The effect of angiotensin-I converting enzyme inhibition on insulin action in healthy volunteers

Insulin receptor substrate-2 (Irs2) in endothelial cells plays a crucial role in insulin secretion

Endothelial cells are considered to be essential for normal pancreatic β-cell function. The current study attempted to demonstrate the role of insulin receptor substrate-2 (Irs2) in endothelial cells with regard to insulin secretion. Endothelial cell-specific Irs2 knockout (ETIrs2KO) mice exhibited impaired glucose-induced, arginine-induced, and glucagon-induced insulin secretion and showed glucose intolerance. In batch incubation and perifusion experiments using isolated islets, glucose-induced insulin secretion was not significantly different between the control and the ETIrs2KO mice. In contrast, in perfusion experiments, glucose-induced insulin secretion was significantly impaired in the ETIrs2KO mice. The islet blood flow was significantly impaired in the ETIrs2KO mice. After the treatment of these knockout mice with enalapril maleate, which improved the islet blood flow, glucose-stimulated insulin secretion was almost completely restored to levels equal to those in the control mice. These data suggest that Irs2 deletion in endothelial cells leads to a decreased islet blood flow, which may cause impaired glucose-induced insulin secretion. Thus, Irs2 in endothelial cells may serve as a novel therapeutic target for preventing and ameliorating type 2 diabetes and metabolic syndrome.

Effect of Renin-Angiotensin system inhibition on cardiovascular events in older hypertensive patients with metabolic syndrome

OBJECTIVE

Metabolic syndrome (MetS) is associated with cardiovascular disease (CVD). Insulin resistance has been hypothesized as the underlying feature of MetS. Angiotensin converting enzyme inhibitors (ACEI) and angiotensin receptor blockers (ARB) are widely used antihypertensives that may improve insulin sensitivity. The aim of the study is to evaluate the effect of ACEI/ARB on incident CVD events in older hypertensive patients with MetS.

MATERIALS/METHODS

We used the Cardiovascular Health Study, a prospective cohort study of individuals>65years of age to evaluate ACEI/ARB use and time to CVD events (including coronary and cerebrovascular events). The study included 777 subjects who had hypertension and ATP III-defined MetS, but free of CVD and diabetes at baseline. Cox regression models were used to evaluate the effect of ACEI/ARB as compared to other antihypertensives on the time to the first CVD events.

RESULTS

ACEI/ARB use was associated with a decreased risk of CVD events (adjusted HR=0.658, 95 % C.I. [0.436-0.993]) compared to other antihypertensives. When CVD endpoints were evaluated separately, use of ACEI/ARB was associated with lower rates of angioplasty and coronary events (HR of 0.129 and 0.530 respectively, with 95 % CI [0.017-0.952] and [0.321-0.875]).

CONCLUSIONS

ACEI/ARB use was associated with a lower risk of CVD events in older hypertensive patients with MetS, primarily due to a reduction in coronary events. The potential protective effect of ACEI/ARB on CVD events in older individuals with MetS will need further confirmation from prospective studies.

Angiotensin, bradykinin and the endothelium

Angiotensins and kinins are endogenous peptides with diverse biological actions; as such, they represent current and future targets of therapeutic intervention. The field of angiotensin biology has changed significantly over the last 50 years. Our original understanding of the crucial role of angiotensin II in the regulation of vascular tone and electrolyte homeostasis has been expanded to include the discovery of new angiotensins, their important role in cardiovascular inflammation and the development of clinically useful synthesis inhibitors and receptor antagonists. While less applied progress has been achieved in the kinin field, there are continuous discoveries in bradykinin physiology and in the complexity of kinin interactions with other proteins. The present review focuses on mechanisms and interactions of angiotensins and kinins that deal specifically with vascular endothelium.

Influence of nebivolol and enalapril on metabolic parameters and arterial stiffness in hypertensive type 2 diabetic patients

OBJECTIVE

To compare the effects of a cardioselective beta-blocker (nebivolol) with those of an angiotensin-converting enzyme inhibitor (enalapril) on parameters of insulin sensitivity, peripheral blood flow and arterial stiffness during one extended glucose clamp experiment.

DESIGN

A randomized, double-blind crossover trial, consisting of two 12-week treatment phases separated by a 4-week wash-out phase.

