Impaired glucose metabolism and reduced insulin clearance in elderly hypertensives

Targeting Insulin-Degrading Enzyme in Insulin Clearance

Hepatic insulin clearance, a physiological process that in response to nutritional cues clears ~50–80% of circulating insulin, is emerging as an important factor in our understanding of the pathogenesis of type 2 diabetes mellitus (T2DM). Insulin-degrading enzyme (IDE) is a highly conserved Zn²⁺-metalloprotease that degrades insulin and several other intermediate-size peptides. Both, insulin clearance and IDE activity are reduced in diabetic patients, albeit the cause-effect relationship in humans remains unproven. Because historically IDE has been proposed as the main enzyme involved in insulin degradation, efforts in the development of IDE inhibitors as therapeutics in diabetic patients has attracted attention during the last decades. In this review, we retrace the path from Mirsky’s seminal discovery of IDE to the present, highlighting the pros and cons of the development of IDE inhibitors as a pharmacological approach to treating diabetic patients.

Community-based study on summer-winter difference in insulin resistance in Kin-Chen, Kinmen, Taiwan

BACKGROUND

The aim of this community-based study was to explore the summer-winter difference in insulin resistance in Kin-Chen, Kinmen.

METHODS

A total of 2,412 residents aged 40 and over was enrolled in a mass survey in Kin-Chen, Kinmen, by the Yang-Ming Crusade, a volunteer organization of well-trained medical students from National Yang-Ming University. All participants were investigated in winter (first phase, January and February, before Chinese New Year) and summer (secondary phase, July and August) in 2002. Structured questionnaires, demographic and physical data, lifestyle, and blood chemistry parameters were collected.

RESULTS

Higher levels of fasting insulin, HOMA-insulin resistance and triglycerides, but lower levels of high-density lipoprotein cholesterol were found in summer than in winter. The prevalence of metabolic syndrome was higher in summer than in winter, with differences of 7.7% in both genders (p = 0.0092 in men, p = 0.0037 in women). Body mass index (BMI), age and physical activity were significantly correlated with metabolic syndrome. After controlling for BMI and other risk profiles, summer was independently and positively associated with fasting insulin and insulin resistance regardless of metabolic syndrome.

CONCLUSION

Fasting insulin, insulin resistance and prevalence of metabolic syndrome were higher in summer than in winter. BMI and season were 2 major determinants of the variation in fasting insulin. The contextual impacts of seasonal variation in shaping metabolic syndrome or insulin resistance in populations need to be reemphasized.

Possible role of oxidative stress in the pathogenesis of hypertension

Recently oxidative stress has been proposed as the cause of hypertension. An imbalance in superoxide and nitric oxide production may account for reduced vasodilation, which in turn can favor the development of hypertension. In vitro and in human studies support this hypothesis. The supplementation of antioxidants, particularly in the form of fresh fruit and vegetables, reduces blood pressure, supporting a role for free radicals in hypertension.

Insulin resistance and insulin pulsatility in essential hypertension

OBJECTIVE

Studies in normal humans and in patients with type 2 diabetes mellitus have demonstrated a close inverse relationship between peripheral insulin sensitivity and the frequency of short-term insulin secretory pulses in the systemic circulation. Our objective was to study this relationship in essential hypertension.

DESIGN

Study of insulin sensitivity and insulin pulse characteristics in hypertensive subjects and normotensive controls using well-established techniques.

METHODS

Twelve subjects with essential hypertension and 12 age- and sex-matched normotensive controls were recruited. Insulin action was measured using the glucose clamp technique combined with isotope dilution methodology. Insulin pulsatility in the peripheral circulation was assessed by sampling every 2 min for 90 min after an overnight fast Pulses were identified using the computer program Pulsar.

RESULTS

Insulin sensitivity index (glucose infusion rate/ serum insulin) was lower in the hypertensive patients (P= 0.01) and fasting insulin was increased (P= 0.008) compared to controls. The frequency and amplitude of insulin pulses were similar in the two groups. Insulin pulse frequency and insulin sensitivity were inversely related in the normotensive group (r= -0.68, P= 0.015), but not in the hypertensive group (r= -0.23, P= 0.48). Insulin clearance was reduced in the hypertensive group (P= 0.03), and was inversely related to insulin pulse frequency in the two groups combined (r = -0.51, P= 0.01).

