Platelet function, platelet prostanoids and vascular prostacyclin in diabetes mellitus

THE EFFECT OF DIABETES ON CARDIOVASCULAR SYSTEM

Diabetes mellitus (DM) is a critical and long-term disorder due to the insufficient production of insulin by the pancreas or ineffective use of insulin by the body. Importantly, cardiovascular disease (CVD) has long been thought to be linked with diabetes. Despite more diabetic individuals surviving from better medications and treatments, there has been significant rise in the morbidity and mortality from CVD. Indeed, the classification of DM based on the electrocardiogram signals of the heart will be an advantageous system. Further, computer-aided classification of DM with integrated algorithms may enhance the execution of the system. In this paper, we have reviewed various studies using heart rate variability signals for automated classification of diabetes. Furthermore, the different techniques used to extract the features and the efficiency of the classification systems are discussed.

Hepatic protein tyrosine phosphatase 1B (PTP1B) deficiency protects against obesity-induced endothelial dysfunction

Growing evidence suggests that hepatic-insulin resistance is sufficient to promote progression to cardiovascular disease. We have shown previously that liver-specific protein-tyrosine-phosphatase 1B (PTP1B) deficiency improves hepatic-insulin sensitivity and whole-body glucose homeostasis. The aim of this study was to investigate the impact of liver-specific PTP1B-deficiency (L-PTP1B-/-) on cardiac and peripheral vascular function, with special emphasis on endothelial function in the context of high-fat diet (HFD)-induced obesity. L-PTP1B-/- mice exhibited an improved glucose and lipid homeostasis and increased insulin sensitivity, without changes in body weight. HFD-feeding increased systolic blood pressure (BP) in both L-PTP1B-/- and control littermates; however, this was significantly lower in L-PTP1B-/- mice. HFD-feeding increased diastolic BP in control mice only, whilst the L-PTP1B-/- mice were completely protected. The analysis of the function of the left ventricle (LV) revealed that HFD-feeding decreased LV fractional shortening in control animals, which was not observed in L-PTP1B-/- mice. Importantly, HFD feeding significantly impaired endothelium-dependent vasorelaxation in response to acetylcholine in aortas from control mice, whilst L-PTP1B-/- mice were fully protected. This was associated with alterations in eNOS phosphorylation. Selective inhibition of COX-2, using NS-398, decreased the contractile response in response to serotonin (5-HT) only in vessels from control mice. HFD-fed control mice released enhanced levels of prostaglandin E, a vasoconstrictor metabolite; whilst both chow- and HFD-fed L-PTP1B-/- mice released higher levels of prostacylin, a vasorelaxant metabolite. Our data indicate that hepatic-PTP1B inhibition protects against HFD-induced endothelial dysfunction, underscoring the potential of peripheral PTP1B inhibitors in reduction of obesity-associated cardiovascular risk in addition to its anti-diabetic effects.

Effect of omega-3 fatty acids on haemostatic functions in urocortin-treated obese rats

Urocortin 1 (UCN1) decreases food intake. We investigated the effects of UCN1 and omega-3 fatty acids (FA) on metabolic and coagulation parameters in high fat diet (HFD)-fed rats. Fifty male Sprague Dawley rats were divided into five groups; control, HFD, HFD with omega-3 FA, HFD with UCN1, and HFD with UCN1 and omega-3 FA. Food intake, body weight (BW), body mass index (BMI), Lee index, glucose, insulin, HOMA-IR, triglycerides, cholesterol, low (LDL) and high (HDL) density lipoproteins, fibrinogen, plasminogen activator inhibitor 1 (PAI-1), fibrin degradation product (FDP), clotting time, bleeding time, prothrombin time (PT), activated partial thromboplastin time (aPTT), and platelet aggregation were measured. Food intake, BW, BMI, Lee index, glucose, insulin, HOMA-IR, triglycerides, cholesterol, LDL, fibrinogen, platelet aggregation, PAI-1, and FDP increased while bleeding and clotting times, PT, and aPTT decreased in HFD rats. UCN1 decreased food intake, BW, BMI, Lee index, bleeding and clotting times, PT, and aPTT and increased fibrinogen, PAI-1, FDP, and platelet aggregation in HFD rats. Omega-3 FA decreased food intake, BW, BMI, Lee index, platelet aggregation, glucose, insulin, HOMA-IR, triglycerides, and increased HDL and bleeding time in HFD rats. We concluded that UCN1 worsens the hypercoagulable state in HFD rats while omega-3 FA improve the insulin resistance and decrease the platelet aggregation in those rats.