METHODS

Patients with type 2 diabetes and arterial hypertension were randomly assigned to one of two treatment sequences (nebivolol-enalapril, enalapril-nebivolol). Haemodynamic, metabolic and other laboratory measurements were carried out on the first and last day of each treatment period by means of a glucose clamp experiment that also involved the measurement of blood flow and arterial stiffness.

RESULTS

Twelve patients were included in this study, of which two dropped out early. Efficacy parameters were therefore available for 10 patients. There was no significant difference in any of the primary efficacy parameters. Moreover, the effects on blood pressure did not significantly differ between both treatments. Six adverse events happened during treatment with nebivolol compared with two during treatment with enalapril, but only one was regarded as possibly related to the treatment.

CONCLUSIONS

This pilot study shows that the combined measurement of insulin sensitivity, blood flow and arterial stiffness is feasible. Nebivolol and enalapril did not show different effects with regard to these parameters in hypertensive diabetic patients. If these results are confirmed in larger clinical trials, this would argue against the reservations against beta-blockers as drugs of first choice in patients with diabetes because of potential metabolic side-effects.

Why blockade of the renin–angiotensin system reduces the incidence of new-onset diabetes

Recent trials have suggested that inhibitors of the renin-angiotensin system (RAS), such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor blockers (ARBs), may reduce the incidence of new-onset diabetes in patients with or without hypertension and at high risk of developing diabetes. In this review, we critically evaluate the evidence from recent clinical trials for such a potential preventive effect of ACE inhibitors and ARBs, including a meta-analysis of these recent trials. The reduced incidence of diabetes in patients at high risk of developing diabetes by ACE inhibitors or ARBs has been explained by haemodynamic effects, such as improved delivery of insulin and glucose to the peripheral skeletal muscle, and non-haemodynamic effects, including direct effects on glucose transport and insulin signalling pathways, all of which decrease insulin resistance. There is now evidence that the pancreas may contain an in situ active RAS, which appears to be upregulated in an animal model of type 2 diabetes. Thus, ACE inhibitors and ARBs may act by attenuating the deleterious effect of angiotensin II on vasoconstriction, fibrosis, inflammation, apoptosis and beta-cell death in the pancreas, thereby protecting a critical beta-cell mass essential for insulin production. New evidence is presented that ACE inhibitors and ARBs may delay or prevent the development of insulin resistance and diabetes, for which novel mechanisms are suggested. The actions of agents that interrupt the RAS on insulin resistance, obesity and diabetes warrant further investigation in other animal models. Prospective clinical studies with the primary endpoint of the prevention of diabetes are now indicated to (i) further explore whether the inhibitors of the RAS are superior compared to other antihypertensive agents such as calcium channel blockers (CCBs) and (ii) to evaluate the potential beneficial effects of combination antihypertensive regimens on the development of diabetes.

Evidence for a direct effect of captopril on early steps of insulin action in BC3H-1 myocytes

Captopril, an angiotensin-converting enzyme (ACE) inhibitor, has been reported to improve insulin sensitivity. However, despite extensive investigation, the mechanisms responsible for this effect are not fully understood. Reduction of plasma angiotensin II and inhibition of kininase II have been suggested to contribute to improve insulin sensitivity. Insulin binding was measured at tracer insulin concentration in intact cells with or without captopril treatment. Specific binding, expressed as percent of total insulin added, was not different in control and captopril-treated cells. However, captopril treatment caused an increase in insulin-induced insulin receptor substrate-1 (IRS-1) phosphorylation accompanied by an increased association of IRS-1 with phosphoinositide-3 kinase (PI-3 kinase), despite no change on insulin receptor (IR) autophosphorylation. There was also an increased threonine kinase B (AKT) phosphorylation in captopril-treated cells followed by enhanced basal and insulin-stimulated glucose uptake. These results indicate that captopril treatment has a direct effect on early phosphorylation events induced by insulin in BC3H-1 myocytes.