CONCLUSIONS

Insulin action was not related to insulin pulse frequency in essential hypertension, in contrast to the situation in normal man.

Correlation of fasting serum C-peptide and insulin with markers of metabolic syndrome-X in a homogenous Chinese population with normal glucose tolerance

We surveyed 1447 men and 1800 women aged 30 years (mean 46.7 years) with normal glucose tolerance in Kin-Chen, Kinmen. Correlations of fasting serum insulin and C-peptide with various clinical and biochemical parameters were analyzed by multiple linear regression analysis. Women had significantly higher levels of insulin than men (98+/-43 vs. 91+/-43 pM, p<0.0001), yet they also had a more favorable cardiovascular risk profile. Insulin was positively associated with the female sex, height, body mass index, waist-to-hip ratio, serum triglyceride, total cholesterol, uric acid, and fasting plasma glucose, and was negatively associated with age, smoking, and high-density lipoprotein cholesterol. Independent correlates for C-peptide were similar to those of insulin, except for the addition of mean blood pressure and the exclusion of age and total cholesterol. Significant interaction of sex-body mass index (coefficient = -0.0051, p = 0.0232) was detected for C-peptide only. In conclusion, both fasting serum insulin and C-peptide are quantitatively associated with cardiovascular risk factors in this homogeneous Chinese population with normal glucose tolerance. The female sex is independently associated with higher insulin and C-peptide levels, and the strength of the positive association between the female sex and C-peptide reduces when the body mass index increases.

Acute effects of blood pressure elevation on insulin clearance in normotensive healthy subjects

Reduced clearance of insulin from plasma contributes to the hyperinsulinemia associated with essential hypertension (EH); however, the association between impaired insulin clearance and EH remains unexplained. Whether elevated blood pressure (BP) affects insulin clearance is unknown; therefore, we used the hyperinsulinemic euglycemic clamp to determine the effects of BP elevation on insulin clearance and sensitivity in eight healthy volunteers. Placebo infusion increased mean BP by 2.6+/-1.6 mm Hg, which was significantly less than rises produced by phenylephrine, an alpha1-adrenoceptor agonist (+11+/-1.8 mmHg, P<.05), or by angiotensin II (+13+/-1.3 mmHg, P<.01). Although beta-adrenoceptor stimulation with isoproterenol did not change mean BP (+3.6 mm Hg, P=NS), it significantly increased systolic pressure (+23+/-2.8 mm Hg versus +2.3+/-4.6 mm Hg with placebo P<.01). Insulin secretion (ie, C-peptide concentrations) was not affected by any of the treatments; however, phenylephrine significantly reduced the metabolic clearance rate of insulin (MCRinsulin) (16.6+/-1.0 mL/kg per minute with placebo versus 13.6+/-0.7 mL/kg per minute with phenylephrine, P<.01) and thereby increased plasma insulin concentrations (66+/-5.1 microU/mL with placebo versus 79+/-4.1 microU/mL with phenylephrine, P<.05). Phenylephrine also increased glucose utilization (42+/-5.8 micromol/kg per minute during placebo versus 58+/-4.8 micromol/kg per minute during phenylephrine, P<.05); however, this was proportional to the increased insulin concentrations; therefore, insulin sensitivity was unchanged. MCRinsulin and plasma insulin concentrations were not affected by angiotensin II; however, glucose utilization increased to 51+/-2.7 micromol/kg per minute (P<.01 versus placebo), indicating insulin sensitivity was increased. MCRinsulin was unaffected by isoproterenol. Thus, alpha-adrenergic stimulation but not increased BP per se is a potent regulator of insulin clearance and plasma insulin concentrations.

Diabetes mellitus, hypertension, and cardiovascular disease: which role for oxidative stress?

