Biomedical subjects
Dimitri P Mikhailidis
Publications and source records attributed to Dimitri P Mikhailidis.
Ghrelin can bind to a species of high density lipoprotein associated with paraoxonase.
Ghrelin is a 28-residue peptide hormone that is principally released from the stomach during fasting and prior to eating. Two forms are present in human plasma: the unmodified peptide and a less abundant acylated version, in which octanoic acid is attached to the third residue, a serine, via an ester linkage. The acylated form of ghrelin acts as a ligand for the growth hormone secretagogue receptor and can stimulate the release of growth hormone from the pituitary gland. It also initiates behavioral and metabolic adaptations to fasting. Here we show that an immobilized form of ghrelin specifically binds a species of high density lipoprotein associated with the plasma esterase, paraoxonase, and clusterin. Both free ghrelin and paraoxon, a substrate for paraoxonase, can inhibit this interaction. An endogenous species of ghrelin is found to co-purify with high density lipoprotein during density gradient centrifugation and subsequent gel filtration. This interaction links the orexigenic peptide hormone ghrelin to lipid transport and metabolism. Furthermore, the interaction of the esterified hormone ghrelin with a species of HDL containing an esterase suggests a possible mechanism for the conversion of ghrelin to des-acyl ghrelin.
Direct measurement of hepatic tissue hypoxia by using a novel tcpO2/pCO2 monitoring system in comparison with near-infrared spectroscopy.
BACKGROUND/AIMS: Hepatic hypoxia occurs during liver surgery and transplantation and it may also appear within liver tumours, correlating with prognosis and efficacy of the treatment. The present study measured liver tissue hypoxia by directly using near-infrared spectroscopy (NIRS) and a novel tcpO2/pCO2 monitoring system. METHODS: Graded hypoxia was achieved in a rabbit model by a stepwise reduction of the fraction of inspired oxygen (FiO2) from 0.3 to 0.0. Animals were allowed to recover from hypoxia at FiO2 of 3.0 indicated by normalised arterial blood gas values. Hepatic tissue oxyhaemoglobin (HbO2), deoxyhaemoglobin (Hb), cytochrome oxidase (Cyt Ox), oxygen partial pressure (pO2) and carbon dioxide partial pressure (pCO2) were measured continuously with the help of NIRS and a Clark-type surface tcpO2/pCO2 monitoring system, throughout the period of hypoxaemia. RESULTS: There was an immediate reduction in hepatic HbO2 with hypoxia and a simultaneous increase in hepatic Hb. Similarly, hepatic tissue pO2 decreased significantly but tissue pCO2 remained unchanged until the FiO2 was below 0.1. Hepatic HbO2 showed a positive correlation with tissue pO2 (r = 0.53, P < 0.001). Hepatic Hb showed a negative correlation with tissue pO2 (r = 0.47, P < 0.001). Hepatic Cyt Ox decreased significantly with an FiO2 of 0.1 or less and showed a positive correlation with hepatic tissue pO2 (r = 0.64, P < 0.001). A significant correlation was found between hepatic tissue pO2 and arterial blood pO2 (r = 0.44, P < 0.001). Arterial blood pCO2 also correlated with hepatic tissue pCO2 (r = 0.53, P < 0.001) measured by the tcpO2/pCO2 monitoring system. CONCLUSION: The data from the present study suggest that, like NIRS, the tcpO2/pCO2 monitoring system can be reliably used for the direct monitoring of hepatic tissue oxygenation in vivo.
Both the ADP receptors P2Y1 and P2Y12, play a role in controlling shape change in human platelets.
