Mechanisms, significance and treatment of vascular dysfunction in type 2 diabetes mellitus: focus on lipid-regulating therapy

Endothelial dysfunction and increased arterial stiffness occur early in the pathogenesis of diabetic vasculopathy. They are both powerful independent predictors of cardiovascular risk. Advances in non-invasive methodologies have led to widespread clinical investigation of these abnormalities in diabetes mellitus, generating a wealth of new knowledge concerning the mechanisms of vascular dysfunction, risk factor associations and potential treatment targets. Endothelial dysfunction primarily reflects decreased availability of nitric oxide (NO), a critical endothelium-derived vasoactive factor with vasodilatory and anti-atherosclerotic properties. Techniques for assessing endothelial dysfunction include ultrasonographic measurement of flow-mediated vasodilatation of the brachial artery and plethysmography measurement of forearm blood flow responses to vasoactive agents. Arterial stiffness may be assessed using pulse wave analysis to generate measures of pulse wave velocity, arterial compliance and wave reflection. The pathogenesis of endothelial dysfunction in type 2 diabetes is multifactorial, with principal contributors being oxidative stress, dyslipidaemia and hyperglycaemia. Elevated blood glucose levels drive production of reactive oxidant species (ROS) via multiple pathways, resulting in uncoupling of mitochondrial oxidative phosphorylation and endothelial NO synthase (eNOS) activity, reducing NO availability and generating further ROS. Hyperglycaemia also contributes to accelerated arterial stiffening by increasing formation of advanced glycation end-products (AGEs), which alter vessel wall structure and function. Diabetic dyslipidaemia is characterised by accumulation of triglyceride-rich lipoproteins, small dense low-density lipoprotein (LDL) particles, reduced high-density lipoprotein (HDL)-cholesterol and increased postprandial free fatty acid flux. These lipid abnormalities contribute to increasing oxidative stress and may directly inhibit eNOS activity. Although lipid-regulating agents such as HMG-CoA reductase inhibitors (statins), fibric acid derivatives (fibrates) and fish oils are used to treat diabetic dyslipidaemia, their impact on vascular function is less clear. Studies in type 2 diabetes have yielded inconsistent results, but this may reflect sampling variation and the potential over-riding influence of oxidative stress, dysglycaemia and insulin resistance on endothelial dysfunction. Results of positive intervention trials suggest that improvement in vascular function is mediated by both lipid and non-lipid mechanisms, including anti-inflammatory, anti-oxidative and direct effects on the arterial wall. Other treatments, such as renin-angiotensin-aldosterone system antagonists, insulin sensitisers and lifestyle-based interventions, have shown beneficial effects on vascular function in type 2 diabetes. Novel approaches, targeting eNOS and AGEs, are under development, as are new lipid-regulating therapies that more effectively lower LDL-cholesterol and raise HDL-cholesterol. Combination therapy may potentially increase therapeutic efficacy and permit use of lower doses, thereby reducing the risk of adverse drug effects and interactions. Concomitant treatments that specifically target oxidative stress may also improve endothelial dysfunction in diabetes. Vascular function studies can be used to explore the therapeutic potential and mechanisms of action of new and established interventions, and provide useful surrogate measures for cardiovascular endpoints in clinical trials.     