Effects of exercise training and ACE inhibition on insulin action in rat skeletal muscle

Our laboratory has demonstrated (Steen MS, Foianini KR, Youngblood EB, Kinnick TR, Jacob S, and Henriksen EJ, J Appl Physiol 86: 2044-2051, 1999) that exercise training and treatment with the angiotensin-converting enzyme (ACE) inhibitor trandolapril interact to improve insulin action in insulin-resistant obese Zucker rats. The present study was undertaken to determine whether a similar interactive effect of these interventions is manifest in an animal model of normal insulin sensitivity. Lean Zucker (Fa/-) rats were assigned to either a sedentary, trandolapril-treated (1 mg. kg(-1). day(-1) for 6 wk), exercise-trained (treadmill running for 6 wk), or combined trandolapril-treated and exercise-trained group. Exercise training alone or in combination with trandolapril significantly (P < 0.05) increased peak oxygen consumption by 26-32%. Compared with sedentary controls, exercise training alone or in combination with ACE inhibitor caused smaller areas under the curve for glucose (27-37%) and insulin (41-44%) responses during an oral glucose tolerance test. Exercise training alone or in combination with trandolapril also improved insulin-stimulated glucose transport in isolated epitrochlearis (33-50%) and soleus (58-66%) muscles. The increases due to exercise training alone or in combination with trandolapril were associated with enhanced muscle GLUT-4 protein levels and total hexokinase activities. However, there was no interactive effect of exercise training and ACE inhibition observed on insulin action. These results indicate that, in rats with normal insulin sensitivity, exercise training improves oral glucose tolerance and insulin-stimulated muscle glucose transport, whereas ACE inhibition has no effect. Moreover, the beneficial interactive effects of exercise training and ACE inhibition on these parameters are not apparent in lean Zucker rats and, therefore, are restricted to conditions of insulin resistance.

Improvement of insulin resistance in essential hypertension by long-acting Ca antagonist benidipine

To investigate whether the long-acting Ca channel blocker, benidipine improves insulin resistance in patients with essential hypertension, insulin sensitivity was measured using the steady state plasma glucose (SSPG) method in 11 or 14 nonobese and nondiabetic hypertensive subjects before and after treatment with benidipine or placebo, respectively, and 11 healthy control subjects. SSPG level was significantly higher in two hypertensive groups, indicating reduced insulin sensitivity than in controls. SSPG level significantly decreased after benidipine treatment, with a decrease of blood pressure. SSPG level and blood pressure did not change in the placebo group. As for oral glucose tolerance test, the area under the curve of insulin diminished significantly after benidipine treatment. SSPG level significantly correlated with intra-platelet Ca2+ concentrations in 9 hypertensive subjects. The long-acting Ca channel blocker benidipine has partially improved insulin resistance in essential hypertension, contributing to the prevention of atherosclerosis associated with insulin resistance.

ACE inhibition but not angiotensin II antagonism reduces plasma fibrinogen and insulin resistance in overweight hypertensive patients

The aim of this study was to compare the effects of the angiotensin-converting enzyme (ACE) inhibitor perindopril and the angiotensin II antagonist losartan on insulin sensitivity and plasma fibrinogen in overweight hypertensive patients. Twenty-eight overweight mild to moderate [diastolic blood pressure (DBP) >90 and <110 mm Hg] hypertensives aged 43-64 years, after a 4-week placebo period, were randomized to perindopril, 4 mg o.d., or losartan, 50 mg o.d., for 6 weeks. Then, after a new placebo period, patients were crossed to the alternative regimen for further 6 weeks. At the end of the placebo and of the treatment periods, blood pressure was measured, plasma fibrinogen was evaluated, and insulin sensitivity was assessed by the euglycemic, hyperinsulinemic clamp technique. Glucose infusion rate (GIR) during the last 30 min of clamp and total glucose requirement (TGR) were evaluated. Both perindopril and losartan reduced SBP (by a mean of 20.2 mm Hg, p < 0.001 vs. placebo; and 15.8 mm Hg, p = 0.002 vs. placebo, respectively) and DBP (by a mean of 15.2 mm Hg, p = 0.001 vs. placebo, and 11.8 mm Hg, p = 0.01 vs. placebo respectively), with no difference between the two treatments. GIR was significantly increased by perindopril (+2.91 mg/min/kg, p = 0.042 vs. placebo), but not by losartan (+0.28 mg/min/kg, NS). TGR was not modified by losartan but was increased by perindopril (+9.3 g, p = 0.042 vs. placebo). Plasma fibrinogen levels were reduced by perindopril (-53.4 mg/dl, p = 0.022 vs. placebo) but not by losartan (-16.8 mg/dl, NS). The perindopril-induced decrease in fibrinogen was correlated with the increase in GIR (r = 0.39; p < 0.01). These findings suggest that fibrinogen decrease produced by the ACE inhibitor is related to its action on insulin sensitivity, which seems to be dependent not on angiotensin II blockade but rather on other mechanisms.