Surface expression of collagen receptor Fc receptor-gamma/glycoprotein VI is enhanced on platelets in type 2 diabetes and mediates release of CD40 ligand and activation of endothelial cells

Diabetes is associated with an enhanced collagen-mediated platelet activation that contributes significantly to thromboischemic complications. In this study, the platelet collagen receptor glycoprotein VI (GPVI) was studied in 385 patients with type 2 diabetes. Surface expression of the platelet Fc receptor that forms a functional complex with GPVI was significantly increased in patients with diabetes compared with those without diabetes (P = 0.02). Fc receptor expression correlated with GPVI expression and was found to be independently associated with diabetes (r = 0.529, P < 0.001). Stimulation of GPVI through a specific anti-GPVI monoclonal antibody significantly enhanced surface expression of CD40L (P = 0.006). Because CD40L is a potent platelet-derived cytokine that is involved in thrombosis and atherosclerosis, we evaluated the effect of GPVI-mediated release of CD40L on activation of endothelial cells. Coincubation of GPVI-stimulated platelets resulted in substantial enhanced endothelial surface expression of CD62P, alphavbeta3, and intercellular adhesion molecule 1 (P < 0.05) and secretion of monocyte chemoattractant protein 1 of cultured human umbilical vein endothelial cells (P < 0.01). These results suggest that the function of collagen receptor GPVI is altered in type 2 diabetes and may play an important role in atherothrombotic complications. Inhibition of GPVI may be a promising pharmacological target in the treatment of high-risk diabetic patients.

Endothelial dysfunction and hypertension in diabetes mellitus

Although the net balance between vasoconstriction and vasodilation determines the tone of the blood vessel, the vasodilatory-vasoconstrictive response following a challenge may also be determined by the intrinsic mechanical and biological properties of the vascular smooth muscle. This article reviews the action of nitric oxide, acetylcholine,and insulin; the effects of hyperglycemia, increase in free fatty acids, diabetes, and obesity on the vasculature; treatment of hypertension in diabetes; the benefits of lowering blood pressure and intensity of blood pressure control; and compares specific antihypertensive therapies on cardiovascular outcomes. The effects of antihypertensive therapy on microvascular complications, choice of antihypertensive regimen, antioxidant and anti-inflammatory effects of antihypertensive drugs, and the potential role of antidiabetic drugs in the treatment of hypertension are also presented.

Vascular reactivity and thiazolidinediones

Endothelial dysfunction is a critical factor in the development of vascular disease in patients with diabetes mellitus. Maintenance of the vascular tone and luminal diameter of a blood vessel is dependent on the net balance of vasoconstrictor and vasodilator forces. In both diabetes and obesity, vascular reactivity is abnormal. After ischemia, carbon dioxide challenge, thermal challenge, or exercise, individuals with diabetes do not exhibit the increase in blood flow or vasodilation observed in persons without diabetes. The mechanisms involved in abnormal reactivity may include both the endothelium and vascular smooth muscle. Major vasodilator factors that act on vascular smooth muscle cells are nitric oxide, prostacyclin, and hyperpolarizing factor. The main vasoconstrictors are endothelin, angiotensin II, norepinephrine, serotonin, and thromboxane A(2). In patients with diabetes, there is an increase in vasoconstrictors and a decrease in vasodilators. Thiazolidinediones (TZDs) improve vasodilative responses, which may be of importance in the treatment of vascular disease. The TZDs have anti-inflammatory effects and suppress free fatty acids and reactive oxygen species at the endothelial level, which may contribute to the improved vascular reactivity observed in patients treated with these agents. In addition, these effects of TZDs may have implications for reducing the incidence and severity of atherosclerosis in the long term.

