Kristian Rett

Endothelial Dysfunction Is Detectable in Young Normotensive First-Degree Relatives of Subjects With Type 2 Diabetes in Association With Insulin Resistance

BACKGROUND Endothelial dysfunction (ED) is regarded as an early step in the development of atherosclerosis. Among the pathogenetic factors leading to atherosclerosis, the role of insulin resistance and hyperinsulinemia as independent risk factors is still under debate. In this study, we examined the association between ED and insulin resistance in normotensive and normoglycemic first-degree relatives (FDRs) of patients with type 2 diabetes mellitus (DM). METHODS AND RESULTS Endothelium-dependent and -independent vasodilation of the brachial artery was measured with high-resolution ultrasound (13 MHz) in 53 normotensive FDRs (21 men, 32 women; mean age, 35 years) with normal oral glucose tolerance, 10 age- and sex-matched normal control subjects, and 25 DM patients (mean age, 57 years). According to the tertiles of the clamp-derived glucose metabolic clearance rate (MCR), the FDRs were further classified as insulin resistant with an MCR or =7.8 mL. kg(-1). min(-1), and borderline with an MCR of 5.9 to 7.7 mL. kg(-1). min(-1). Flow-associated dilation was 4.1+/-0.9% in insulin-resistant FDRs, 6.7+/-1.1% in borderline FDRs, 9.0+/-1.2% in insulin-sensitive FDRs (P=0.002), 7.7+/-2.9% in control subjects (P=NS versus FDRs), and 3.8+/-1.0% in DM patients (P=0.03). In multiple regression analysis, low MCR was significantly correlated with ED independent of age, sex, smoking, body mass index, percent body fat, serum insulin, and lipids. CONCLUSIONS There is a significant association between ED and insulin resistance in young FDRs of DM subjects independent of the classic cardiovascular risk factors.

Antihypertensive Therapy and Insulin Sensitivity: Do we have to Redefine the Role of β-Blocking Agents?

Essential hypertension is, at least in many subjects, associated with a decrease in insulin sensitivity, whereas glycemic control is (still) normal. Metaanalyses of hypertension intervention studies revealed different efficacy of treatment on cerebral (cerebrovascular accidents [CVA]) and cardiac (coronary heart disease [CHD]) morbidity and mortality. Although CVA were reduced to an extent similar to that anticipated, the decrease in CHD was less than expected. These differences are likely to be caused by the different impact of concomitant cardiovascular risk factors, such as dyslipidemia, impaired glucose tolerance, and non-insulin-dependent diabetes mellitus on CHD and CVA. Frequently these cardiovascular risk factors are ineffectively controlled in hypertensive patients, and moreover, some of the widely used antihypertensive agents have unfavorable side effects and further deteriorate these particular metabolic risk factors. Therefore, the metabolic side effects of antihypertensive treatment have received more attention. During the past few years, studies demonstrated that most antihypertensive agents modify insulin sensitivity in parallel with alterations in the atherogenic lipid profile. Alpha1-blockers and angiotensin converting enzyme inhibitors were shown to either have no impact on or even improve insulin resistance and the profile of atherogenic lipids, whereas most of the calcium channel blockers were found to be metabolically inert. The diuretics and beta-adrenoreceptor antagonists further decrease insulin sensitivity and worsen dyslipidemia. The mechanisms by which beta-adrenoreceptor antagonist treatment exert its disadvantageous effects are not fully understood, but several possibilities exist: significant body weight gain, reduction in enzyme activities (muscle lipoprotein lipase and lecithin cholesterol acyltransferase), alterations in insulin clearance and insulin secretion, and, probably most important, reduced peripheral blood flow due to increase in total peripheral vascular resistance. Recent metabolic studies found beneficial effects of the newer vasodilating beta-blockers, such as dilevalol, carvedilol and celiprolol, on insulin sensitivity and the atherogenic risk factors. In many hypertensive patients, elevated sympathetic nerve activity and insulin resistance are a deleterious combination. Although conventional beta-blocker treatment was able to take care of the former, the latter got worse; the newer vasodilating beta-blocker generation seems to be capable of successfully treating both of them.

