Improvement in endothelial function by angiotensin-converting enzyme inhibition in non-insulin-dependent diabetes mellitus

The effect of combined aerobic and resistance exercise training on vascular function in type 2 diabetes

OBJECTIVES

The purpose of this study was to examine whether exercise training stimulates a generalized improvement in vascular function in patients with type 2 diabetes mellitus.

BACKGROUND

Exercise is often recommended for patients with type 2 diabetes to improve physical conditioning and glycemic control. This study examined the effect of eight weeks of exercise training on conduit and resistance vessel function in patients with type 2 diabetes, using a randomized crossover design.

METHODS

Both resistance vessel endothelium-dependent and -independent functions were determined by forearm plethysmography and intrabrachial infusions of acetylcholine (ACh) and sodium nitroprusside (SNP), respectively, in 16 patients with type 2 diabetes. Conduit vessel endothelial function was assessed in 15 of these patients using high-resolution ultrasound and flow-mediated dilation of the brachial artery; glyceryl trinitrate (GTN) was used as an endothelium-independent dilator.

RESULTS

Flow-mediated dilation increased from 1.7 +/- 0.5% to 5.0 +/- 0.4% following training (p < 0.001). The forearm blood flow ratio to ACh was significantly improved (analysis of variance, p < 0.05). Responses to SNP and GTN were unchanged. Endothelium-dependent vasodilation was enhanced in both conduit and resistance vessels.

CONCLUSIONS

If endothelial dysfunction is an integral component of the pathogenesis of vascular disease, as currently believed, this study supports the value of an exercise program in the management of type 2 diabetes.

Improved analysis of brachial artery ultrasound using a novel edge-detection software system

Brachial artery ultrasound is commonly employed for noninvasive assessment of endothelial function. However, analysis is observer dependent and susceptible to errors. We describe studies on a computerized edge-detection and wall-tracking software program to allow more accurate and reproducible measurement. In study 1, three purpose-built Perspex phantom arteries, 3.00, 4.00, and 6.00 mm in diameter, were measured with the software. There was a mean bias of 11 microm (P < 0.001 at each level) between known and measured values; the mean resolving power of the software was estimated as 8.3 microm. In study 2, the mean intraobserver coefficient of variation of repeated measures of flow-mediated dilation (FMD) using the software (6.7%) was significantly lower than that for traditional manual measurements using the intima-lumen interfaces (24.8%, P < 0.05) and intima-media interfaces (32.5%, P < 0.05). In study 3, 24 healthy volunteers underwent repeat testing twice within 1 wk; the coefficients of variation for between-visit reproducibility of FMD and response to glyceryl trinitrate using the software were 14.7 and 17.6%, respectively. Assuming 80% power and an alpha of 0.05, eight subjects with matched controls would be required, in a parallel designed study, to detect an absolute 2.5% change in FMD. In summary, we have developed a semiautomated computerized vascular ultrasound analysis system that will improve the power of clinical intervention studies to detect small changes in arterial diameter.

Better microvascular function on long-term treatment with lisinopril than with nifedipine in renal transplant recipients

BACKGROUND

The prevalence of hypertension in renal transplant recipients is high but the pathophysiology is poorly defined. Impaired endothelial function may be a factor of major importance. The present study addresses the effects of long-term treatment with either lisinopril or slow-release nifedipine on microvascular function and plasma endothelin in renal transplant recipients on cyclosporin A (CsA).

METHODS

Seventy-five hypertensive renal transplant recipients were double-blind randomized to receive slow-release nifedipine (NIF, n=40) or lisinopril (LIS, n=35). Ten normotensive, age-matched recipients served as controls. All patients received CsA-based immunosuppressive therapy including prednisolone and azathioprine. Microvascular function was assessed in the forearm skin vasculature, using laser Doppler flowmetry in combination with post-occlusive reactive hyperaemia and endothelial-dependent function during local acetylcholine (ACh) stimulation.

RESULTS

The analysis of microvascular function (AUC(rh)) showed that nifedipine-treated patients had significantly lower responses compared with lisinopril-treated patients (20+/-17 and 43+/-20 AU x min respectively, P=0.0016). Endothelial function was borderline significantly lower in the NIF group compared with the LIS group (640+/-345 and 817+/-404 AU x min respectively, P=0.056). The responses in the LIS group were comparable with those in non-hypertensive controls (AUC(rh) was 37+/-16 and AUC(ACh) was 994+/-566 AU x min). Plasma endothelin-1 concentrations were significantly higher in the NIF group compared with the LIS group (0.44+/-0.19 vs. 0.34+/-0.10 fmol/ml respectively, P=0.048), and were 0.29+/-0.09 fmol/ml in the control patients. AUC(ACh) was associated with plasma endothelin-1 (P=0.0053), while AUC(rh) was not (P=0.080).

