Uncoupling of Ca2+ sparks from BK channels in cerebral arteries underlies hypoperfusion in hypertension-induced vascular dementia

The deficit in cerebral blood flow (CBF) seen in patients with hypertension-induced vascular dementia is increasingly viewed as a therapeutic target for disease-modifying therapy. Progress is limited, however, due to uncertainty surrounding the mechanisms through which elevated blood pressure reduces CBF. To investigate this, we used the BPH/2 mouse, a polygenic model of hypertension. At 8 mo of age, hypertensive mice exhibited reduced CBF and cognitive impairment, mimicking the human presentation of vascular dementia. Small cerebral resistance arteries that run across the surface of the brain (pial arteries) showed enhanced pressure-induced constriction due to diminished activity of large-conductance Ca2+-activated K+ (BK) channels-key vasodilatory ion channels of cerebral vascular smooth muscle cells. Activation of BK channels by transient intracellular Ca2+ signals from the sarcoplasmic reticulum (SR), termed Ca2+ sparks, leads to hyperpolarization and vasodilation. Combining patch-clamp electrophysiology, high-speed confocal imaging, and proximity ligation assays, we demonstrated that this vasodilatory mechanism is uncoupled in hypertensive mice, an effect attributable to physical separation of the plasma membrane from the SR rather than altered properties of BK channels or Ca2+ sparks, which remained intact. This pathogenic mechanism is responsible for the observed increase in constriction and can now be targeted as a possible avenue for restoring healthy CBF in vascular dementia.

Perspectives on Cognitive Phenotypes and Models of Vascular Disease

Clinical investigations have established that vascular-associated medical conditions are significant risk factors for various kinds of dementia. And yet, we are unable to associate certain types of vascular deficiencies with specific cognitive impairments. The reasons for this are many, not the least of which are that most vascular disorders are multi-factorial and the development of vascular dementia in humans is often a multi-year or multi-decade progression. To better study vascular disease and its underlying causes, the National Heart, Lung, and Blood Institute of the National Institutes of Health has invested considerable resources in the development of animal models that recapitulate various aspects of human vascular disease. Many of these models, mainly in the mouse, are based on genetic mutations, frequently using single-gene mutations to examine the role of specific proteins in vascular function. These models could serve as useful tools for understanding the association of specific vascular signaling pathways with specific neurological and cognitive impairments related to dementia. To advance the state of the vascular dementia field and improve the information sharing between the vascular biology and neurobehavioral research communities, National Heart, Lung, and Blood Institute convened a workshop to bring in scientists from these knowledge domains to discuss the potential utility of establishing a comprehensive phenotypic cognitive assessment of a selected set of existing mouse models, representative of the spectrum of vascular disorders, with particular attention focused on age, sex, and rigor and reproducibility. The workshop highlighted the potential of associating well-characterized vascular disease models, with validated cognitive outcomes, that can be used to link specific vascular signaling pathways with specific cognitive and neurobehavioral deficits.

Differential restoration of functional hyperemia by antihypertensive drug classes in hypertension-related cerebral small vessel disease

Dementia resulting from small vessel diseases (SVDs) of the brain is an emerging epidemic for which there is no treatment. Hypertension is the major risk factor for SVDs, but how hypertension damages the brain microcirculation is unclear. Here, we show that chronic hypertension in a mouse model progressively disrupts on-demand delivery of blood to metabolically active areas of the brain (functional hyperemia) through diminished activity of the capillary endothelial cell inward-rectifier potassium channel, Kir2.1. Despite similar efficacy in reducing blood pressure, amlodipine, a voltage-dependent calcium-channel blocker, prevented hypertension-related damage to functional hyperemia whereas losartan, an angiotensin II type 1 receptor blocker, did not. We attribute this drug class effect to losartan-induced aldosterone breakthrough, a phenomenon triggered by pharmacological interruption of the renin-angiotensin pathway leading to elevated plasma aldosterone levels. This hypothesis is supported by the finding that combining losartan with the aldosterone receptor antagonist eplerenone prevented the hypertension-related decline in functional hyperemia. Collectively, these data suggest Kir2.1 as a possible therapeutic target in vascular dementia and indicate that concurrent mineralocorticoid aldosterone receptor blockade may aid in protecting against late-life cognitive decline in hypertensive patients treated with angiotensin II type 1 receptor blockers.