The role of platelets in peripheral vascular disease

Platelets play a major role in acute ischaemic syndromes and peripheral vascular disease. They are involved in the development and progression of atherosclerosis, native vessel and graft thrombosis. They have a central role in the development of restenosis and reocclusion after peripheral percutaneous transluminal angioplasty. Antiplatelet therapy has been shown to be beneficial in patients undergoing peripheral vascular surgery or radiological intervention. Yet current routine therapy, namely aspirin and dipyridamole are limited in their mode of action and efficacy. Recent developments in the understanding of platelet function has led to the development of new more potent drugs such as clopidogrel. Combination of drugs and more specific investigation of individual platelet function may well result in improved bypass and angioplasty patency rates. The results of proposed large randomised controlled trials on the role and safety of aspirin and clopidogrel are awaited with interest. Given the importance of platelets in peripheral vascular disease highlighted in this review, achieving an optimal safe anti-platelet effect for each patient with peripheral vascular disease should be the target of future research.

Relationship between thromboxane/prostacyclin ratio and diabetic vascular complications

To elucidate the relationship between the thromboxane A2/prostacyclin (TXA2/PGI2) ratio and diabetic complications, the levels of 11-dehydro-thromboxane B2 and 2,3-dinor-6-keto-prostaglandin F1alpha, the urinary metabolites of thromboxane A2 and prostacyclin, were measured in diabetics by gas chromatography/selected ion monitoring. We compared the TXA2/PGI2 ratio in healthy volunteers and diabetics. The TXA2/PGI2 ratio of diabetics was significantly higher than that of healthy volunteers and we could reconfirm the hypercoagulable condition in diabetics. We also investigated the difference of TXA2/PGI2 levels in diabetics with retinopathy and neuropathy. The TXA2/PGI2 ratio of diabetics with retinopathy showed significantly higher level than without retinopathy. However, the TXA2/PGI2 ratio of diabetics with neuropathy was the same as without neuropathy. These results suggest that the TXA2/PGI2 ratio reflects the pathological conditions of diabetes, especially the change of vasculature. The monitoring and improvement of TXA2/PGI2 ratio could be useful for the prevention of diabetic vascular complications.

Novel platelet antiaggregating substances

We describe herein a novel action of four stable analogs of the hepoxilins. These analogs inhibit to different degrees, the aggregation of washed human platelets evoked by collagen. One of the analogs, PBT-3, is particularly effective with an IC(50) = 8 x 10(-7) M. The other analogs are about 5-fold less active, but all analogs are about 500-fold more active than the native hepoxilins. The PBT analogs inhibit the collagen-enhanced formation of thromboxane A(2) and HHT but do not affect the formation of 12-HETE or the release of arachidonic acid except at doses higher than those needed to block platelet aggregation. These results demonstrate that these novel compounds may have potential for development into drugs in the treatment of thromboxane-mediated cardiovascular disease.

Hyperglycemia potentiates collagen-induced platelet activation through mitochondrial superoxide overproduction

Alteration of platelet function contributes to microthrombus formation and may play an important role in the pathogenesis of diabetic micro- and macroangiopathies. However, the molecular mechanism for platelet dysfunction observed in patients with diabetes has not been fully elucidated. In this study, the direct effects of hyperglycemia on platelet function in vitro were investigated. Hyperglycemia increased reactive oxygen species generation in human platelets, and this effect was additive with that of collagen. Thenoyltrifluoroacetone (TTFA), an inhibitor of mitochondrial electron transport chain complex II, and carbonyl cyanide m-chlorophenylhydrazone (CCCP), an uncoupler of oxidative phosphorylation, completely prevented the effects of hyperglycemia, suggesting that reactive oxygen species arise from the mitochondrial electron transport chain. Hyperglycemia potentiated both platelet aggregation and the subsequent release of platelet-derived growth factor AB induced by a nonaggregating subthreshold concentration of collagen, which were also completely inhibited by TTFA or CCCP. Furthermore, hyperglycemia was found to inhibit protein tyrosine phosphatase (PTP) activity and increase phosphorylation of the tyrosine kinase Syk in platelets exposed to collagen. Hyperglycemia-induced PTP inhibition and Syk phosphorylation were found to be completely prevented by TTFA, CCCP, or Mn(III)tetrakis (4-benzoic acid) porphyrin, a stable cell-permeable superoxide dismutase mimetic. These results suggest that hyperglycemia-induced mitochondrial superoxide generation may play an important role in platelet dysfunction observed in patients with diabetes.

