by changes in Ins(1,4,5)P3-receptor [Ins(1,4,5)P3-R] density or affinity, we performed the following study. In common carotid (Com), circle of Willis, and main branch anterior, middle, and posterior cerebral arteries (MBC) from normoxic fetal (approximately 140 days), newborn (3-5 days), and adult sheep and fetal and adult sheep acclimatized to high altitude, we quantified Ins(1,4,5)P3-R with [3H]Ins(1,4,5)P3. In normoxic Com, Ins(1,4,5)P3-R density values (fmol/mg protein) in fetus, newborn, and adult were 8 +/- 53, 150 +/- 18, and 357 +/- 21, respectively (P < 0.05). In normoxic MBC cerebral arteries, the receptor density values in the three age groups were 115 +/- 15, 105 +/- 9, 99 +/- 5 fmol/mg protein, respectively. For fetal and adult Com, high-altitude, long-term hypoxemia was associated with decreases in Ins(1,4,5)P3-R density of 32 (to 58 +/- 5) and 70% (to 109 +/- 12), respectively, from control values (P < 0.01). In MBC cerebral arteries of fetus and adult, hypoxic-associated decreases in Ins(1,4,5)P3-R density from control were 80 (to 23 +/- 3) and 47% (to 53 +/- 7), respectively (P < 0.01). Ins(1,4,5)P3 binding affinity to the receptor averaged 11.8 +/- 0.5 nM and did not vary significantly as a function of vessel type, developmental age, or hypoxia. In Com, but not in MBC, Ins(1,4,5)P3-R density increased dramatically with developmental age. This suggests that differences in Ins(1,4,5)P3-R density values may account, in part, for differences in contractile responses of the two artery types in the several age groups. In response to long-term, high-altitude hypoxia, Ins(1,4,5)P3-R density values in both fetal and adult Com and MBC decreased significantly, as did their NE-induced contraction. This suggests a cellular basis for changes in cerebrovascular contractility in response to long-term hypoxia and that Ins(1,4,5)P3-R may play a role in acclimatization responses to high altitude.

Effects of losartan on cerebral arteries in stroke-prone spontaneously hypertensive rats.

To investigate in young salt-loaded stroke-prone spontaneously hypertensive rats (SHR-SP) the effects of a long-term administration of the angiotensin II AT1 receptor antagonist losartan [1 mg/kg (L1) and 10 mg/kg (L10) per day, from 5 to 20 weeks of age] on the structural and functional characteristics of the middle cerebral artery. Morphological measurements and isometric tension recordings (myograph, contractile responses to potassium chloride and serotonin, relaxant responses to bradykinin and sodium nitroprusside) were performed on isolated vessels from randomly selected control and losartan-treated SHR-SP and age-matched Wistar-Kyoto (WKY) rats killed at ages 6-7, 10-11 and 16-17 weeks. Whereas all control SHR-SP had died within 18 weeks of being born, losartan at both doses afforded full protection against stroke and mortality. Losartan limited malignant hypertension development dose-dependently. Age-related increases in cerebral arterial wall thickness and wall:lumen ratio were not affected (L1) or limited slightly (L10) by losartan. In control SHR-SP, contractile responses of cerebral arteries to agonists decreased with ageing and stroke occurrence and were significantly smaller than those of age-matched WKY rat arteries. Losartan limited the cerebrovascular contractility impairment dose-dependently in SHR-SP but did not affect the WKY rat cerebral artery contractility. In addition, losartan limited the age-related alteration of the endothelium-dependent relaxation of cerebral arteries observed in control SHR-SP dose-dependently. In SHR-SP, losartan prevented stroke and improved the cerebral artery's smooth muscle and endothelial cell functions, which are altered during ageing and impaired even more dramatically by stroke occurrence.

Long-Term Hypoxic-Induced Changes in Inositol 1,4,5-Trisphosphate Receptor Density in Fetal and Adult Ovine Cerebral Arteries. † 1344

Background. In response to high altitude long-term hypoxia, the cerebral arteries of fetal and adult sheep show decreased contractile responses to K+ depolarization and amines (Am. J. Physiol. 264:R65-R72, 1993). This is associated with markedly attenuated density ofα1-adrenergic receptors and their second messenger inositol 1,4,5-trisphosphate (InsP3) (submitted). To test the hypothesis that hypoxic-induced changes in developmental and vessel specific cerebral artery contractility are mediated, in part, by changes in IP3-receptor(IP3R) density and/or affinity, we performed the following study.Methods. In aorta, common carotid artery, circle of Willis, and cerebral arteries from both near-term fetuses and adult ewes, we quantified IP3R density (fmol/mg protein) and affinity (nM) by saturation binding with [3H]IP3 (n=4 to 6 assays per group. * indicates a p value of < 0.05 for hypoxia vs normoxia). Results. For fetal and adult aorta, long-term hypoxemia was associated with decreases of in IP3R density of 46% (109 to 59) and 66%* (181 to 62), respectively. In the common carotid artery, IP3-R density for fetus and adult decreased 32%* (85 to 58) and 69%* (357 to 109), respectively. In cerebral arteries of fetus and adult, hypoxic-associated IP3-R densities decreased 80%* (128 to 25) and 46%* (99 to 53), respectively. The IP3 binding affinity was not changed among the groups studied. Conclusions. 1) IP3R values in fetal and adult aorta, common carotid, and combined anterior, middle, and posterior cerebral arteries, all decreased significantly in response to long-term hypoxia. 2) The decrease in fetal IP3-receptor density was of the same order of magnitude as that of the adult. This suggests that despite“buffering” by the maternal organism, fetal vascular tissues sensed hypoxia in a manner similar to that of the adult. 3) These findings suggest a cellular basis for changes in cerebrovascular contractility in response to long-term hypoxia, and that IP3-receptors may play a role in certain acclimatization responses to high altitude. (Supported in part by USPHS Grant HD 03807)

Maturation enhances the sensitivity of ovine cerebral arteries to the ATP-sensitive potassium channel activator lemakalim.

A wide variety of previous studies have demonstrated that arterial reactivity and contractility change dramatically during maturation. In light of recent findings that binding sites for glibenclamide, a ligand for ATP-sensitive potassium (KATP) channels, become more abundant with age in many tissues, the present studies examine the hypothesis that maturational changes in vascular reactivity involve changes in arterial electrophysiologic characteristics. To test this hypothesis, we determined the dose-response relation to lemakalim, a selective activator of KATP channels, in isolated endothelium-denuded segments of the second (2B, internal diameter approximately 200 microns) and fourth (4B, internal diameter approximately 125 microns) branches of middle cerebral arteries taken from newborn (3-7 d old) and adult sheep. At 100 microM, lemakalim completely relaxed serotonin-induced tone in all arteries. However, -log ED50 values were 29 to 43 times greater in adult (2B, 7.15 +/- 0.38; 4B, 6.61 +/- 0.42) than in newborn (2B, 5.52 +/- 0.25; 4B, 5.15 +/- 0.24) segments. Correspondingly, Hill values were significantly smaller in adults (2B, 0.47 +/- 0.17; 4B, 0.71 +/- 0.30) than in newborns (2B, 1.40 +/- 0.35; 4B, 3.30 +/- 0.92). These findings demonstrate that KATP channels are less sensitive to activation in newborn than in adult cerebral arteries. Given the important influence of KATP channels on vascular tone, and their possible role in many cardiovascular responses, the present data suggest that maturational increases in the activity of KATP channels contribute significantly to age-related changes in cerebrovascular contractility.