Cardiovascular co-morbidities, inflammation and cerebral small vessel disease

Cerebral small vessel disease (cSVD) is the most common cause of vascular cognitive impairment and affects all levels of the brain's vasculature. Features include diverse structural and functional changes affecting small arteries and capillaries that lead to a decline in cerebral perfusion. Due to an aging population, incidence of cerebral small vessel disease (cSVD) is continually rising. Despite its prevalence and its ability to cause multiple debilitating illnesses, such as stroke and dementia, there are currently no therapeutic strategies for the treatment of cSVD. In the healthy brain, interactions between neuronal, vascular and inflammatory cells are required for normal functioning. When these interactions are disturbed, chronic pathological inflammation can ensue. The interplay between cSVD and inflammation has attracted much recent interest and this review discusses chronic cardiovascular diseases, particularly hypertension, and explores how the associated inflammation may impact on the structure and function of the small arteries of the brain in cSVD. Molecular approaches in animal studies are linked to clinical outcomes in patients and novel hypotheses regarding inflammation and cSVD are proposed that will hopefully stimulate further discussion and study in this important area.

Disruption of Pressure-Induced Ca2+ Spark Vasoregulation of Resistance Arteries, Rather Than Endothelial Dysfunction, Underlies Obesity-Related Hypertension

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Obesity-related hypertension is one of the world’s leading causes of death and yet little is understood as to how it develops. As a result, effective targeted therapies are lacking and pharmacological treatment is unfocused. To investigate underlying microvascular mechanisms, we studied small artery dysfunction in a high fat–fed mouse model of obesity. Pressure-induced constriction and responses to endothelial and vascular smooth muscle agonists were studied using myography; the corresponding intracellular Ca²⁺ signaling pathways were examined using confocal microscopy. Principally, we observed that the enhanced basal tone of mesenteric resistance arteries was due to failure of intraluminal pressure-induced Ca²⁺ spark activation of the large conductance Ca²⁺ activated K⁺ potassium channel (BK) within vascular smooth muscle cells. Specifically, the uncoupling site of this mechanotransduction pathway was at the sarcoplasmic reticulum, distal to intraluminal pressure-induced oxidation of Protein Kinase G. In contrast, the vasodilatory function of the endothelium and the underlying endothelial IP-3 and TRPV4 (vanilloid 4 transient receptor potential ion channel) Ca²⁺ signaling pathways were not affected by the high-fat diet or the elevated blood pressure. There were no structural alterations of the arterial wall. Our work emphasizes the importance of the intricate cellular pathway by which intraluminal pressure maintains Ca²⁺ spark vasoregulation in the origin of obesity-related hypertension and suggests previously unsuspected avenues for pharmacological intervention.

"A Step and a Ceiling": mechanical properties of Ca2+ spark vasoregulation in resistance arteries by pressure-induced oxidative activation of PKG

We investigated the biomechanical relationship between intraluminal pressure within small mesenteric resistance arteries, oxidant activation of PKG, Ca2+ sparks, and BK channel vasoregulation. Mesenteric resistance arteries from wild type (WT) and genetically modified mice with PKG resistance to oxidative activation were studied using wire and pressure myography. Ca2+ sparks and Ca2+ transients within vascular smooth muscle cells of intact arteries were characterized using high-speed confocal microscopy of intact arteries. Arteries were studied under conditions of varying intraluminal pressure and oxidation. Intraluminal pressure specifically, rather than the generic stretch of the artery, was necessary to activate the oxidative pathway. We demonstrated a graded step activation profile for the generation of Ca2+ sparks and also a functional "ceiling" for this pressure --sensitive oxidative pathway. During steady state pressure - induced constriction, any additional Ca2+ sensitive-K+ channel functional availability was independent of oxidant activated PKG. There was an increase in the amplitude, but not the Area under the Curve (AUC) of the caffeine-induced Ca2+ transient in pressurized arteries from mice with oxidant-resistant PKG compared with wild type. Overall, we surmise that intraluminal pressure within resistance arteries controls Ca2+ spark vasoregulation through a tightly controlled pathway with a graded onset switch. The pathway, underpinned by oxidant activation of PKG, cannot be further boosted by additional pressure or oxidation once active. We propose that these restrictive characteristics of pressure-induced Ca2+ spark vasoregulation confer stability for the artery in order to provide a constant flow independent of additional pressure fluctuations or exogenous oxidants.