Interactions of captopril and verapamil on glucose tolerance and insulin action in an animal model of insulin resistance

We have shown previously that the combination of a long-acting, non-sulfhydryl-containing angiotensin-converting enzyme (ACE) inhibitor (trandolapril) and the Ca2+ channel blocker verapamil improve insulin-stimulated glucose transport in skeletal muscle of the obese Zucker rat, a model of insulin resistance, hyperinsulinemia, and dyslipidemia. In the present study, we investigated the interactions of chronic treatment (28 days) with verapamil (20 mg/kg) and a short-acting, sulfhydryl-containing ACE inhibitor (captopril, 50 mg/kg) in combination on insulinemia, lipidemia, glucose tolerance, and insulin action on skeletal muscle glucose transport (2-deoxyglucose uptake in epitrochlearis) in lean and obese Zucker rats. In lean animals, verapamil alone and in combination with captopril actually increased (P < .05) plasma insulin, whereas in obese animals, verapamil alone worsened the hyperinsulinemia already present, and this effect was abolished by cotreatment with captopril. Captopril alone or in combination with verapamil reduced plasma free fatty acid (FFA) levels in obese rats, but not in lean rats. Captopril alone reduced the glucose-insulin index in obese animals given an oral glucose load, and this was associated with a significant increase in insulin-mediated muscle glucose transport. The greatest improvement in these responses was elicited in obese animals receiving combined captopril and verapamil treatment, and was associated with increases in muscle GLUT-4 glucose transporter protein and hexokinase and citrate synthase activities. In conclusion, these findings indicate that the short-acting, sulfhydryl-containing ACE inhibitor captopril can elicit beneficial metabolic effects on the hyperinsulinemia, dyslipidemia, glucose intolerance, and insulin resistance of muscle glucose transport of the obese Zucker rat. Moreover, there is a positive interactive effect on these pathophysiological parameters between captopril and verapamil in this animal model of insulin resistance.

Comparison of effects of quinapril and metoprolol on glycaemic control, serum lipids, blood pressure, albuminuria and quality of life in non-insulin-dependent diabetes mellitus patients with hypertension. Swedish Quinapril Group

OBJECTIVE

To compare the long-term effects of the angiotensin-converting enzyme (ACE)-inhibitor quinapril and the cardioselective beta-adrenergic blocking agent metoprolol on glycaemic control, with glycosylated haemoglobin (HbA1c) as the principal variable, in non-insulin-dependent diabetes mellitus (NIDDM) patients with hypertension.

DESIGN

A randomized, double-blind, double-dummy, multicentre study during 6 months preceded by a 4 week wash-out and a 3 week run-in placebo period. Quinapril (20 mg) and metoprolol (100 mg, conventional tablets) were given once daily. No change was made in the treatment of diabetes (diet and hypoglycaemic agents).

SUBJECTS

Seventy-two patients fulfilling the criteria were randomized and entered the double-blind period. Twelve patients did not complete the study. Sixty patients, 26 on quinapril and 34 on metoprolol, were available for the final analysis.

MAIN OUTCOME MEASURES

The effect was assessed by changes in HbA1c, the fasting serum glucose and the post-load serum glucose, C-peptide and insulin levels during the oral glucose tolerance test.