The PPARγ2 amino acid polymorphism Pro 12 Ala is prevalent in offspring of Type II diabetic patients and is associated to increased insulin sensitivity in a subgroup of obese subjects

Aims/hypothesis. Recently a mutation in the coding sequence of the adipocyte specific isoform peroxisome proliferator-activated receptor γ2 (PPARγ2) was described, leading to the substitution of Proline to Alanine at codon 12. Mutations in PPARγ2 could have a role in people who are at increased risk for the development of obesity and Type II (non-insulin-dependent) diabetes mellitus. Methods. Non-diabetic first-degree relatives (n = 108) of subjects with Type II diabetes were characterized by oral glucose tolerance tests and euglycaemic hyperinsulinaemic glucose clamp to determine insulin sensitivity. Results. We found 75 (69 %) probands without the PPARγ ProAla12 substitution, 28 heterozygotes (26 %) and 5 (4 %) homozygotes. When the whole group was analysed for an association between the mutation and insulin sensitivity, no statistical significance could be shown. Only in the group with severe obesity more than 30 kg/m2, an association (p = 0.016) of the polymorphism with an increase in insulin sensitivity was found. Conclusion/interpretation. These observations suggest that the mutation in the PPARγ2 molecule may have a role in subgroups prone to the development of obesity and Type II diabetes. [Diabetologia (1999) 42: 758–762]

Insulin-induced glucose transporter (GLUT1 and GLUT4) translocation in cardiac muscle tissue is mimicked by bradykinin.

The effect of bradykinin on glucose transporter translocation in isolated rat heart was compared with the effect of insulin. Hearts from male obese (fa/fa) Zucker rats were perfused under normoxic conditions and constant pressure in a classic Langendorff preparation with 12 mmol/l glucose as substrate, and a set of functional parameters was measured simultaneously. Bradykinin was administered at a concentration (10−11 mmol/l) that did not increase coronary flow. Insulin was used at a concentration (8 × 10−8 mmol/1) known to maximally stimulate glucose transport in this model. After 15 min of perfusion with insulin or bradykinin, subcellular membrane fractions of the heart were prepared, and distribution of glucose transporter protein (GLUT1 and GLUT4) in fractions enriched with surface membranes (transverse tubules [TTs] and sarcolemmal membranes [PMs]) and with low-density microsomal membranes (LDMs) were determined by immunoblotting with the respective antibodies. Both glucose transporter isoforms were translocated after stimulation with insulin (increased transporter protein content in the PM+TT-enriched fraction with a concomitant decrease in the LDM-enriched fraction) and, to a smaller extent, also with bradykinin. These data suggest that in hearts of insulin-resistant obese (fa/fa) Zucker rats, bradykinin interacts with or facilitates the translocation process of both GLUT1 and GLUT4.

Potential Role of Bradykinin in Forearm Muscle Metabolism in Humans

Using the euglycemic-hyperinsulinemic glucose clamp and the human forearm technique, we have demonstrated that the improved glucose disposal rate observed after the administration of an angiotensin-converting enzyme (ACE) inhibitor such as captopril may be primarily due to increased muscle glucose uptake (MGU). These results are not surprising because ACE, which is identical to the bradykinin (BK)-degrading kininase II, is abundantly present in muscle tissue, and its inhibition has been observed to elicit the observed metabolic actions via elevated tissue concentrations of BK and through a BK B2 receptor site in muscle and/or endothelial tissue. These findings are supported by several previous studies. Exogenous BK applied into the brachial artery of the human forearm not only augmented muscle blood flow (MBF) but also enhanced the rate of MGU. In another investigation, during rhythmic voluntary contraction, both MBF and MGU increased in response to the higher energy expenditure, and the release of BK rose in the blood vessel, draining the working muscle tissue. Inhibition of the activity of the BK-generating protease in muscle tissue (kallikrein) with aprotinin significantly diminished these functional responses during contraction. Applying the same kallikrein inhibitor during the infusion of insulin into the brachial artery significantly reduced the effect of insulin on glucose uptake into forearm muscle. This is of interest, because in recent studies insulin has been suggested to elicit its actions on MBF and MGU via the accelerated release of endothelium-derived nitric oxide, the generation of which is also stimulated by BK in a concentration-dependent manner. This new evidence obtained from in vitro and in vivo studies sheds new light on the discussion of whether BK may play a role in energy metabolism of skeletal muscle tissue.