Lack of direct effect of adiponectin on vascular smooth muscle cell BKCa channels or Ca2+ signaling in the regulation of small artery pressure-induced constriction

The aim of this study was to investigate mechanisms by which adiponectin influences vascular Ca²⁺ signaling, K⁺ channel activity and thus contractile tone of small arteries. Vasodilation to adiponectin was studied in mesenteric resistance arteries constricted with intraluminal pressure. Ca²⁺ signals were characterized using high speed confocal microscopy of intact arteries. Patch clamp investigated the effect of adiponectin on individual VSMC potassium (K⁺) channel currents. Adiponectin dilated arteries constricted with pressure‐induced tone by approximately 5% and the induced vasodilation was only transient. The dilation to adiponectin was reduced by pharmacological interruption of the Ca²⁺ spark/large conductance activated K⁺ (BK) channel pathway but from a physiological perspective, interpretation of the data was limited by the small effect. Neither Adiponectin nor the presence of intact perivascular adipose tissue (PVAT) influenced Ca²⁺ spark or Ca²⁺ wave frequency or characteristics. Studied using a perforated patch approach, Adiponectin marginally increased current through the VSMC BK channel but this effect was lost using the whole cell technique with dialysis of the cytoplasm. Adiponectin did not change the frequency or amplitude of Ca²⁺ spark‐induced transient outward currents (STOC). Overall, our study shows that Adiponectin induces only a small and transient dilation of pressure constricted mesenteric arteries. This vasodilatory effect is likely to be independent of Ca²⁺ sparks or direct BK channel activation.

Abnormal Remodeling of Subcutaneous Small Arteries Is Associated With Early Diastolic Impairment in Metabolic Syndrome

Background

Small artery pathophysiology is frequently invoked as a cause of obesity‐related diastolic heart failure. However, evidence to support this hypothesis is scant, particularly in humans.

Methods and Results

To address this, we studied human small artery structure and function in obesity and looked for correlations between vascular parameters and diastolic function. Seventeen obese patients with metabolic syndrome and 5 control participants underwent echocardiography and subcutaneous gluteal fat biopsy. Small arteries were isolated from the biopsy and pressure myography was used to study endothelial function and wall structure. In comparison with the control group, small arteries from obese participants exhibited significant endothelial dysfunction, assessed as the vasodilatory response to acetylcholine and also pathological growth of the wall. For the obese participants, multiple regression analysis revealed an association between left atrial volume and both the small artery wall thickness (β=0.718, P=0.02) and wall‐to‐lumen ratio (β=0.605, P=0.02). Furthermore, the E:E′ ratio was associated with wall‐to‐lumen ratio (β=0.596, P=0.02) and inversely associated with interleukin‐6 (β=−0.868, P=0.03). By contrast, endothelial function did not correlate with any of the echocardiographic parameters studied.

Conclusions

Although the small arteries studied were not cardiac in origin, our results support a role for small artery remodeling in the development of diastolic dysfunction in humans. Further direct examination of the structure and function of the myocardial resistance vasculature is now warranted, to elucidate the temporal association between metabolic risk factors, small artery injury, and diastolic impairment.

Pressure-induced oxidative activation of PKG enables vasoregulation by Ca2+ sparks and BK channels

Activation of Ca²⁺-sensitive, large-conductance potassium (BK) channels in vascular smooth muscle cells (VSMCs) by local, ryanodine receptor-mediated Ca²⁺ signals (Ca²⁺ sparks) acts as a brake on pressure-induced (myogenic) vasoconstriction-a fundamental mechanism that regulates blood flow in small resistance arteries. We report that physiological intraluminal pressure within resistance arteries activated cGMP-dependent protein kinase (PKG) in VSMCs through oxidant-induced formation of an intermolecular disulfide bond between cysteine residues. Oxidant-activated PKG was required to trigger Ca²⁺ sparks, BK channel activity, and vasodilation in response to pressure. VSMCs from arteries from mice expressing a form of PKG that could not be activated by oxidants showed reduced Ca²⁺ spark frequency, and arterial preparations from these mice had decreased pressure-induced activation of BK channels. Thus, the absence of oxidative activation of PKG disabled the BK channel-mediated negative feedback regulation of vasoconstriction. Our results support the concept of a negative feedback control mechanism that regulates arterial diameter through mechanosensitive production of oxidants to activate PKG and enhance Ca²⁺ sparks.

Effects of Obesity on Perivascular Adipose Tissue Vasorelaxant Function: Nitric Oxide, Inflammation and Elevated Systemic Blood Pressure

Introduction

Perivascular adipose tissue (PVAT) surrounds most vessels in the human body. Healthy PVAT has a vasorelaxant effect which is not observed in obesity. We assessed the contribution of nitric oxide (NO), inflammation and endothelium to obesity-induced PVAT damage.

