Plasma glucose and insulin responses to a graded iv infusion of glucose were compared in two groups of glucose-tolerant women divided on the basis of their insulin sensitivity. Resistance to insulinmediated glucose disposal was measured using the insulin suppression test, and the women studied were chosen to represent the highest and lowest quartiles of insulin resistance seen in the normal population. The sensitivity of the pancreatic -cell to glucose was assessed by measuring the glucose, insulin, and C peptide concentrations in response to continuous graded iv infusions of glucose at rates of 1, 2, 3, 4, 6, and 8 mg/kg⅐min for 40 min each. In addition, insulin secretion rates in response to the graded glucose infusion, calculated over each sampling period, were derived from deconvolution of peripheral plasma C peptide concentrations, using a two-compartment model of C peptide kinetics and standard parameters for C peptide clearance. Although plasma glucose concentrations were only slightly higher throughout the glucose infusion, the insulin concentrations were approximately doubled in the insulin-resistant subjects. When expressed as a function of the molar increments in plasma glucose achieved during the glucose infusion studies, the insulin-resistant women had a 90% higher (684 Ϯ 55 vs. 360 Ϯ 36 pmol/L⅐mmol/L; P Ͻ 0.001) total integrated plasma insulin response as the glucose concentration was increased from 5 to 9 mmol/L. However, the total integrated insulin secretory rate was only increased by 37% (1494 Ϯ 133 vs. 1093 Ϯ 125 pmol/mmol/L⅐min; P Ͻ 0.05) in the insulin-resistant group. This discrepancy suggested that insulin clearance was lower in the insulin-resistant subjects, and the calculation of this value, as the ratio of the total secretion of insulin to the area under the plasma insulin curve, was significantly lower in the insulin-resistant group (1.25 Ϯ 0.05 vs. 1.87 Ϯ 0.16 L/min⅐m 2 ; P Ͻ 0.005). These results show that the hyperinsulinemia of insulin resistance results from an increase in insulin secretion secondary to a shift to the left of the glucose-stimulated insulin response curve as well as a decrease in insulin clearance. (J Clin Endocrinol Metab 82: 1834 -1838, 1997 P REVIOUS STUDIES have demonstrated a highly significant relationship in normoglycemic individuals between resistance to insulin-mediated glucose disposal and the plasma insulin response to oral glucose (1-3). Although the plasma glucose concentration is increased in normal individuals proportionate to the degree of resistance to insulinmediated glucose disposal, the plasma insulin response to oral glucose is increased out of proportion to the coexisting plasma glucose concentration (1-3). Thus, the pancreatic -cell appears to adapt to and compensate for the chronic state of insulin resistance by secreting more insulin in response to a given increment in plasma glucose than would be the case in an insulin-sensitive individual. In this study we have used the graded glucose infusion protocol devised by Polonsky and a...
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