RESULTS

In the quinapril group, the fasting serum glucose, oral glucose tolerance and the C-peptide and insulin responses, determined as the incremental area under the curves (AUC), showed no change, but the mean HbA1c level increased from 6.2 +/- 1.1% to 6.5 +/- 1.3% (P < 0.05). In the metoprolol group, the rise in the mean level of HbA1c, from 6.3 +/- 1.0% to 6.8 +/- 1.3% (P < 0.01), tended to be more marked than after quinapril, although there was no significant difference between the increments. The mean fasting serum glucose showed an increase from 9.1 +/- 1.9 mM to 10.1 +/- 2.8 mM (P < 0.01) which correlated significantly with the duration of diabetes (P < 0.01) and the increase in fasting serum triglycerides (P < 0.001). Moreover, in the metoprolol group we found significant decreases in the oral glucose tolerance as well as C-peptide and insulin responses to the glucose load.

CONCLUSIONS

Treatment with quinapril for 6 months appears to have advantages over metoprolol in NIDDM patients with hypertension. Although treatment with quinapril or metoprolol over 6 months was concomitant with a rise in the HbA1c, increased fasting blood glucose, decreased oral glucose tolerance and decreased C-peptide and insulin responses to a glucose challenge were observed only in patients treated with metoprolol.

Effect of chronic bradykinin administration on insulin action in an animal model of insulin resistance

The nonapeptide bradykinin (BK) has been implicated as the mediator of the beneficial effect of angiotensin-converting enzyme inhibitors on insulin-stimulated glucose transport in insulin-resistant skeletal muscle. In the present study, the effects of chronic in vivo BK treatment of obese Zucker (fa/fa) rats, a model of glucose intolerance and severe insulin resistance, on whole body glucose tolerance and skeletal muscle glucose transport activity stimulated by insulin or contractions were investigated. BK was administered subcutaneously (twice daily at 40 microg/kg body wt) for 14 consecutive days. Compared with a saline-treated obese group, the BK-treated obese animals had significantly (P < 0.05) lower fasting plasma levels of insulin (20%) and free fatty acids (26%), whereas plasma glucose was not different. During a 1 g/kg body wt oral glucose tolerance test, the glucose and insulin responses [incremental areas under the curve (AUC)] were 21 and 29% lower, respectively, in the BK-treated obese group. The glucose-insulin index, the product of the glucose and insulin AUCs and an indirect index of in vivo insulin action, was 52% lower in the BK-treated obese group compared with the obese control group. Moreover, 2-deoxyglucose uptake in the isolated epitrochlearis muscle stimulated by a maximally effective dose of insulin (2 mU/ml) was 52% greater in the BK-treated obese group. Contraction-stimulated (10 tetani) 2-deoxyglucose uptake was also enhanced by 35% as a result of the BK treatment. In conclusion, these findings indicate that in the severely insulin-resistant obese Zucker rat, chronic in vivo treatment with BK can significantly improve whole body glucose tolerance, possibly as a result of the enhanced insulin-stimulated skeletal muscle glucose transport activity observed in these animals.

Effects of trandolapril and verapamil on glucose transport in insulin-resistant rat skeletal muscle

We have used an animal model of insulin resistance-the obese Zucker (fa/fa) rat-to test whether oral administration of the non-sulfhydryl-containing angiotensin-converting enzyme (ACE) inhibitor, trandolapril, alone or in combination with the Ca2+-channel blocker, verapamil, can induce a beneficial effect on insulin-stimulated glucose transport and metabolism in skeletal muscle. Insulin-stimulated 2-deoxyglucose (2-DG) uptake in the isolated epitrochlearis muscle was less than 50% as great in obese animals compared with lean (Fa/-) controls (P < .05), but was significantly improved in the obese group by both short-term (6 hours, +33%) and long-term (14 days,+70%) oral treatment with trandolapril. Verapamil treatment alone did not alter insulin-stimulated 2-DG uptake in muscle, but simultaneous administration of verapamil and trandolapril resulted in the most pronounced effect on insulin-stimulated 2-DG uptake (+106%). Long-term treatment with trandolapril alone and in combination with verapamil significantly increased muscle glycogen (+26% to 27%), glucose transporter GLUT-4 protein (+27% to 31%), and hexokinase activity (+21% to 49%), and decreased plasma insulin levels (-23% to -29%). Muscle citrate synthase activity was enhanced only when trandolapril and verapamil were administered in combination (+24%). We conclude that the long-acting, non-sulfhydryl-containing ACE inhibitor, trandolapril, alone and in combination with the Ca2+-channel blocker, verapamil, can significantly improve insulin-stimulated glucose transport activity in skeletal muscle of the insulin-resistant obese Zucker rat, and that this improvement is associated with favorable adaptive responses in GLUT-4 protein levels, glycogen storage, and activities of relevant intracellular enzymes of glucose catabolism.