Methods

Rats were fed a high-fat diet or normal chow. PVAT function was assessed using wire myography. Skeletonised and PVAT-intact mesenteric vessels were prepared with and without endothelium. Vessels were incubated with L-NNA or superoxide dismutase (SOD) and catalase. Gluteal fat biopsies were performed on 10 obese and 10 control individuals, and adipose tissue was assessed using proteomic analysis.

Results

In the animals, there were significant correlations between weight and blood pressure (BP; r = 0.5, p = 0.02), weight and PVAT function (r = 0.51, p = 0.02), and PVAT function and BP (r = 0.53, p = 0.01). PVAT-intact vessel segments from healthy animals constricted significantly less than segments from obese animals (p < 0.05). In a healthy state, there was preservation of the PVAT vasorelaxant function after endothelium removal (p < 0.05). In endothelium-denuded vessels, L-NNA attenuated the PVAT vasorelaxant function in control vessels (p < 0.0001). In obesity, incubation with SOD and catalase attenuated PVAT-intact vessel contractility in the presence and absence of endothelium (p < 0.001). In obese humans, SOD [Cu-Zn] (SOD1; fold change −2.4), peroxiredoxin-1 (fold change −2.15) and adiponectin (fold change −2.1) were present in lower abundances than in healthy controls.

Conclusions

Incubation with SOD and catalase restores PVAT vasorelaxant function in animal obesity. In the rodent model, obesity-induced PVAT damage is independent of endothelium and is in part due to reduced NO bioavailability within PVAT. Loss of PVAT function correlates with rising BP in our animal obesity model. In keeping with our hypothesis of inflammation-induced damage to PVAT function in obesity, there are lower levels of SOD1, peroxiredoxin-1 and adiponectin in obese human PVAT.

Retinal arterial hypertrophy: the new LVH?

Prevention of target organ damage represents the El Dorado for clinicians who treat hypertension. Although many of the cardiovascular sequelae of chronic hypertension are due to large artery atherosclerosis, an equal number are due to small artery dysfunction. These microvascular complications include eye disease (retinopathy), kidney failure, diastolic dysfunction of the heart and small vessel brain disease leading to stroke syndromes, dementia and even depression. Examination of the retinal vasculature represents the only way to reliably derive information regarding small arteries responsible for these diverse pathologies. This review aims to summarise the rapidly accruing evidence indicating that easily observable abnormalities of retinal arteries reflect target organ damage elsewhere in the body of hypertensive patients. In tandem, we also present putative mechanisms by which hypertension and diabetes fundamentally change small artery structure and function and how these processes may lead to target organ damage.

Effects of bariatric surgery on human small artery function: evidence for reduction in perivascular adipocyte inflammation, and the restoration of normal anticontractile activity despite persistent obesity

Objectives

The aim of this study was to investigate the effects of bariatric surgery on small artery function and the mechanisms underlying this.

Background

In lean healthy humans, perivascular adipose tissue (PVAT) exerts an anticontractile effect on adjacent small arteries, but this is lost in obesity-associated conditions such as the metabolic syndrome and type II diabetes where there is evidence of adipocyte inflammation and increased oxidative stress.

Methods

Segments of small subcutaneous artery and perivascular fat were harvested from severely obese individuals before (n = 20) and 6 months after bariatric surgery (n = 15). Small artery contractile function was examined in vitro with wire myography, and perivascular adipose tissue (PVAT) morphology was assessed with immunohistochemistry.

Results

The anticontractile activity of PVAT was lost in obese patients before surgery when compared with healthy volunteers and was restored 6 months after bariatric surgery. In vitro protocols with superoxide dismutase and catalase rescued PVAT anticontractile function in tissue from obese individuals before surgery. The improvement in anticontractile function after surgery was accompanied by improvements in insulin sensitivity, serum glycemic indexes, inflammatory cytokines, adipokine profile, and systolic blood pressure together with increased PVAT adiponectin and nitric oxide bioavailability and reduced macrophage infiltration and inflammation. These changes were observed despite the patients remaining severely obese.

Conclusions

Bariatric surgery and its attendant improvements in weight, blood pressure, inflammation, and metabolism collectively reverse the obesity-induced alteration to PVAT anticontractile function. This reversal is attributable to reductions in local adipose inflammation and oxidative stress with improved adiponectin and nitric oxide bioavailability.

