Fasting insulin concentration is highly correlated with quantitative insulin sensitivity check index

Na+-sensitive elevation in blood pressure is ENaC independent in diet-induced obesity and insulin resistance

The majority of patients with obesity, insulin resistance, and metabolic syndrome have hypertension, but the mechanisms of hypertension are poorly understood. In these patients, impaired sodium excretion is critical for the genesis of Na(+)-sensitive hypertension, and prior studies have proposed a role for the epithelial Na(+) channel (ENaC) in this syndrome. We characterized high fat-fed mice as a model in which to study the contribution of ENaC-mediated Na(+) reabsorption in obesity and insulin resistance. High fat-fed mice demonstrated impaired Na(+) excretion and elevated blood pressure, which was significantly higher on a high-Na(+) diet compared with low fat-fed control mice. However, high fat-fed mice had no increase in ENaC activity as measured by Na(+) transport across microperfused cortical collecting ducts, electrolyte excretion, or blood pressure. In addition, we found no difference in endogenous urinary aldosterone excretion between groups on a normal or high-Na(+) diet. High fat-fed mice provide a model of metabolic syndrome, recapitulating obesity, insulin resistance, impaired natriuresis, and a Na(+)-sensitive elevation in blood pressure. Surprisingly, in contrast to previous studies, our data demonstrate that high fat feeding of mice impairs natriuresis and produces elevated blood pressure that is independent of ENaC activity and likely caused by increased Na(+) reabsorption upstream of the aldosterone-sensitive distal nephron.

Assessing the predictive accuracy of QUICKI as a surrogate index for insulin sensitivity using a calibration model

The quantitative insulin-sensitivity check index (QUICKI) has an excellent linear correlation with the glucose clamp index of insulin sensitivity (SI(Clamp)) that is better than that of many other surrogate indexes. However, correlation between a surrogate and reference standard may improve as variability between subjects in a cohort increases (i.e., with an increased range of values). Correlation may be excellent even when prediction of reference values by the surrogate is poor. Thus, it is important to evaluate the ability of QUICKI to accurately predict insulin sensitivity as determined by the reference glucose clamp method. In the present study, we used a calibration model to compare the ability of QUICKI and other simple surrogates to predict SI(Clamp). Predictive accuracy was evaluated by both root mean squared error of prediction as well as a more robust leave-one-out cross-validation-type root mean squared error of prediction (CVPE). Based on data from 116 glucose clamps obtained from nonobese, obese, type 2 diabetic, and hypertensive subjects, we found that QUICKI and log (homeostasis model assessment [HOMA]) were both excellent at predicting SI(Clamp) (CVPE = 1.45 and 1.51, respectively) and significantly better than HOMA, 1/HOMA, and fasting insulin (CVPE = 3.17, P < 0.001; 1.67, P < 0.02; and 2.85, P < 0.001, respectively). QUICKI and log(HOMA) also had the narrowest distribution of residuals (measured SI(Clamp) - predicted SI(Clamp)). In a subset of subjects (n = 78) who also underwent a frequently sampled intravenous glucose tolerance test with minimal model analysis, QUICKI was significantly better than the minimal model index of insulin sensitivity (SI(MM)) at predicting SI(Clamp) (CVPE = 1.54 vs. 1.98, P = 0.001). We conclude that QUICKI and log(HOMA) are among the most accurate surrogate indexes for determining insulin sensitivity in humans.