Two types of ADP receptors, P2Y(1) and P2Y(12) are involved in platelet aggregation. The P2X(1) receptor is also present but its role, in terms of platelet function, is not yet defined. The aim of this study was to establish if the ADP receptors, P2Y(1,) P2Y(12) and P2X(1) play a role in controlling platelet shape change (PSC) in human platelets. PSC is an early phase of platelet activation that precedes aggregation. Using a high-resolution channelyzer, PSC was assessed by measuring the median platelet volume (MPV). The P2Y(1) receptor antagonist MRS 2179 (1.06 - 10.25 micro mol/l) blocked ADP-induced PSC (by 100%). The median IC(50) was 3.16 micro mol/l. MRS 2179 also significantly (P = 0.01) inhibited PSC induced by the combination of ADP + serotonin (5HT). The P2Y(12) receptor antagonist AR-C69931MX significantly inhibited (at 10s, P = 0.009; 15 s, P = 0.001 and 30 s, P = 0.015) ADP-induced PSC. The P2X(1) receptor antagonist TNP-ATP had no significant effect on ADP- or ADP + 5HT-induced PSC. We conclude that the IC(50) of a P2Y(1)-blocker can be derived because of the high-resolution and reproducibility of the channelyzer technique. In addition to the P2Y(1) purinoceptor, the P2Y(12)receptor appears to be involved in ADP-induced PSC since this process was significantly inhibited by AR-C69931MX. The channelyzer technique may be more reliable than optical aggregometry to assess PSC.
Influence of platelet count and activity on thromboelastography parameters.
It has been suggested that thromboelastography TEG) can help in limiting or directing the appropriate use of blood products during surgery. However, the contribution of platelets to the TEG profile has not been studied in detail. Blood was taken from eight healthy subjects and eight patients with peripheral arterial disease (PAD). Immunomagnetic separation was achieved by the addition of Dynabeads labeled with a CD41 murine antibody (to the GPIIb/IIIa receptor) to achieve 90-100% depletion of platelets from blood. This was then titrated with whole blood to achieve platelet counts of approximately 0, 25 and 50% of the original count to compare with whole blood using TEG. Platelet function was also assessed by spontaneous platelet aggregometry (SPA) at baseline and at the 50% dilution. SPA, maximum amplitude (MA) and K time were significantly different in PAD patients compared to controls (P < 0.05). In both controls and PAD patients there was a strong linear correlation between Log10 [platelet count] and MA (r = 0.97 for controls, r = 0.89 for PAD) and K time (r = -0.86 for controls, r = -0.68 for PAD). Correlation between Log10 [platelet count] and R time was poor in both groups. The MA and K TEG parameters may be most useful for assessing platelet transfusion requirements.
Mean platelet volume as an indicator of platelet activation: methodological issues.
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The epsilon 2 and 4 alleles of apolipoprotein E and ischemic vascular events in the Greek population--implications for the interpretation of similar studies.
The authors investigated whether apolipoprotein (apo) E polymorphism has an allelic and/or genotypic impact on the risk of an ischemic vascular event (IVE) in Greek patients with cardiovascular diseases (CVD). They compared apo E polymorphisms in 1) a group of 165 patients with IVE [IVE(+)], of whom 107 had survived a myocardial infarction and 58 an ischemic stroke; 2) a group of 165 patients, matched with the first group for age and gender, with angiographically confirmed coronary artery disease but without IVE [IVE(-)]; 3) a group of 240 healthy younger individuals with no family history of CVD. The apo epsilon2 allele was 5.2-fold less frequent in the IVE(+) group compared to the IVE(-) group (1.2% vs 6.2%, p = 0.001). The frequency of the epsilon2 allele in healthy subjects was 8.1%, which is 6.7-fold higher than in the IVE(+) group (p < 0.001), and more than twice as high compared to all CVD patients (p = 0.001). No significant differences in epsilon4 allele frequencies were observed between IVE(+) and IVE(-) patients (9.8% vs 8.4%) or between patients with CVD and healthy subjects (9.1% vs 10.2%). The epsilon4 allele was not associated with an increased risk for CVD or IVE. In contrast, an inverse and beneficial association of the epsilon2 allele with IVE was observed among Greek patients with CVD. These results suggest that the epsilon4 and epsilon2 alleles have a variable significance in terms of predicting the risk of vascular events in different populations. Therefore, it is important to carry out "local" studies.
Apolipoprotein E polymorphism and atherosclerosis.