Effect of DT-TX 30, a combined thromboxane synthase inhibitor and thromboxane receptor antagonist, on retinal vascularity in experimental diabetes mellitus

Combined thromboxane synthase inhibitors and thromboxane receptors antagonists have been shown to have a beneficial effect on different models of thrombosis in vivo. We studied the action of one of these compounds (DT-TX 30) compared with dazoxiben (a thromboxane synthase inhibitor) on retinal vascularity in streptozotocin-diabetic rats. Ten nondiabetic animals were treated with isotonic saline, 30 (10 animals per group) were given 0.4, 4, and 8 mg kg(-1) per day of DT-TX 30 (p.o.) and 30 (10 animals per group) were given 10, 50, and 100 mg kg(-1) per day of dazoxiben (p.o.) over a 90-day study period. DT-TX 30 caused a dose-dependent decrease of platelet aggregation and thromboxane B2 synthesis. There was an increase of 9, 65, and 166% in the synthesis of prostacyclin after treatment with 0.4, 4, and 8 mg kg(-1) per day, respectively. Retinal vascularity increased in 51, 72, and 182% of animals in response to the three doses used. Synthesis of prostacyclin and the degree of retinal vascularity showed a linear correlation (r2=0.6528,p<0.00001). Dazoxiben, at doses that inhibited thromboxane synthesis as much as DT-TX 30, increased prostacyclin production and retinal vascularity with less potency than DT-TX 30. In conclusion, the antagonism of thromboxane receptors may be an important additional effect to the inhibition of thromboxane synthase in the prevention of ischemic retinal lesions in experimental diabetes.

Differential alterations of spontaneous and stimulated 45Ca(2+) uptake by platelets from patients with type I and type II diabetes mellitus

Diabetes mellitus (DM) is associated with hyperaggregability of platelets. Although the mechanisms underlying this abnormality remain unknown, Ca(2+) imbalance has been implicated. Both activators (alpha-adrenoceptor agonists, collagen, and ADP) and inhibitors (beta-adrenoceptor agonists, iloprost and dibutyryl cAMP) of platelet function, respectively, elicit the uptake of [45Ca(2+)] in human platelets. It was determined that the [45Ca(2+)] uptake methods employed reflected signal transduction events at the plasma membrane rather than absolute changes of Ca(2+) fluxes or levels of cytosolic Ca(2+). In the present study, basal (unstimulated) [45Ca(2+)] uptake by platelets from both type I and type II diabetic patients was significantly enhanced when compared to age-matched controls. When basal values were subtracted from stimulated values, there were highly significant decreases in [45Ca(2+)] uptake in platelets from type I diabetic patients compared to controls when stimulated with adrenaline, isoprenaline, noradrenaline, collagen, A23187, or iloprost. In contrast, when basal values were subtracted from stimulated values there were significant increases in [45Ca(2+)] uptake by platelets from type II diabetic patients when stimulated with adrenaline, isoprenaline, noradrenaline, A23187, iloprost, and collagen. It is concluded that in type I and type II DM there are differential alterations in [45Ca(2+)] sequestration linked to inhibitors and stimulators of platelet activation. These data indicate that the hyperaggregability of platelets that is associated with both type I and type II DM may be due to an aetiology other than Ca(2+) mobilization linked to signal transduction.

The role of prostaglandins in the aetiology and treatment of erectile dysfunction

Both animal and human penile tissue synthesize prostaglandins (PGs). Furthermore, intracavernous injection of certain PGs elicits erection in men with erectile dysfunction (ED). It is also well established that PGs are involved in the pathophysiology of atherosclerosis and diabetes mellitus (DM). Since atherosclerosis and DM are major risk factors for ED, it has been suggested that the disruption of PG synthesis in penile tissues and related vasculature may play a role in the pathogenesis of ED. In this review, we discuss the role of PGs in normal penile erection as well as on the pathophysiology and treatment.

Effects of aspirin treatment on survival in non-insulin-dependent diabetic patients with coronary artery disease. Israeli Bezafibrate Infarction Prevention Study Group

PURPOSE

The benefit of aspirin treatment among diabetic patients with chronic coronary artery disease is not well established. The purpose of this study was to assess the effect of aspirin on cardiac and total mortality in a large cohort of diabetic patients with established coronary artery disease and to compare it with the effect of aspirin in nondiabetic counterparts.