Four week administration of an ACE inhibitor and a cardioselective beta-blocker in healthy volunteers: no influence on insulin sensitivity

In most, but not all, studies antihypertensive treatment with angiotensin converting enzyme inhibitors (ACE inhibitors) improves insulin sensitivity, whereas beta-blockers decrease insulin sensitivity. However, there was a significant increase in body weight with beta-blockers and changes in the body potassium homeostasis with ACE inhibitors. In order to compare the drug specific metabolic effects of an ACE inhibitor and a cardioselective beta-blocker controlling these factors, we measured insulin sensitivity in a randomized, double-blind cross-over study in 22 healthy volunteers (age 27 +/- 3 years; BMI 22.0 +/- 1.5 kg m-2 (mean +/- SD)) during euglycaemic glucose clamps before and after 4 weeks' administration of 5 mg Lisinopril or 5 mg Bisoprolol. Both drug phases were separated by 4 weeks of no drug administration. During the insulin sensitivity measurements potassium concentrations were clamped at basal levels by means of a variable i.v. potassium infusion. Body weight was monitored at weekly intervals and kept constant within +/- 1 kg of the subjects' baseline weight throughout the entire study period. Insulin sensitivity did not change significantly during either drug administration period. The insulin sensitivity index of the 22 volunteers after administration of the ACE inhibitor was 7.9 +/- 2.4 mL min-1 m2 microU-1 mL-1 (basal index 8.3 +/- 1.9 mL min-1 m2 microU-1 mL-1, and 7.5 +/- 2.1 mL min-1 m2 microU-1 mL-1 after administration of the beta-blocker (basal index 8.2 +/- 1.9 mL min-1 m2 microU-1 mL-1; NS).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of angiotensin II receptor blockade on insulin sensitivity and sympathetic nervous system activity in primary hypertension

The objective of this study was to investigate the effect of Losartan (NK-954, DuP-753), a new selective angiotensin II receptor antagonist, on insulin sensitivity and sympathetic nervous system activity in patients with severe primary hypertension. Five patients with a record of diastolic blood pressure (DBP) > or = 115 mmHg, currently either untreated or with DBP > 95 mmHg on antihypertensive treatment, were examined in an open study with the euglycemic glucose clamp examination before and after being treated with Losartan for an average of 6 weeks. The glucose disposal rate increased from 6.2 +/- 2.6 to 7.9 +/- 2.6 mg/kg x min (27%, p < 0.05) during treatment with Losartan. The insulin sensitivity index (glucose disposal rate divided by mean insulin concentration during clamp) increased from 7.7 +/- 4.5 to 10.1 +/- 4.1 arbitrary units (30%, p < 0.05). Plasma noradrenaline decreased from 1.87 +/- 0.53 to 1.11 +/- 0.13 nmol/l (40%, p < 0.05), while plasma adrenaline was unchanged (0.23 +/- 0.10 vs. 0.22 +/- 0.11 nmol/l, n.s.). Mean blood pressure decreased from 132 +/- 10 to 119 +/- 13 mmHg (p < 0.05) and heart rate was unchanged during treatment with Losartan. Thus, antihypertensive treatment with the new selective angiotensin II receptor antagonist Losartan seems to improve insulin sensitivity. A decrease in plasma noradrenaline on Losartan suggests a sympathicolytic effect which together with vasodilation may explain the fall in blood pressure and the improvement in insulin sensitivity.