The relationship between apolipoprotein (apo) E and vascular disease has been the subject of a considerable amount of research. However, this relationship is far from clearly defined. This deficiency appears to be due to a multitude of factors. Among these are differences in ethnicity, age (and possibly gender), diagnostic criteria, and environmental factors (eg, diet and smoking) that have contributed to the contradictory findings. Several diseases and their treatment may also influence this relationship. There are also documented interactions between apo E genotypes and other genes or vascular risk factors. One possible clinically relevant application of identifying the apo E genotype could be to assess the response to a particular drug treatment. It may also be that apo E polymorphism will become a good predictor of vascular death (eg, from myocardial infarction or stroke) rather than an indicator of the risk of developing vascular disease but without an acute ischemic event. More research is required to define the place of apo E genotyping in the management of vascular disease in its various forms. Whatever the future brings, the evaluation of apo E genotypes will need to be rapid, cheap, and technically undemanding before this investigation becomes widely available and clinically relevant.
The effect of a loading dose (300 mg) of clopidogrel on platelet function in patients with peripheral arterial disease.
Clopidogrel acts on the P2Y12 adenosine diphosphate (ADP) purinergic receptors on human platelets. The aim of this study was to establish if a loading dose of clopidogrel inhibits platelet activation in patients with peripheral arterial disease (PAD). Two indices of platelet activation were considered: platelet shape change (PSC) and aggregation. Citrated blood was collected from ten PAD patients who were not on aspirin, at baseline (0 hours) and 2 and 4 hours after these patients ingested a loading dose (300 mg) of clopidogrel. ADP (5 micromo/L)-induced platelet aggregation in whole blood was inhibited after 2 hours (free platelet count, 47% +/- 19% vs. 68% +/- 15%; p < or = 0.001) and 4 hours (47% +/- 19% vs. 66% +/- 16%; p < or = 0.001). There was also a significant inhibition of 5- hydroxytryptamine (SHT, 5.0 micromol/L)-induced platelet aggregation at 2 hours. This trend was also observed for 10-micomol/L ADP-induced aggregation. ADP (0.3-0.4 micromol/L)-induced PSC was significantly inhibited at 4 hours (increase in median platelet volume, 6.3%, 1.8-10.7 vs. 1.2%, 0-5.3; p = 0.01). 5HT (0.5 micromol/L)-induced PSC at 4 hours was also significantly inhibited (8.1, 5.3-10.6 vs. 3.0, 0-8.2; p = 0.03). A loading dose of clopidogrel (300 mg) inhibits platelet activation in PAD patients, as early as 2 hours. To the authors' knowledge, no other study considered the effect of a loading dose of clopidogrel in PAD.
Value of thromboelastography in the assessment of platelet function.
Thromboelastography (TEG) is a useful measure of coagulation. Modified TEG (that is with the addition of a GP IIb/IIIa receptor antagonist) has been used to assess the contribution of the fibrinogen-platelet interaction to TEG parameters (in particular the maximum amplitude, MA). Modified TEG was compared with other investigations of platelet function to assess its sensitivity in both normal subjects and in patients with peripheral arterial disease (PAD), a condition associated with activated platelets. Blood was collected from eight healthy subjects and 12 PAD patients. Platelet function was measured by TEG, flow cytometry (using PAC-1, P-selectin, GP IIIa and GP Ib murine antibodies) and platelet aggregometry (spontaneous and ADP-induced) in the presence and absence of tirofiban (a GP IIb/IIIa receptor antagonist). TEG showed a statistically significant reduction in MA with tirofiban at 0.4 mg/L and an increase in k (kinetic time; which indicates how fast clot strength is increasing once clotting starts) at 0.2 and 0.4 mg/L of tirofiban in both healthy subjects and PAD patients. Flow cytometry showed a significant decrease in the PAC-1 binding index (at 0.2 mg/L). This finding was compatible with the significant reductions found in spontaneous and ADP-induced platelet aggregation. However, aggregometry and flow cytometry were more sensitive indicators of platelet inhibition than the TEG parameters. TEG does not provide a comprehensive or sensitive reflection of impaired platelet function. If TEG is used as an index of severely impaired platelet function, we recommend that the k parameter should be used as well as MA. TEG should be supplemented by other methods of platelet function assessment wherever possible.