PATIENTS AND METHODS

In this observational study among patients screened for participation in the Bezafibrate Infarction Prevention Study, the effects of aspirin treatment in 2,368 non-insulin-dependent diabetic patients with coronary artery disease were compared to those in 8,586 nondiabetic patients. Hazard ratios (HR) and 95% confidence intervals (CI) were estimated with proportional hazards models.

RESULTS

Fifty-two percent of diabetic patients and 56% of nondiabetic patients reported aspirin therapy. After 5.1 +/- 1.3 (mean +/- SD) years of follow-up, the absolute benefit per 100 patients treated with aspirin was greater in diabetic patients than in nondiabetic patients (cardiac mortality benefit: 5.0 versus 2.1, and all-cause mortality benefit: 7.8 versus 4.1). Overall cardiac mortality among diabetic patients treated with aspirin was 10.9% versus 15.9% in the nonaspirin group (P < 0.001), and all-cause mortality was 18.4% and 26.2% (P < 0.001). After adjustment for possible confounders, treatment with aspirin was an independent predictor of reduced overall cardiac (HR = 0.8; 95% CI: 0.6-1.0) and all-cause mortality (HR = 0.8; 95% CI: 0.7-0.9) among diabetic patients, similar to those in nondiabetic patients.

CONCLUSION

Treatment with aspirin was associated with a significant reduction in cardiac and total mortality among non-insulin-dependent diabetic patients with coronary artery disease. The absolute benefit of aspirin was greater in diabetic patients than in those without diabetes.

Effect of WEB 2086-BS, an antagonist of platelet-activating factor receptors, on retinal vascularity in diabetic rats

Specific antagonists of platelet-activating factor (PAF) receptors inhibit platelet aggregation and thromboxane synthesis. These two processes have been implicated in the course of diabetic retinopathy. We assessed the effect of a specific PAF receptor antagonist, WEB 2086-BS (3-(4-(2-chlorophenyl)-9-methyl-6H-thieno(3,2-f) (1,2,4 triazolo-(4,3-a(1,4)-diazepine-2-yl)-1-(4-morpholinyl)-1-propanone) on retinal vascularity in a model of experimental streptozocin-induced diabetes in rats. Rats were divided into five experimental groups (10 animals/group): group I, non-diabetic group II, untreated diabetic group III, diabetic given 1 mg/kg per day of WEB 2086-BS (p.o.) group IV, diabetic given 5 mg/kg per day (p.o.) and group V, diabetic given 10 mg/kg per day (p.o.). After 3-month treatment, platelet aggregometry, platelet synthesis of thromboxane B2, aortic production of 6-keto-prostaglandin F1alpha, platelet and vascular lipid peroxidation, and percentage of the retinal area occupied by horseradish peroxidase-labeled vessels were measured. Untreated diabetic rats showed an increase in platelet reactivity, reduced 6-keto-prostaglandin F1alpha production, increased thromboxane B2 and lipid peroxides, and a decrease in the percentage of retinal area occupied by horseradish peroxidase-labeled vessels. WEB 2086-BS produced a decrease in platelet aggregation induced by collagen in whole blood, in thromboxane B2 synthesis and lipid peroxide production, and an increase in the percentage of retinal area occupied by horseradish peroxidase-labeled vessels (13.9+/-1.1% in group II and 9.9+/-0.8% in group V). There was a statistically significant linear correlation (Y= -0.72 + 137X, r2 = 0.7247, P < 0.0007) between thromboxane B2 values and the percentages of retinal area occupied by horseradish peroxidase-labeled vessels in the groups of animals treated with WEB 2086-BS.

Effect of camonagrel, a selective thromboxane synthase inhibitor, on retinal vascularization in experimental diabetes

Platelet hyperactivity accompanied by an increased synthesis of thromboxane and/or a decreased prostacyclin production are important factors in ischemic diabetic retinopathy. We studied the effect of camonagrel and dazoxiben, two thromboxane synthase inhibitors, on retinal vascularization in a model of streptozotocin-induced diabetes in rats. Ten nondiabetic rats, 10 diabetic animals treated with saline (i.e., not treated), and 60 diabetic animals treated with dazoxiben or camonagrel (10, 50 or 100 mg kg(-1) day(-1) p.o.) were studied. All treatments lasted for 90 days. Dazoxiben and camonagrel produced a dose-dependent reduction in platelet aggregation and thromboxane synthesis. Dazoxiben increased prostacylin synthesis by 78% at 100 mg kg(-1) day(-1), and camonagrel by 154%. Dazoxiben increased retinal vascularity by 74%, and by 183% after camonagrel treatment. Prostacyclin synthesis showed a direct linear correlation with the degree of retinal vascularization (r2=0.6733, P < 0.00001). We conclude that an increased prostacyclin synthesis may have a greater influence than the inhibition of thromboxane synthesis in preventing ischemic diabetic retinopathy in experimental diabetes. Camonagrel may be an alternative treatment in the prevention of these lesions.