Beta-blocking agents in patients with insulin resistance: effects of vasodilating beta-blockers

Essential hypertension is--at least in many subjects--associated with a decrease in insulin sensitivity, while glycaemic control is (still) normal. It seems that in hypertensive patients, two major functions of insulin are impaired: there is insulin resistance of peripheral glucose uptake (primarily skeletal muscle) and insulin resistance of insulin-stimulated vasodilation. In view of some retrospective data and meta-analyses, which showed a less than expected reduction in coronary events (coronary paradox), the metabolic side effects of the antihypertensive treatment have received more attention. Many groups have shown that conventional antihypertensive treatment, both with beta-blockers and/or diuretics, decreases insulin sensitivity by various mechanisms. While low-dose diuretics seem to be free of these metabolic effects, there is no evidence for this in the beta-adrenergic blockers. However, recent metabolic studies evaluated the effects of vasodilating beta-blockers, such as dilevalol, carvedilol and celiprolol, on insulin sensitivity and the atherogenic risk factors. None of them decreased insulin sensitivity, as has been described for the beta-blockers with and without beta1 selectivity. This supports the idea that peripheral vascular resistance and peripheral blood flow play a central role in mediating the metabolic side effects of the beta-blocking agents, as the vasodilating action (either via beta2 stimulation or alpha1-blockade) seems to more than offset the detrimental effects of the blockade of beta (or beta1) receptors. Further studies are needed to elucidate the relevance of the radical scavenging properties of these agents and their connection to their metabolic effects. Therefore, the beneficial characteristics of these newer beta-adrenoreceptor blockers suggest that the vasodilating beta-blocking agents could be advantageous for hypertensive patients with insulin resistance or type 2 diabetes.

The tumour necrosis factor alpha –238 G → A and –308 G → A promoter polymorphisms are not associated with insulin sensitivity and insulin secretion in young healthy relatives of Type II Diabetic patients

Aims/hypothesis. Tumour necrosis factor-α (TNF-α) is believed to influence skeletal muscle insulin resistance. Two G → A transitions in the promoter region of TNF-α at position –238 and –308 have been identified that could play a part in transcriptional regulation of the gene. Insulin resistance is an independent familial trait that predicts the development of Type II (non-insulin-dependent) diabetes mellitus. We investigated the influence on insulin sensitivity and insulin secretion of both polymorphisms in a cohort of young healthy relatives of patients with Type II diabetes.¶Methods. We examined 109 first-degree relatives of Caucasian patients with a history of Type II diabetes, who underwent extensive metabolical and anthropometrical phenotyping, and determined the TNF-α–238 and –308 G→ A promoter polymorphisms.¶Results. For the –238 polymorphism, 83 probands (76.1 %) were homozygous for the G-allele, 25 probands (22.9 %) were heterozygous and 1 proband (0.9 %) was homozygous for the A-allele. For the –308 polymorphism, 83 probands (76.1 %) were homozygous for the G-allele, 24 probands (22.0 %) were heterozygous and 2 probands (1.18 %) were homozygous for the A-allele. Probands with and without the polymorphism did not differ in insulin sensitivity (p = 0.78), insulin-concentrations and C-peptide concentrations in oral glucose tolerance tests (p > 0.05).¶Conclusions/interpretation. We could not detect an association between insulin sensitivity or insulin secretion and TNF-α promoter polymorphisms in our cohort. The polymorphisms occur at the same frequencies in probands with either low or high insulin sensitivity. [Diabetologia (2000) 43: 181–184]

Lipolysis in skeletal muscle is rapidly regulated by low physiological doses of insulin

Aims/hypothesis. Both patients with Type II (non-insulin-dependent) diabetes mellitus and normoglycaemic, insulin resistant subjects were shown to have an increased lipid content in skeletal muscle, which correlates negatively with insulin sensitivity. Recently, it was shown that during a hyperinsulinaemic euglycaemic clamp interstitial glycerol was reduced not only in adipose tissue but also in skeletal muscle. To assess whether lipolysis of muscular lipids is also regulated by low physiological concentrations of insulin, we used the microdialysis technique in combination with a 3-step hyperinsulinaemic glucose clamp. Methods. Nineteen lean, healthy subjects (12 m/7 f) underwent a glucose clamp with various doses of insulin (GC I = 0.1, GC II = 0.25 and GC III = 1.0mU · kg–1· min–1). Two double lumen microdialysis catheters each were inserted in the paraumbilical subcutaneous adipose tissue and in skeletal muscle (tibialis anterior) to measure interstitial glycerol concentration (index of lipolysis) and ethanol outflow (index of tissue flow). Results. During the different steps of the glucose clamp, glycerol in adipose tissue was reduced to 81 ± 7 % (GC I), 55 ± 8 % (GC II) and 25 ± 5 % (GC III), respectively, of basal. In contrast, glycerol in skeletal muscle declined to 73 ± 5 % (GC I) and to 57 ± 6 % (GC II) but was not further reduced at GC III. Tissue flow was higher in the skeletal muscle and remained unchanged in both compartments throughout the experiment. Conclusion/interpretation. This study confirms the presence of glycerol release in skeletal muscle. Lipolysis in skeletal muscle and adipose tissue are suppressed similarly by minute and physiological increases in insulin but differently by supraphysiological increases. Inadequate suppression of intramuscular lipolysis resulting in increased availability of non-esterified fatty acids, could represent a potential mechanism involved in the pathogenesis of impaired glucose disposal, i. e. insulin resistance, in muscle. [Diabetologia (1999) 42: 1171–1174]