Platelets as predictors of vascular risk: is there a practical index of platelet activity?
Activated platelets play a role in the pathogenesis of coronary heart disease (CHD). Following activation, platelets change shape, aggregate, and release several bioactive substances. The aim of this review is to identify if there is a simple and cost-effective method that indicates platelet activation and predicts the risk of CHD and vascular events. The rationale for identifying high-risk patients is to reduce their risk of vascular events by administering appropriate and effective antiplatelet treatment, like aspirin, clopidogrel, or combination regimens. Many laboratory tests estimating platelet activity have been described. Some are relatively simple, such as spontaneous or agonist-induced platelet aggregation. Other tests include measuring the mean platelet volume (MPV) or plasma soluble P-selectin levels. Some more complex tests include flow cytometry to determine platelet GP IIb/IIIa receptors, platelet surface P-selectin, platelet-monocyte aggregates, and microparticles. Only few prospective studies assessed the predictive value of platelet activation in healthy individuals. Although the MPV seems an 'easy' method, there are insufficient data supporting its ability to predict the risk of a vascular event in healthy adults. Platelet aggregation, in whole blood or in platelet-rich plasma was not consistently predictive of vascular risk. Soluble P-selectin measurement is a promising method but it needs further evaluation. Flow cytometry methods are costly, time-consuming, and need specialized equipment. Thus, they are unlikely to be useful in estimating the risk in large numbers of patients. There is as yet no ideal test for the detection of platelet activation. Each currently available test has merits and disadvantages. Simple methods such as the MPV and the determination of platelet release products need further evaluation.
Early vascular benefits of statin therapy.
Large-scale trials established that statin administration in hypercholesterolaemic individuals and patients with coronary heart disease (CHD) significantly reduces the risk of vascular events and death. This benefit was primarily attributed to their actions on lipids. This review focuses on the benefits (clinical and experimental) of statins observed soon (approximately 12 weeks) after their administration. Statins rapidly increase nitric oxide production and improve endothelial function (e.g. increased flow-mediated dilatation). Similarly, antioxidant properties decrease the susceptibility of low density lipoprotein cholesterol to oxidation. Statins inhibit the migration of macrophages and smooth muscle cell proliferation leading to an antiproliferative effect and the stabilisation of atherosclerotic plaques. Anti-inflammatory effects include a reduction in serum C-reactive protein levels, inflammatory and proinflammatory cytokines (e.g. IL-6, IL-8), adhesion molecules (e.g. ICAM-1, VCAM-1) and other acute phase proteins. Statins influence the haemostatic system. They reduce tissue factor expression and platelet activity, whereas fibrinolysis can be enhanced. Statins improve microalbuminuria, renal function, hypertension and arterial wall stiffness. A significant reduction of the carotid intima media thickness (IMT) was also reported early after statin treatment. These early effects of statins probably contribute to the significant reduction in vascular events seen in some 'short-term' studies. There is a need to further elucidate the rapid and non-lipid lowering properties of statins.
Statins and renal function in patients with diabetes mellitus.
Lipids may adversely affect renal function. The recently published MRC/BHF Heart Protection Study (HPS) subgroup analysis showed that simvastatin significantly reduced the fall in glomerular filtration rate in high-risk patients with and without diabetes mellitus. These findings are in line with those of smaller earlier studies, including the GREek Atorvastatin and Coronary heart disease Evaluation (GREACE) Study. Lipid lowering trials need to consider that changes in renal function may occur. Renal and ischaemic heart disease may progress in parallel and statins may be beneficial to both organs.
Gamma-glutamyltransferase and vascular disease.
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The effects of lipid-regulating therapy on haemostatic parameters.