Hyperactivity and increased hydrogen peroxide formation in platelets of NIDDM patients

Free radical activity may contribute to atherosclerotic lesions which in diabetic subjects may frequently lead to vascular complications. It is known that oxidative stress is associated to diabetes. Protein glycation and glucose oxidation could be possible source of free radicals. 28 non insulin dependent diabetic subjects (NIDDM) were examined. 20 healthy subjects matched for age, sex and for the presence of hypertension and hyperlipidemia were also studied. Hydrogen peroxide, measured by intracellular levels of the fluorescent 2,7-dichloro-fluorescein (DCF), was considered as indicative parameter of free radical production. The results showed that in resting platelets the basal level of hydrogen peroxide was significantly higher in diabetic subjects than in controls. Moreover, after stimulation with thrombin, collagen, phorbol myristate acetate (PMA) and platelet activating factor (PAF), platelets of diabetic subjects generated significantly higher amounts of hydrogen peroxide than controls. Moreover, platelet aggregation induced by adenosine 5'-diphosphate (ADP) and plasma beta TG levels were higher in diabetics than in controls. In diabetic patients platelet free radical production and functional activity are increased and therefore could play a role in the elevated thrombotic risk described in diabetes.

Vascular reactivity and platelet aggregability during the first five years of insulin-dependent diabetes in children

Skin microvascular reactivity and platelet aggregation in response to collagen and adenosine diphosphate (ADP) was studied prospectively in a population-based cohort of children with newly acquired type 1 diabetes mellitus (IDDM), who have now been followed up longitudinally for 5 years. The skin microvascular vasodilation in response to ischaemia was recorded by means of transcutaneous oximetry at 37 degrees C and compared with that in a control group of 58 healthy children. Platelet aggregation was compared with 42 healthy control children. Sixty months after diagnosis, the same degree of impairment of vasodilatory capacity was noted as previously recorded at 24 months and on admission, on all these occasions being significantly lower than the control value and the results from the 12-month follow-up. A sex difference was noted, diabetic girls both during and after puberty exhibiting a greater impairment of vasodilator capacity as compared with diabetic boys. The same degree of platelet aggregation in response to collagen was noted 60 months after diagnosis as had been recorded previously at 24 and 12 months, all significantly increased as compared with the results from admission, but not different from those in controls. By contrast, the aggregation in response to ADP was now lower than that observed on admission. No statistically significant gender difference was noted. The implication of the impaired skin microvascular vasodilation and of the changing pattern of platelet aggregation for later diabetic angiopathy needs to be evaluated in the coming decade.

Platelets from patients with diabetes mellitus have impaired ability to mediate vasodilation

OBJECTIVES

The purpose of this study was to examine vasomotor responses mediated by platelets from patients with diabetes mellitus.

BACKGROUND

Diabetes mellitus is associated with increased cardiovascular morbidity and mortality, which in part may be due to a variety of abnormalities reported in diabetic platelets. However, the effects of diabetic platelets on vasomotor tone have not been characterized.

METHODS

We compared platelet-mediated vasodilation elicited by platelets isolated from 30 healthy volunteers and 29 patients with diabetes mellitus as they were perfused through a preconstricted normal rabbit carotid artery.

RESULTS

Platelets from the diabetic patients mediated an impaired dilatory response in comparison with normal platelets: 2.7 +/- 2% versus 15.8 +/- 3.4% (p < 0.001) and 4.1 +/- 2.7% versus 32.7 +/- 3.3% (p < 0.001) (mean +/- SEM) increase in vessel diameter, for 5 X 10(7) and 1 X 10(8) platelets per milliliter perfused, respectively. The degree of impairment was similar for type I (insulin-dependent) and type II (non-insulin-dependent) diabetes mellitus. Normal platelets incubated in high D-glucose concentrations lost their ability to mediate dilation in a concentration-dependent and time-dependent manner. This was not true for incubation of normal platelets in high concentrations of L-glucose or insulin. However, there was not a significant correlation between glucose control in the diabetic patients and the ability of their platelets to mediate vasodilation.