CYP1B1基因敲除对成年小鼠肝脏脂肪代谢的影响及可能机制

目的探讨CYP1B1对高脂膳食诱导的成年小鼠脂肪代谢的作用。方法 CYP1B1基因敲除(KO)和野生型(WT)雄性成年C57/BL小鼠(6 w龄)各16只,给予低脂(LFD,30%)、高脂肪(HFD,60%)饲料共6 w。小鼠处死后取血清、附睾脂肪和肝脏组织检测相应的生化和分子生物学指标。结果 6 w高脂膳食后,KO小鼠能量摄入总量稍高于WT小鼠,但其体重增量和附睾脂肪组织重量均显著低于WT小鼠;WT小鼠脂肪细胞直径明显大于KO小鼠,且血糖、血清及肝脏组织中甘油三酯(TG)水平亦明显高于KO小鼠;肝脏组织RT-PCR结果显示,CYP1B1基因敲除后,启动脂肪形成的核因子及脂肪合成相关基因如CD36、SREBP1c、SCD1等表达下降,而调控脂肪氧化分解的基因如CPT-1α,UCP-2表达显著上升;蛋白印迹结果显示,CYP1B1基因敲除增强腺苷-磷酸激酶(AMPK)的磷酸化。结论 CYP1B1基因敲除对成年小鼠营养性肥胖的保护作用可能与AMPK磷酸化增强并调控肝脏中脂肪代谢相关基因的表达有关。     

N-NITRO-L-ARGININE AND INDOMETHACIN DO NOT AFFECT ENDOTHELIN-INDUCED CONSTRICTION OF LARGE AND SMALL CORONARY ARTERIES IN THE ANAESTHETIZED GREYHOUND

• 1 The aim of this study was to investigate whether endothelin-l (ET-1)-induced constriction of large and small coronary arteries in the anaesthetized greyhound is modulated by the endogenous release of nitric oxide or prostanoids.
• 2 ET-1 (1–100 ng/kg) and the α₁-adrenoceptor agonist phenylephrine (0.5-2 pg/kg), when injected directly into the circumflex coronary artery, caused dose-dependent decreases in epicardial coronary artery diameter and coronary vascular conductance without affecting systemic arterial pressure or the rate and force of cardiac contraction.
• 3 Inhibition of NO synthesis with N-nitro-L-arginine (NOLA, 5 mg/kg, i.c.) decreased coronary artery diameter, coronary conductance and heart rate and increased arterial pressure. The coronary vasoconstrictor response to ET-1 was unaffected by NOLA. By contrast, NOLA significantly increased the phenylephrine-induced constriction of the epicardial coronary artery but not the resistance vessels.
• 4 Indomethacin (5 mg/kg, i.v.), an inhibitor of cyclo-oxygenase, significantly decreased epicardial coronary artery diameter but did not affect coronary conductance. Indomethacin had no effect on the coronary vascular responses to ET-1 or phenylephrine. Combined treatment with NOLA plus indomethacin also failed to affect the coronary vasoconstrictor effects of ET-1.
• 5 Basal release of NO and vasodilator prostanoids modulated resting coronary vascular tone but did not Influence the vasoconstrictor responses to endothelin in either large or small coronary arteries.

     

The effect of ischaemia on endothelium-dependent vasodilatation and adrenoceptor-mediated vasoconstriction in rat isolated hearts.

1. The aim of this study was to investigate whether global ischaemia and reperfusion in rat isolated hearts affects endothelium-dependent vasodilatation and adrenoceptor-mediated vasoconstriction. In addition, it was first determined whether inhibition of the actions of nitric oxide (NO) influenced the responses to alpha-adrenoceptor agonists in the rat coronary vasculature. 2. In rat isolated, Langendorff perfused hearts, inhibition of NO with haemoglobin (Hb, 6 microM) significantly inhibited the vasodilator responses to the endothelium-dependent vasodilators, acetylcholine (ACh, 3-100 pmol), carbachol (CCh, 10-300 pmol), bradykinin (Bk, 1-30 pmol) and histamine (0.3-10 nmol) but did not affect responses to the endothelium-independent vasodilator, sodium nitroprusside (SNP, 0.01-1 nmol). 3. Inhibition of the action of NO by Hb significantly enhanced the vasoconstrictor response to the non-selective alpha-adrenoceptor agonist, noradrenaline (NA, 0.1-10 nmol) and the alpha 2-adrenoceptor agonist, B-HT 920 (0.001-1 mumol) but had no effect on the vascular response to the alpha 1-adrenoceptor agonist, methoxamine (MTX, 10-300 nmol). 4. In the perfused hearts ischaemia, induced by 30 min perfusion at 5% of the normal rate of flow, followed by 15 min of reperfusion (ischaemia/reperfusion) selectively impaired the vasodilator responses to ACh and CCh which act by muscarinic receptor stimulation but did not affect responses to the other endothelium-dependent vasodilators Bk and histamine or to the endothelium-independent dilator SNP. 5. After ischaemia/reperfusion the coronary vasoconstrictor responses to B-HT 920 were slightly but significantly enhanced whereas the responses to NA and MTX were unaffected. 6. Thus, in the rat isolated heart, low flow induced-ischaemia and reperfusion causes a selective impairment of muscarinic receptor-mediated vasodilatation but does not impair responses to all endothelium-dependent vasodilators. Enhanced constrictor responses to noradrenaline and B-HT 920 in the presence of Hb indicates that endogenous NO modulates the constriction of coronary resistance vessels in response to stimulation of alpha 2-adrenoceptors. Ischaemia and reperfusion in this isolated vascular bed caused only a small increase in the coronary vasoconstrictor response to alpha 2-adrenoceptor stimulation. It appears that in the rat isolated heart the degree of endothelial dysfunction caused by ischaemia/reperfusion is insufficient to cause a functionally significant change in alpha-adrenoceptor-mediated constriction.     