CONCLUSIONS

The study indicates that long-term treatment with lisinopril, when compared with nifedipine, yields a more beneficial effect on microvascular function in hypertensive renal transplant recipients on CsA. The beneficial microvascular effect may be mediated in part by an endothelin-1-associated effect on the endothelium.

Diabetes and endothelial dysfunction: a clinical perspective

The main etiology for mortality and a great percent of morbidity in patients with diabetes mellitus is atherosclerosis. A hypothesis for the initial lesion of atherosclerosis is endothelial dysfunction, defined pragmatically as changes in the concentration of the chemical messengers produced by the endothelial cell and/or by blunting of the nitric oxide-dependent vasodilatory response to acetylcholine or hyperemia. Endothelial dysfunction has been documented in patients with diabetes and in individuals with insulin resistance or at high risk for developing type 2 diabetes. Factors associated with endothelial dysfunction in diabetes include activation of protein kinase C, overexpression of growth factors and/or cytokines, and oxidative stress. Several therapeutic interventions have been tested in clinical trials aimed at improving endothelial function in patients with diabetes. Insulin sensitizers may have a beneficial effect in the short term, but the virtual absence of trials with cardiovascular end-points preclude any definitive conclusion. Two trials offer optimism that treatment with ACE inhibitors may have a positive impact on the progression of atherosclerosis. Although widely used, the effect of hypolipidemic agents on endothelial function in diabetes is not clear. The role of antioxidant therapy is controversial. No data have been published regarding the effects of hormonal replacement therapy on endothelial dysfunction in postmenopausal women with type 2 diabetes.

Quinapril treatment enhances vascular sensitivity to insulin

OBJECTIVE

Insulin-mediated vasodilation has been shown to be impaired with hypertension and aggravated by dietary sodium restriction. Whether vascular insulin resistance in this setting could be improved by antihypertensive therapy was unknown. Therefore, we determined the effect of therapy with an angiotensin-converting enzyme inhibitor, on vascular sensitivity to insulin in hypertensive subjects fed a sodium-restricted diet.

DESIGN AND METHODS

The effects of 3 months of therapy with quinapril on vascular sensitivity to insulin was assessed in 11 hypertensive subjects using a randomized, placebo-controlled, double-blind design. Vascular sensitivity to insulin was assessed by determination of the insulin ED50 from dorsal hand vein linear variable differential transformer in phenylephrine-preconstricted vessels. Subjects were maintained on a 75 mmol/day sodium diet for 3 days prior to each study.

RESULTS

Quinapril therapy significantly improved vascular sensitivity to insulin, as assessed by a decrease in the ED50 for insulin (ED50 insulin: placebo = 501 +/- 189 [muU/ min; quinapril = 276 +/- 100 muU/min P < 0.05). Isoproterenol-mediated relaxation was also enhanced by quinapril treatment (maximal isoproterenol-mediated relaxation: Placebo = 92 +/- 15% of baseline distension; Quinapril = 151 +/- 31% P < 0.05).

CONCLUSIONS

The current study suggests the hypothesis that the ACE-inhibitor quinapril has beneficial effects on vascular function in general, and on insulin-mediated vascular responses, in particular.

Vascular Effects of ACE Inhibition Independent of the Renin-Angiotensin System in Hypertensive Renovascular Disease : A Randomized, Double-Blind, Crossover Trial

-To evaluate whether ACE inhibition and angiotensin II type 1 blockade exert beneficial effects on NO availability independent of their blood pressure-lowering effect, we used a double-blind crossover design to study vascular function in 18 patients with hypertensive renovascular disease during 6 weeks of therapy with enalapril (Ena) and valsartan (Val) compared with non-renin-angiotensin system-mediated treatment with the alpha(1)-blocker doxazosin (Dox). Control measurements were performed in 13 age-matched volunteers. Forearm blood flow was assessed with venous occlusion plethysmography, and serotonin and nitroprusside were used as endothelium-dependent and -independent vasodilators, respectively. Blood pressure was similar during all treatment periods. Serotonin-induced vasodilation was decreased in patients during Dox treatment (n=12) compared with control subjects (n=13) (increase 42+/-20% versus 107+/-65%, P:<0.05). Crossover from Dox to Val (n=6) had no effect on serotonin response (increase 50+/-14%), but crossover to Ena (n=6) caused a significant improvement (increase 79+/-39%, P:<0.05 versus Dox). In an assessment of all patients, serotonin-induced vasodilation during Ena (n=12, increase 75+/-31%) was increased compared with both Val and Dox (43+/-14% and 42+/-20%, respectively; both P:<0.05 versus Ena). The nitroprusside response remained unaltered during all treatment periods. In conclusion, ACE inhibition improves the impaired endothelium-dependent vascular function in patients with hypertensive renovascular disease. This effect is unrelated to blood pressure-lowering or angiotensin II-mediated effects.