There is growing evidence that the components of the haemostatic system play a significant role in the development and progression of atherosclerosis and its complications. Lipid-lowering interventions have been associated with a significant reduction of morbidity and mortality. However, the improvement in cardiovascular risk seen in several clinical trials is incompletely explained by cholesterol reduction. Therefore, the benefit from lipid lowering drugs may involve non-lipid mechanisms. These include beneficial effects on the arterial wall, improved endothelial function and a favourable influence on blood rheology and thrombogenesis. In this review, we consider the influence of lipid-lowering interventions on rheological and haemostatic parameters as well as the potential clinical relevance of these effects.
Decreased urinary bladder apoptosis in a rabbit model of diabetes mellitus.
Urinary bladder dysfunction is a recognised complication of diabetes mellitus (DM). This is thought to be partly related to altered bladder morphology as DM is associated with increased bladder weight. In DM, increased cellular proliferation is well established. However, there is evidence that in other pathological states affecting the urinary tract, altered apoptosis may also play a role. We therefore used a rabbit model to investigate whether there are any changes in bladder apoptosis with DM. Diabetes was induced in adult New Zealand white rabbits. Age-matched controls were also used. After 6 months, the bladders were excised and weighed. The TUNEL technique was used to detect and quantify apoptosis in both DM and age-matched control bladders. Diabetes was confirmed as this group had significantly (P<0.001) elevated serum glucose-compared to controls. The bladder weights were also significantly (P<0.001) greater in the DM rabbits. Apoptosis was significantly (P<0.001) decreased in the urothelial cells of the DM bladders. Our results confirm previous findings that DM is associated with increased bladder weight. Although this is associated with increased cellular proliferation, we have demonstrated that decreased apoptosis may also play an important role. Therefore, decreased apoptosis may be important in the pathophysiology of DM cystopathy.
A review of the carotid and femoral intima-media thickness as an indicator of the presence of peripheral vascular disease and cardiovascular risk factors.
Peripheral vascular disease (PVD) is a common condition often associated with cardiovascular risk factors and events. With the aid of B-mode ultrasound scanning, evidence is emerging that these risk factors and events are significantly related to an increased carotid and femoral intima-media thickness (IMT). More importantly, treatment of these risk factors is associated with a decrease or a diminished progression of the IMT, paralleled by a reduction in cardiovascular events and an improvement in the symptoms associated with PVD. This evidence is particularly strong for lipid lowering therapy. Additional predictors of cardiovascular risk like the IMT, could now influence the decision to intervene with medication.
A cross-sectional study of the effects of hormon replacement therapy on the cardiovascular disease risk profile in healthy postmenopausal women.
OBJECTIVE: To assess risk factors for cardiovascular disease in healthy postmenopausal women who had been uninterruptedly on menopausal hormone replacement therapy (HRT) for at least 5 years or who had not received any HRT. DESIGN: Cross-sectional study. SETTING: The Royal Free Hospital and The Middlesex Hospital. PATIENT(S): A total of 256 healthy postmenopausal women were analyzed: 73 were taking tibolone, 60 were taking transdermal E(2), 58 were taking conjugated equine estrogens (E), and 65 were not taking any menopausal therapy. INTERVENTION(S): Cardiovascular disease risk factors measurement. MAIN OUTCOME MEASURE(S): Total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides, lipoprotein(a), insulin, glycated hemoglobin, high sensitivity C-reactive protein, fibrinogen, total antioxidants, and endothelin-1. RESULT(S): The different types of HRT induced disparate changes in the various markers of cardiovascular disease. Significantly higher high sensitivity C-reactive protein concentrations were found in women receiving conjugated equine E and tibolone than in women who were not taking any therapy. Glycated hemoglobin was significantly lower in women receiving transdermal E(2) and tibolone compared to women not on HRT. Women on tibolone had significantly higher systolic blood pressure. CONCLUSION(S): Because high sensitivity C-reactive protein has recently emerged as an important predictor of cardiovascular disease, the higher high sensitivity C-reactive protein levels observed in women on conjugated equine estrogens and on tibolone have potential important clinical implications.