Cerebrovascular damage in late-life depression is associated with structural and functional abnormalities of subcutaneous small arteries

Late-life depression is increasingly viewed as a vascular illness because of patients exhibiting characteristic white matter brain lesions and in vivo large artery endothelial dysfunction. However, the "vascular depression" hypothesis pertains to the microvasculature, and this circulation has not been studied in this context. Our objective was to examine structure and function of small subcutaneous arteries in patients with late-life depression. Thus, 16 patients aged 71.8±4.0 years with late-life depression were compared with 15 control participants aged 72.1±5.9 years. There were similar cardiovascular profiles between the 2 groups. All of the participants underwent MRI brain scans and subcutaneous gluteal fat biopsy from which small arteries were isolated and studied using pressure myography. Cerebral microvascular damage in depressed patients was confirmed by assessment of basal ganglia Virchow-Robin space scores (depressed patients 3.9±1.7 versus controls: 2.5±1.6; P=0.01). Contractility to norepinephrine was equivalent in both groups, but relaxation of the small arteries to acetylcholine was significantly reduced in depressed patients (84.0±4.0%) compared with control participants (96.0±1.4%; P=0.012). This difference in arterial relaxation was reduced but not entirely eliminated when NO synthase was inhibited. Depressed patients also exhibited hypertrophic wall growth with an increase in medial cross-sectional area (P=0.035, multiple ANOVA and wall thickness; P=0.04, multiple ANOVA). In conclusion, despite similar cardiovascular profiles, depressed patients with cerebral microvascular damage show abnormalities of subcutaneous small artery structure and function.

Vascular function in older adults with depressive disorder

BACKGROUND

Cerebrovascular disease plays an important role in depressive disorder, especially in older adults. An understanding of vascular function in depression is important etiologically and to develop innovative treatments that may improve prognosis by ameliorating vascular damage.

METHODS

This study assessed endothelial function, arterial stiffness, and atherosclerosis in a variety of vessel beds in 25 elderly subjects with depressive disorder compared with 21 nondepressed control subjects. Subjects underwent pulse wave velocity, pulse wave analysis, carotid intima media thickness analysis, and magnetic resonance imaging. A subset (16 patients and 15 control subjects) had assessment of biopsied small artery dilatation to acetylcholine to further assess endothelial function.

RESULTS

The mean sample age was 72.4 years with an average age at onset for depression of 60 years. Mean carotid intima media thickness was significantly higher in depressed subjects (p < .01). Pulse wave velocity was 1.6 m/sec higher in depressed subjects (borderline significance). There was a significant reduction in the dilatation response to acetylcholine in preconstricted small arteries (p = .01). On magnetic resonance imaging, depressed subjects had significantly more dilated Virchow-Robin spaces in the basal ganglia (p = .01). Depressed subjects had greater volume of white matter lesions in all regions, but this did not reach statistical significance. There were no baseline differences in vascular risk.

CONCLUSIONS

Depression in the elderly is associated with poorer endothelial function and more atherosclerosis. This is associated with a greater white matter hyperintensities lesion load and basal ganglia microangiopathy. The use of vasoprotective drugs to improve endothelial function or retard atherosclerosis as depression-modifying agents should be explored.

Local inflammation and hypoxia abolish the protective anticontractile properties of perivascular fat in obese patients

BACKGROUND

Inflammation in adipose tissue has been implicated in vascular dysfunction, but the local mechanisms by which this occurs are unknown.

METHODS AND RESULTS

Small arteries with and without perivascular adipose tissue were taken from subcutaneous gluteal fat biopsy samples and studied with wire myography and immunohistochemistry. We established that healthy adipose tissue around human small arteries secretes factors that influence vasodilation by increasing nitric oxide bioavailability. However, in perivascular fat from obese subjects with metabolic syndrome (waist circumference 111+/-2.8 versus 91.1+/-3.5 cm in control subjects, P<0.001; insulin sensitivity 41+/-5.9% versus 121+/-18.6% in control subjects, P<0.001), the loss of this dilator effect was accompanied by an increase in adipocyte area (1786+/-346 versus 673+/-60 mum(2), P<0.01) and immunohistochemical evidence of inflammation (tumor necrosis factor receptor 1 12.4+/-1.1% versus 6.7+/-1%, P<0.001). Application of the cytokines tumor necrosis factor receptor-alpha and interleukin-6 to perivascular fat around healthy blood vessels reduced dilator activity, resulting in the obese phenotype. These effects could be reversed with free radical scavengers or cytokine antagonists. Similarly, induction of hypoxia stimulated inflammation and resulted in loss of anticontractile capacity, which could be rescued by catalase and superoxide dismutase or cytokine antagonists. Incubation with a soluble fragment of adiponectin type 1 receptor or inhibition of nitric oxide synthase blocked the vasodilator effect of healthy perivascular adipose tissue.