Losartan, an angiotensin type 1 receptor antagonist, improves endothelial function in non-insulin-dependent diabetes

OBJECTIVES

The present study examined the effect on forearm endothelial function of an angiotensin II type 1 receptor antagonist, losartan, in subjects with non-insulin-dependent diabetes mellitus (NIDDM).

BACKGROUND

Angiotensin-converting enzyme (ACE) inhibition with enalapril improves acetylcholine (ACh)-dependent endothelial function in patients with NIDDM. This could be mediated through angiotensin II and the type 1 receptor or could be due to inhibition of kininase II and a bradykinin preserving effect. It is therefore relevant to determine whether a type 1 receptor antagonist improves endothelial function.

METHODS

The influence of losartan (50 mg daily for four weeks) on endothelium-dependent and independent vasodilator function was determined in 9 NIDDM subjects using a double-blinded placebo-controlled crossover protocol. Forearm blood flow was measured using strain-gauge plethysmography.

RESULTS

Losartan significantly decreased infused arm vascular resistance in response to three incremental doses of intrabrachial acetylcholine (p < 0.05, ANOVA). The forearm blood flow ratio (flow in infused to noninfused arm) was also increased (p < 0.01). Responses to sodium nitroprusside and monomethyl arginine were not significantly changed.

CONCLUSIONS

Losartan administration at 50 mg per day improved endothelium-dependent dilation of resistance vessels in patients with NIDDM. That is, blockade of the angiotensin II type 1 receptors improves endothelial function in NIDDM. At least some of the similarly beneficial effect of ACE inhibition is probably mediated also through the angiotensin II-type 1 receptor pathway. The use of a type 1 receptor antagonist seems a reasonable alternative to an ACE inhibitor to maintain endothelial function in NIDDM subjects.

Effect of aerobic and resistance exercise training on vascular function in heart failure

Exercise training of a muscle group improves local vascular function in subjects with chronic heart failure (CHF). We studied forearm resistance vessel function in 12 patients with CHF in response to an 8-wk exercise program, which specifically excluded forearm exercise, using a crossover design. Forearm blood flow (FBF) was measured using strain-gauge plethysmography. Responses to three dose levels of intra-arterial acetylcholine were significantly augmented after exercise training when analyzed in terms of absolute flows (7.0 +/- 1.8 to 10.9 +/- 2.1 ml x 100 ml(-1) x min(-1) for the highest dose, P < 0.05 by ANOVA), forearm vascular resistance (21.5 +/- 5.0 to 15.3 +/- 3.9 ml x 100 ml forearm(-1) x min(-1), P < 0.01), or FBF ratios (P < 0.01, ANOVA). FBF ratio responses to sodium nitroprusside were also significantly increased after training (P < 0.05, ANOVA). Reactive hyperemic flow significantly increased in both upper limbs after training (27.9 +/- 2.7 to 33.5 +/- 3.1 ml x 100 ml(-1) x min(-1), infused limb; P < 0.05 by paired t-test). Exercise training improves endothelium-dependent and -independent vascular function and peak vasodilator capacity in patients with CHF. These effects on the vasculature are generalized, as they were evident in a vascular bed not directly involved in the exercise stimulus.

Endothelial dysfunction in diabetes

Endothelial dysfunction plays a key role in the pathogenesis of diabetic vascular disease. The endothelium controls the tone of the underlying vascular smooth muscle through the production of vasodilator mediators. The endothelium-derived relaxing factors (EDRF) comprise nitric oxide (NO), prostacyclin, and a still elusive endothelium-derived hyperpolarizing factor (EDHF). Impaired endothelium-dependent vasodilation has been demonstrated in various vascular beds of different animal models of diabetes and in humans with type 1 and 2 diabetes. Several mechanisms of endothelial dysfunction have been reported, including impaired signal transduction or substrate availibility, impaired release of EDRF, increased destruction of EDRF, enhanced release of endothelium-derived constricting factors and decreased sensitivity of the vascular smooth muscle to EDRF. The principal mediators of hyperglycaemia-induced endothelial dysfunction may be activation of protein kinase C, increased activity of the polyol pathway, non-enzymatic glycation and oxidative stress. Correction of these pathways, as well as administration of ACE inhibitors and folate, has been shown to improve endothelium-dependent vasodilation in diabetes. Since the mechanisms of endothelial dysfunction appear to differ according to the diabetic model and the vascular bed under study, it is important to select clinically relevant models for future research of endothelial dysfunction.