CONCLUSIONS

Platelets from patients with diabetes mellitus have an impaired ability to mediate vasodilation. This impairment appears to be mediated by high glucose concentration. Further work is needed to elucidate the mechanisms for this abnormality in diabetic platelets.

Platelet turnover in advanced diabetes

There is considerable information to indicate that when there is vessel injury, platelets can play a major role in the development of atherosclerosis and its thromboembolic complications in the non-diabetic population. Diabetes is a disease associated with an increased risk of vascular complications, but the nature of this association is unclear. Platelets from diabetic humans and animals are hypersensitive to agonists in vitro, but it is unclear if this hypersensitivity persists in vivo. Platelets survival and turnover in the circulation have been used as indicators of platelet behaviour in vivo, but platelet survival measurements do not necessarily correlate with platelet sensitivity in vitro and these measurements may be too insensitive to reflect platelet turnover on injured vessels or in thrombi, unless there is extensive vascular disease. Studies in rats and rabbits indicate that diabetes causes increased platelet interaction with experimentally-injured vessels and accumulation in thrombi. It is unclear, however, whether this altered behaviour of platelets at the injured vessel surface reflects changes in the platelets or in the vessel wall and whether it contributes to the increased risk for vascular complications in diabetic patients.

Prevention of platelet dysfunction by vitamin E in diabetic atherosclerosis

Premature atherosclerosis and other vascular disorders are serious complications of diabetes mellitus. Contributing factors include (i) increased peroxidation of LDL leading to foam cell formation, fatty streaks and plaque formation in the arterial wall, and (ii) hyperreactivity of blood platelets leading to increased platelet adhesion and aggregation. Vitamin E may play a protective role as an antioxidant and/or membrane stabilizing agent in either mechanism. In platelets it appears to regulate arachidonic acid metabolism. Decreased vitamin E levels in platelets are associated with increased aggregation. This is reversible by correction of the vitamin E status. In diabetics, platelet vitamin E levels tend to be reduced with concomitant increase in platelet aggregation. Several studies in patients with insulin-dependent diabetes mellitus and, to some extent, in those with non-insulin-dependent diabetes mellitus have shown that supplementation with several hundred IU vitamin E significantly reduced platelet aggregation and lipid peroxidation. In healthy volunteers high-dose supplementation had no notable effect on platelet aggregation. However, doses as low as 200 IU vitamin E significantly reduced platelet adhesion and inhibited the formation of protruding pseudopods typically occurring in activated platelets. In diabetic patients a decrease in the nonenzymatic glycation of proteins by vitamin E supplementation has been observed. Controlled studies are needed to confirm the effect of vitamin E on platelet function in well-defined groups of diabetics, followed by large-scale trials investigating the prevention of diabetic vascular complications as clinical end point.

The role of protein phosphorylation in the regulation of cyclic nucleotide phosphodiesterases

The cyclic nucleotide phosphodiesterases constitute a complex superfamily of enzymes responsible for catalyzing the hydrolysis of cyclic nucleotides. Regulation of cyclic nucleotide phosphodiesterases is one of the two major mechanisms by which intracellular cyclic nucleotide levels are controlled. In many cases the fluctuations in cyclic nucleotide levels in response to hormones is due to the hormone responsiveness of the phosphodiesterase. Isozymes of the cGMP-inhibited, cAMP-specific, calmodulin-stimulated and cGMP-binding phosphodiesterases have been demonstrated to be substrates for protein kinases. Here we review the evidence that hormonally responsive phosphorylation acts to regulate cyclic nucleotide phosphodiesterases. In particular, the cGMP-inhibited phosphodiesterases, which can be phosphorylated by at least two different protein kinases, are activated as a result of phosphorylation. In contrast, phosphorylation of the calmodulin-stimulated phosphodiesterases, which coincides with a decreased sensitivity to activation by calmodulin, results in decreased phosphodiesterase activity.

