Essential Hypertension Is Associated With Decreased Insulin Clearance and Insulin Resistance

Selection of Study Subjects

A total of seventy-five white volunteers (48 hypertensive and 27 normotensive) were studied. Investigators explained the purpose and methods of the study, and all participants gave their informed, written consent. The study was approved by the Institutional Review Board for Human Studies of the University of Texas Southwestern Medical Center at Dallas. The inclusion criteria for the hypertensive subjects were as follows: (1) presence of chronic hypertension with an average sitting diastolic pressure between 95 and 114 mm Hg on two separate readings 1 week apart 3 weeks after discontinuation of all antihypertensive medications; (2) age 21 years or older; (3) weight less than 40% over ideal body weight; (4) absence of angina pectoris, previous myocardial infarction, cardiac failure, arrhythmia, and renal dysfunction; (5) absence of the use of other concomitant medication that could affect insulin sensitivity; and (6) fasting glucose level less than 110 mg/dL. The normotensive subjects were all 21 years of age or older, less than 40% over ideal body weight, and free of any significant disease and took no concomitant medications. Systolic pressure levels were less than 140 mm Hg and diastolic pressure levels less than 85 mm Hg. All subjects had an initial laboratory screening panel that included a complete blood count and a chemistry panel that included determination of liver function and fasting glucose and lipid, electrolyte, serum creatinine, and uric acid levels.

Experimental Protocol

The study participants were admitted to the General Clinical Research Center at Parkland Memorial Hospital the day before the studies, where they were assured of a nonstressful environment. Duplicate samples for fasting insulin and glucose levels were obtained on the day of admission. On the following morning, an insulin infusion study was performed as follows: A small plastic catheter was placed in a hand vein in a retrograde fashion to obtain blood samples. This hand was placed in a warm box at 65°C to obtain arterialized venous blood. A long, thin plastic catheter was placed in an antegrade fashion in an antecubital vein for infusion of insulin and 20% dextrose (17.7% glucose). Plasma insulin level was raised acutely, and a continuous infusion of insulin was maintained at 40 mU per meter squared of body surface area per minute during 2 hours. Dextrose was infused at a variable rate according to the results of plasma glucose levels measured at 5-minute intervals on samples with the use of an enzymatic glucose analyzer. Glucose infusion rates were determined by the protocol of DeFronzo et al.²⁰ With this methodology, plasma glucose levels were maintained at basal levels with less than 5% variation from this level. Infusion pumps (model 2205, Harvard Apparatus) were used during the procedure. Insulin levels were measured in blood samples obtained every 10 minutes during the procedure by a sensitive radioimmunoassay (Diagnostic Products Corp), with intra-assay and interassay coefficients of variation of 2% to 5% and 5% to 10%, respectively. Cross-reactivity with proinsulin with this assay is 40%. We do not expect this cross-reactivity to significantly interfere with the calculation of insulin clearance because during hyperinsulinemic insulin infusions, the contribution of endogenously secreted insulin and proinsulin to the total insulin measured in the assay is in the range of 5%. The mean insulin level between minutes 90 and 120 was defined as the steady-state insulin level. Insulin-mediated glucose uptake (M value) was calculated with the use of the glucose infusion rates during minutes 90 to 120 of the clamp and was expressed in milligrams glucose per kilogram fat-free mass per minute. During insulin infusions, at the levels used in this study, hepatic glucose output has previously been shown to be completely suppressed in nondiabetic subjects.¹¹ C peptide levels were also measured at time 0 and at minutes 90 and 120 during the clamps with a sensitive radioimmunoassay.

Body density was obtained in all subjects using underwater weight. Percentage of body fat was derived from body density, and fat-free mass was calculated by subtracting fat mass from total body weight. Waist-to-hip ratio was calculated by dividing the minimum circumference at the level of the umbilicus and the maximum circumference at the level of the gluteus.

Forty-six of the study subjects underwent two studies at identical insulin infusion rates (40 mU/m² per minute) as part of a different protocol. The intrasubject variability in steady-state insulin levels during these two insulin infusions was 13%.

The metabolic clearance rate of insulin (MCR_i) was calculated as previously published²¹ by dividing the insulin infusion rate by the steady-state insulin concentration, which is obtained by subtracting the estimated residual insulin secretion from the mean insulin concentration between minutes 90 and 120 of the study. The estimated residual insulin secretion is the product of the basal (preinfusion) insulin level multiplied by the ratio of C peptide at baseline to C peptide at minutes 90 to 120 of the insulin infusion. Thus, the formula for MCR_i is as follows²¹ : MCR_i=Insulin Infusion Rate/{[Insulin_90-120]−[Insulin₀]×(C Peptide_90-120/C Peptide₀)}.

Statistical Analysis

Statistical analysis was performed with BMDP software (BMDP Statistical Software). Results are expressed as mean±SE. Blood pressure was treated as a categorical variable (hypertensive, normotensive). The means of MCR_i, M value, steady-state insulin level, and fasting insulin level were compared between the normotensive and hypertensive groups with a two-sample Student's t test. To evaluate the potential influences of obesity, sex, and insulin sensitivity on MCR_i, we used ANCOVA and multiple regression models to analyze MCR_i according to hypertension status with sex, percentage of body fat, M value, and waist-to-hip ratio as covariates. We also assessed the effect of MCR_i, adiposity, M value, sex, and hypertension on fasting insulin levels using multiple regression analysis. Variables for the regression models were selected to minimize multicolinearity. Subsequently, stepwise regression was performed. A value of P<.05 was considered statistically significant.