Appraising and assessing reflection in students' writing on a structured worksheet

BACKGROUND: A variety of teaching and learning techniques intended to engage students in reflection are either in use or are being developed in medical and dental education. In line with evidence-based practice in education, research is needed to appraise the utility and effectiveness of these techniques, so that they may be used with confidence. AIM: To assess whether students completing a 'reflective' learning activity based on a structured worksheet really were reflecting. METHOD: A qualitative, multi-method approach was taken. Worksheets completed by students were examined for evidence of reflection by researchers using two sets of criteria for the assessment of reflection derived from the literature, and by peer judges using their own criteria. The opinions of students completing the activity, regarding its acceptability and utility, were elicited by a questionnaire incorporating a 5-point Likert scale. RESULTS: Results from all methods suggest that students completing the activity were reflecting. Students' opinions of the activity were mainly positive. CONCLUSION: The methods employed may be of use to educators wishing to appraise reflective learning activities or, possibly, to assess student reflection.     

Type 1 diabetes and hypercholesterolaemia reveal the contribution of endothelium-derived hyperpolarizing factor to endothelium-dependent relaxation of the rat aorta

1. The present study evaluated the effect of diabetes, hypercholesterolaemia and their combination on the contribution of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) to relaxation of rat isolated aortic rings and the potential contribution of oxidant stress to the disturbance of endothelial function. 2. Thoracic aortic rings from control, diabetic, hypercholesterolaemic and diabetic plus hypercholesterolaemic rats were suspended in organ baths for tension recording. Generation of superoxide by the aorta was measured using lucigenin-enhanced chemiluminescence. 3. The maximal response to acetylcholine (ACh) was significantly reduced in diabetic or hypercholesterolaemic rats compared with control rats. In rats with diabetes plus hypercholesterolaemia, both the sensitivity and maximal response to ACh was impaired. In control rats, the response to ACh was abolished by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA) or inhibition of soluble guanylate cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). In contrast, in rats with diabetes, hypercholesterolaemia or both, relaxation to ACh was resistant to inhibition by L-NNA or ODQ, but abolished by additional inhibition of K(Ca) channels with charybdotoxin plus apamin. 4. The generation of superoxide was not significantly enhanced in aortic rings from either diabetic or hypercholesterolaemic rats, but was significantly increased in aortic rings from rats with diabetes plus hypercholesterolaemia. 5. These results suggest that when diabetes and hypercholesterolaemia impair endothelium-dependent relaxation, due to a diminished contribution from NO, a compensatory contribution of EDHF to endothelium-dependent relaxation of the aorta is revealed. The attenuation of NO-mediated relaxation, at least in the presence of both diabetes and hypercholesterolaemia, is associated with enhanced superoxide generation.