Accelerated atherosclerotic vascular disease is the leading cause of mortality in patients with diabetes mellitus. Endothelium-derived nitric oxide (NO) is a potent endogenous nitrovasodilator and plays a major role in modulation of vascular tone. Selective impairment of endothelium-dependent relaxation has been demonstrated in aortas of both nondiabetic animals exposed to elevated concentrations of glucose in vitro and insulin-dependent diabetic animals. The impaired NO release in experimentally induced diabetes may be prevented by a number of antioxidants. It has been hypothesized that oxygen-derived free radicals (OFR) generated during both glucose autoxidation and formation of advanced glycosylation end products may interfere with NO action and attenuate its vasodilatory activity. The oxidative injury may also be increased in diabetes mellitus because of a weakened defense due to reduced endogenous antioxidants (vitamin E, reduced glutathione [GSH]). A defective endothelium-dependent vascular relaxation has been found in animal models of hypertension and in hypertensive patients. An imbalance due to reduced production of NO or increased production of free radicals, mainly superoxide anion, may facilitate the development of an arterial functional spasm. Treatment with different antioxidants increases blood flow in the forearm and decreases blood pressure and viscosity in normal humans; vitamin E inhibits nonenzymatic glycosylation, oxidative stress, and red blood cell microviscosity in diabetic patients. Long-term randomized clinical trials of adequate size in secondary and primary prevention could support the free-radical hypothesis for diabetic diabetic vascular complications and the use of antioxidants to reduce the risk of coronary heart disease.

Pyrazinoylguanidine downregulates the glucose fatty-acid cycle in hypertensive, hyperinsulinemic diabetic patients

Eight hypertensive patients with noninsulin dependent diabetes mellitus (NIDDM) were administered the experimental drug pyrazinoylguanidine (PZG) either alone or in combination with calcium-channel or beta-blockers. This treatment appeared to "downregulate" the glucose fatty acid cycle and reduced both systolic and diastolic blood pressures and mean body weight. Patients served as their own controls in this dose-escalation study, which included placebo treatment (baseline) 3 weeks, 300 mg PZG for 3 weeks and 600 mg for 3 weeks. PZG reduced increased serum concentrations of free fatty acids (FFA), glucose, and triglycerides (TG). TG concentrations correlated inversely with serum HDL-cholesterol concentrations. The beta-blockers used by several patients increased their FFA, glucose, insulin and TG concentrations, as well as blunting their response to PZG. The calcium-channel blockers exerted these effects to a much lesser extent. PZG reduced or abolished glycosuria, related to PZG's capacity to decrease hyperglycemia. Withdrawal of PZG restored glycosuria, as blood sugar increased. PZG was well tolerated. No patient reported any adverse effect or missed a weekly clinic visit (12 weeks). PZG deserves further study as supplementary and/or replacement therapy in NIDDM patients who are hypertensive and hyperlipidemic.

Hyperinsulinemia in hypertension: increased secretion, reduced clearance or both?

Peripheral hyperinsulinism is said to be associated and perhaps implicated in the pathogenesis of hypertension. There is however some inconsistency in the evidence of the relationship between insulin and blood pressure. We prospectively investigated glucose metabolism, insulin and C-peptide values and serum lipids in a large sample of hypertensive as compared with age and body habitus-matched normotensive subjects. As a group, the 145 hypertensives (blood pressure: 160/99 +/- 8.5/6.5 mmHg, mean +/- SD) had significantly elevated fasting plasma insulin (p < 0.02), total and LDL-cholesterol (p < 0.01) than 132 normotensive control subjects. The fasting HbA1c (glycated hemoglobin A1c)/insulin ratio, an estimate of insulin sensitivity, was significantly lower (5.15 +/- 1.45) in the hypertensives than normotensives (5.8 +/- 1.5, p < 0.001). Hypertensives had normal fasting C-peptide levels and lower C-peptide/insulin molar ratios, indicating low hepatic insulin extraction. There was no correlation between mean blood pressure (1/3 systolic + 2/3 diastolic) and fasting serum C-peptide (p = 0.14), insulin (p = 0.11), HbA1c/insulin ratio (p = 0.6), C-peptide/insulin ratio (p = 0.22) and HbA1c (p = 0.19), even after adjusting for age, BMI and family history of diabetes. The differences between hypertensives and normotensives persisted after dividing the subjects according to the presence/absence of either obesity or impaired glucose tolerance, but the significance was lost due to the smaller samples of the subgroups. The obese hypertensives with impaired glucose tolerance had the lowest values of insulin sensitivity and clearance in the fasting state.(ABSTRACT TRUNCATED AT 250 WORDS)