CONCLUSIONS

We conclude that adipocytes secrete adiponectin and provide the first functional evidence that it is a physiological modulator of local vascular tone by increasing nitric oxide bioavailability. This capacity is lost in obesity by the development of adipocyte hypertrophy, leading to hypoxia, inflammation, and oxidative stress.

Early rheumatoid arthritis is associated with a deficit in the CD4+CD25high regulatory T cell population in peripheral blood

OBJECTIVE

Our aim was to test the hypothesis that there is a deficit in the CD4+CD25high regulatory T-cell population in early rheumatoid arthritis (RA), either in size or functional activity.

METHODS

Peripheral blood mononuclear cells were examined from subjects with early active RA who had received no previous disease-modifying therapy (n = 43), from individuals with self-limiting reactive arthritis (n = 14), from subjects with stable, well-controlled RA (n = 82) and from healthy controls (n = 72). The frequencies of CD4+CD25high T-cells were quantified using flow cytometry, and function was assessed by the ability to suppress proliferation of CD4+CD25- T-cells. Paired blood and synovial fluid was analysed from a small number of RA and reactive arthritis patients.

RESULTS

There was a smaller proportion of CD4+CD25high T-cells in the peripheral blood of early active RA patients (mean 4.25%) than in patients with reactive arthritis or in controls (mean 5.90 and 5.30%, respectively, P = 0.001 in each case). Frequencies in stable, well-controlled RA (mean 4.63%) were not significantly different from early active RA or controls. There were no differences in suppressor function between groups. Higher frequencies of CD4+CD25high T-cells were found in synovial fluid than blood in both RA and reactive arthritis.

CONCLUSIONS

These data demonstrate a smaller CD4+CD25high regulatory T-cell population in peripheral blood of individuals with early active RA prior to disease-modifying treatment. This may be a contributory factor in the susceptibility to RA and suggests novel approaches to therapy.

Ascites and apparent renal failure treated with a Foley catheter

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Glutathione disulphide as a marker of reperfusion injury in ischaemic skin flaps

Estimation of the oxidised form of glutathione (GSSG) in an ischaemic/reperfused organ is frequently employed as an indicator of oxidative stress created by the production of oxygen free radicals during the reperfusion period. The time course of oxidative stress and tissue damage in 19 ischaemic/reperfused guinea-pig island skin flaps was evaluated. No-flow ischaemia was induced in the flaps for 6 h in 7 animals, and for 8 h in 9 animals (a further 3 animals served as controls without ischaemia). Arterial and venous blood samples were obtained directly from the flap pedicle at baseline, 10, 30, and 60 min following reperfusion. Results suggest that a second focus of oxidative injury, possibly mediated by activated neutrophils, contributes to the overall process of reperfusion injury. Plasma levels of GSSG allow for a more sensitive quantification of oxidant stress within reperfused ischaemic flaps, and may serve as a useful tool in skin flap research.

Effects of an aromatase inhibitor on thymus and kidney and on oestrogen receptors in female MRL/MP-lpr/lpr mice

The effects of an aromatase inhibitor, 4-hydroxyandrostenedione (4-OHA), which blocks oestrogen formation, have been studied in female MRL/MP-lpr/lpr mice which are a model of SLE. At 11.5 weeks, mice were implanted subcutaneously either with empty Silastic implants or with implants containing 25 mg 4-OHA. At 15 weeks, they were sacrificed by decapitation and liver, thymus, kidneys and uterus taken for wet weight, histology and measurement of cytosolic and nuclear oestrogen receptors. Thymus weights were significantly lower in 4-OHA-treated mice although uterus weights were similar in both groups. Also, whereas thymuses from control-treated mice were packed with plasma cells with abundant cytoplasm, those from 4-OHA-treated mice contained T cells with large nuclei. Relative oestrogen receptor abundances were: uterus > liver > thymus, although cytosolic receptors could not be detected in thymus cytosols of MRL mice unless they were treated with the aromatase inhibitor. In kidney, there was histological evidence that inflammation was limited to mesangium in 4-OHA-treated mice. These results support the hypothesis that oestrogens may be involved in the aetiology of murine SLE and provide data suggesting that substances which block oestrogen production in vivo may be useful to treat certain forms of SLE.