Elevated glucose alters A23187-induced release of arachidonic acid from porcine aortic endothelial cells by enhancing reacylation

Cultured porcine aortic endothelial cells were conditioned in normal (5.2 mM) and elevated (15.6 mM) glucose, prelabeled with [14C]arachidonic acid and stimulated with ionophore A23187. Elevated glucose cultures released less radiolabeled products and less [14C]arachidonic acid. Analysis of cellular lipids revealed that elevated glucose reduced net loss of radiolabel from diacylphosphatidylethanolamine, did not affect early phosphatidylinositol hydrolysis, and increased net loss from diacylphosphatidylcholine and alkenylacylphosphatidylethanolamine. Uptake of radiolabel upon stimulation was examined to measure the role of reacylation on the diminished net release of radiolabel in elevated glucose cultures. Enhanced acylation of [3H]arachidonic acid into cellular lipids, especially PI, was observed in stimulated and resting cultures with elevated glucose. Further, pretreatment of the cultures with an acyltransferase inhibitor, thimerosal, prior to A23187 stimulation in radiolabeled cultures, abolished the effects of glucose on eicosanoid and arachidonic acid release. Differences in the ionophore-induced net loss of radiolabel from diacylphosphatidylethanolamine and phosphatidylinositol of the two glucose treatments were diminished by thimerosal exposure, while net loss of radiolabel from diacylphosphatidylcholine and alkenylacylphosphatidylethanolamine were unaffected. The data indicate that elevated glucose alters deacylation and enhances reacylation of arachidonic acid into endothelial cells and particularly into phosphatidylinositol. Enhanced reacylation may explain some of the altered lipid pathways that have been observed in experiments that elevate glucose concentrations or involve diabetes.

'Spontaneous' platelet aggregation in whole blood in diabetic patients with and without microvascular disease

Consistent abnormalities of agonist-induced platelet aggregation, in either whole blood or platelet rich plasma, have not been demonstrated in diabetic patients without microvascular disease. In the present study platelet aggregation in the absence of exogenous agonists ('spontaneous' aggregation) was compared between 22 non-diabetic subjects and 23 Type 1 diabetic patients with (n = 12) and without (n = 11) microvascular disease. 'Spontaneous' aggregation was determined by measuring the percentage fall in single platelet number in aliquots of whole blood shaken for 60 min. Diabetic patients without microvascular disease had fewer single platelets remaining (greater aggregation) than non-diabetic subjects at all time-points (69.7 +/- 6.6 vs 82.3 +/- 7.3% at 60 min p less than 0.001), but more platelets remaining than in diabetic patients with microvascular disease at all time-points (69.7 +/- 6.6 vs 61.0 +/- 7.8% at 60 min p less than 0.02). No significant correlations were observed between platelet aggregation and plasma glucose, blood cell counts, or glycated haemoglobin levels. The study suggests that platelet abnormalities antedate the appearance of microvascular disease in diabetic patients.

Thromboxane production and platelet aggregation in type 2 diabetes mellitus without vascular complications

Diabetic individuals frequently have platelet hyperaggregability and increased thromboxane (TXB2) production. To evaluate whether improvement of metabolic control or changes in fatty acid composition of serum lipids might alter thromboxane (TXB2) formation and platelet function, we followed up 25 newly diagnosed type 2 diabetics without angiopathy for about 6 months. Improvement of metabolic control (HbA1, fell from 12.0 +/- 0.3 to 9.0 +/- 0.3%; p less than 0.01) was associated with significant decrease in total cholesterol, triglycerides, and ratios of total cholesterol/HDL-cholesterol and LDL-cholesterol/HDL-cholesterol. Palmitic acid of phospholipids decreased significantly, whereas eicosapentaenoic acid increased. Regardless of this, the ADP-induced platelet aggregability and sensitivity were not altered. There was no effect whatever on the TXB2 synthesis capacity of clotting whole blood (204.9 +/- 25.0 vs 222.8 +/- 32.0 ng/ml) over 6 months of treatment. Platelet aggregability and TXB2 formation were not correlated to the degree of metabolic control, nor were there any correlations to serum lipids and their fatty acid composition. Thus, we are tempted to speculate that glucose metabolism in diabetes itself does not affect platelet aggregation or TXB2 formation in type 2 diabetes mellitus.