Perfusion-Independent Effect of Bradykinin and Fosinoprilate on Glucose Transport in Langendorff Rat Hearts

Angiotensin-converting enzyme (ACE) inhibitor-stimulated glucose metabolism and perfusion in muscle tissue seem to be, at least in part, mediated by kinins. However, the relative contribution of direct metabolic or secondary hemodynamically induced effects is unclear. It was the aim of this study to characterize the effects of ACE inhibition and bradykinin on glucose transport while changes in cardiocoronary function that might influence glucose transport were minimized. Hearts from Wistar rats were perfused by a Langendorff preparation and a set of functional parameters were simultaneously measured. Bradykinin (10[-11] M) and fosinoprilate (10[-7] M) were administered at concentrations that did not affect coronary flow. Insulin was employed as reference at half-maximal concentration. The nonmetabolizable glucose analog 3-O-[14C]methyl-D-glucose and the nontransportable tracer L-[3H]glucose were coperfused for the calculation of glucose transport. Using a 2-compartment mathematical model we found that the glucose transport rate, which was doubled with insulin, was increased almost 3-fold by either bradykinin or fosinoprilate. In the presence of the B2 bradykinin receptor antagonist HOE 140 (D-Arg[Hyp3,Thi5,D-Tic7,Oic8]-bradykinin; icatibant), the effect of both agents was completely abolished. Both agents also induced minor changes in contractility/relaxation parameters that again were completely neutralized with icatibant. A perfusion-independent but B2-kinin receptor-dependent stimulating effect on glucose transport by either bradykinin or fosinoprilate is concluded. This effect could, in analogy to insulin be due to increased glucose transporter translocation, increased endothelium-derived nitric oxide formation, or--despite constant coronary flow conditions--secondary to altered cardiac function.

Hypertension in the non-insulin-dependent diabetes mellitus syndrome: a critical review of therapeutic intervention.

Background: Since it is not yet clear whether and to what degree treatment of mild hypertension will decrease cardiovascular morbidity and mortality in non-insulin-dependent diabetes (NIDDM), decisions concerning the treatment of hypertensive diabetics are at present based on data from the non-diabetic population. Recent research on causes of diabetes: A large body of recent work on the sequence of events leading from the prediabetic to the hyperglycemic stages of the NIDDM syndrome has suggested that elevated blood pressure and other cardiovascular risk factors may precede NIDDM by many years and that after the onset of NIDDM intervention might be too late to be beneficial. Prospective intervention study in progress: It is not possible to draw firm conclusions that can be applied to the treatment of hypertensive diabetics before the results of the United Kingdom Prospective Diabetes Study/Hypertension in Diabetes Study are published later this decade. Unfortunately, this study does not consider different stages within the NIDDM syndrome. Moreover, World Health Organization criteria are not used for the diagnosis of either NIDDM or hypertension, so that the study subjects are insufficiently characterized. Finally, confounding variables such as racial, ethnic and sex. differences and the individual stage of the NIDDM syndrome (and consequent variation in antidiabetic treatment) have been either not considered or not competely ruled out. These shortcomings seriously threaten the significance of this otherwise important study. Conclusions: As long as there are no diabetes-specific data from adequately sized intervention studies, recommendations for antihypertensive treatment in NIDDM will be based on strategies accepted for the non-diabetic population. These recommendations involve symptom-based diagnostic and therapeutic concepts of both NIDDM and hypertension and ignore recent pathogenetic concepts which could lead to an interdisciplinary and integrated approach to cardiovascular risk management, and possibly to the prevention of end-stage NIDDM syndrome with its advanced macrovascular complications.