Lifelong sequential changes in glucose tolerance and insulin secretion in genetically obese Zucker rats (fa/fa) fed a diabetogenic diet

Life-long sequential changes in glucose tolerance and insulin secretion were investigated in genetically obese Zucker rats (fa/fa) fed a diabetogenic diet rich in lard and sucrose. Comparisons were made with lean littermates (Fa/-) receiving normal chow diet. At 3-month intervals, seven to nine lean and obese rats had two permanent venous catheters implanted, allowing stress- and pain-free sampling of blood before, during, and after substrate administration. Intravenous glucose, iv arginine, and oral glucose tolerance were tested. The obese rats progressively developed hyperglycemia and severe hyperinsulinemia; their basal glycemia reached 8.8 +/- 1.1 vs. 5.8 +/- 0.2 mmol/liter in the lean rats at 46 weeks of age; respective insulinemia was 287.7 +/- 61.9 and 18.1 +/- 2.8 mU/liter (mean +/- SD). In the obese rats a distinct loss in glucose tolerance was seen with progression of age in spite of rising stimulated insulin secretion, which suggests progressive development of insulin resistance without exhaustion of B-cell secretory capacity. Absence of insulin deficiency was also suggested by immunohistochemical staining of pancreatic tissue specimens from obese rats, which showed large populations of insulin-containing cells. Like the obese animals, lean rats exhibited a decrease in insulin sensitivity with age. Relating basal individual glycemia and insulinemia, a rise by 1 mmol/liter in glycemia was associated with a 8.8-fold rise in basal insulinemia in lean rats, but only with a 1.8-fold increase in obese rats. Similar correlations for stimulated glycemia and insulinemia suggest impaired glucose sensitivity of pancreatic B-cells in obese vs. lean rats. In conclusion, hyperglycemia and hyperinsulinemia in insulin-resistant obese Zucker rats on a diabetogenic diet are not characterized by quantitatively deficient B-cell secretory capacity, but, rather, by impaired B-cell sensitivity to glucose with qualitatively intact regulation of glycemia and insulinemia at elevated plasma concentrations.