Insulin resistance--not hyperinsulinemia--is pathogenic in essential hypertension

A correlation between essential hypertension and insulin resistance/hyperinsulinemia is well documented, and there is adequate reason to believe that this association is causal. The common presumption that hyperinsulinemia mediates this connection is based on studies demonstrating various pressor effects of insulin, such as sodium retention, activation of the sympathetic nervous system, and stimulation of renin output. However, a consideration of physiological parameters in essential hypertensives indicates that these insulin-mediated pressor effects are unlikely to play a crucial pathogenic role in most cases of essential hypertension. Moreover, physiological elevation of insulin following a meal is typically associated with a reduction of blood pressure in hypertensives and the elderly. Euglycemic insulin clamps tend to reduce blood pressure in elderly subjects, and prolonged maintenance of hyperinsulinemia in animals does not raise blood pressure. In fact, insulin has long been known to have direct vasodilatory or antipressor effects on resistance vessels, and there is recent evidence that insulin reduces vascular resistance in skeletal muscles to facilitate glycogen storage after a meal. I propose that essential hypertensives experience a net deficit of insulin activity in vascular muscle, and that, in conjunction with other genetic or acquired defects of electrolyte transport, this leads to an increase in basal vascular tone and a hypersensitivity to pressor agents. Correction of insulin resistance usually aids blood pressure control, and in addition may mitigate the excess cardiovascular risk associated with hypertension.

A comparison between enalapril and captopril on insulin sensitivity in normotensive healthy volunteers

BACKGROUND

Captopril has been shown to improve insulin sensitivity in insulin resistant hypertensive individuals and enalapril has been shown to improve insulin sensitivity in a small group of healthy volunteers, but there has been no direct comparison of the effects of the different angiotensin converting enzyme inhibitors (ACEIs) on insulin sensitivity in either insulin sensitive or insulin insensitive populations.

AIM

To compare the impact of two different ACEIs (captopril and enalapril) on insulin mediated glucose uptake in normotensive, non-obese, insulin sensitive subjects.

METHOD

A single blind cross-over study comparing captopril (6.25 mg twice daily) and enalapril (5 mg once daily) for 28 days with a 28 day washout period between drugs. Insulin mediated glucose uptake was measured by means of the euglycaemic hyperinsulinaemic clamp at the start and completion of each period of drug therapy.

RESULTS

Both drugs resulted in elevations of fasting insulin levels (mean difference +/- SEM for combined data, 2.7 +/- 1.8; p < 0.05) and a reduction in insulin mediated glucose uptake (mean difference for combined data, -0.72 +/- 0.37 mg/kg-1 minute-1; p = 0.056). Results were similar for both agents and suggest a class effect.

CONCLUSIONS

The increase in fasting insulin levels, and reduction in insulin mediated glucose uptake in this study are in contrast to findings in obese and hypertensive subjects, and indicate that studies of insulin sensitivity of ACEIs in non-obese, normotensive subjects are inappropriate for predicting likely effects in clinical practice.

Acute hyperinsulinemia induces sodium retention and a blood pressure decline in diabetes mellitus

Hyperinsulinemia supposedly contributes to hypertension in diabetes mellitus. We sought to determine if the renal and cardiovascular effects of insulin are preserved in diabetes despite resistance to its glucose-lowering effect. We studied the effects of two doses of insulin (50 and 500 milliunits/kg.hr-1), using the euglycemic clamp technique, on fractional sodium excretion, blood pressure, and heart rate in two groups of non-insulin-dependent diabetics: eight patients with and eight patients without hypertension. Hypertensive diabetics had higher basal insulin levels than normotensive diabetics (21.8 +/- 2.9 and 14.4 +/- 1.6 milliunits/l, respectively [mean +/- SEM]; p = 0.03). The degree of insulin resistance, but not plasma insulin levels, correlated with the height of mean arterial blood pressure (r = 0.60 and 0.73 at the low and high insulin dose, respectively; p less than 0.05). In contrast, the change in mean arterial blood pressure correlated negatively with the change in endogenous insulin levels during the control experiment (r = -0.41, p less than 0.02). Exogenous insulin induced a similar reduction in fractional sodium excretion in normotensive and hypertensive diabetics (43 +/- 5.9% and 48 +/- 16.4% during the low insulin dose and 57 +/- 9.1% and 62 +/- 12.5% during the high insulin dose, respectively). A decline in blood pressure was noted that correlated with the whole body glucose uptake during the high insulin dose (r = 0.52, p less than 0.05). Since heart rate response and plasma norepinephrine level during the insulin clamp were comparable in both groups, an abnormality of the baroreceptor reflex is suggested. It appears that insulin resistance, but not insulin, is primarily related to hypertension. At the same time, insulin may still exert some effect on blood pressure by way of its renal or vasodilatory, or both, action.