[Relationship between insulin secretion and insulin resistance in nondiabetic subjects with myotonic dystrophy--a...

Hyperinsulinemia in myotonic dystrophy (MyD) is generally considered as a result of insulin resistance. However, as a consequence of abnormality in systemic membrane fluidity in this disease, abnormal insulin clearance or intrinsic pancreatic beta-cell dysfunction could also contribute to the hyperinsulinemia. To clarify the cause of hyperinsulinemia in MyD, we examined the insulin resistance, insulin clearance and the capacity of insulin secretion in 6 patients with MyD. They received a euglycemic hyperinsulinemic (insulin 40 mU/m2/min) clamp study for the quantification of insulin resistance by the glucose infusion rate (GIR). We evaluated their capacity of insulin secretion by the ratio of increment of serum insulin level to that of plasma glucose level 30 minutes after a 75 g oral glucose load (insulinogenic index; I.I.) and the ratio of area under the curve of serum insulin levels to that of plasma glucose levels during a 75 g oral glucose load (AUCIRI/AUCPG). The I.I. (1.1 +/- 0.6 vs 0.4 +/- 0.3, p < 0.05) and the AUCIRI/AUCPG (0.5 +/- 0.2 vs 0.2 +/- 0.1, p < 0.01) in MyD were significantly greater than those of GIR, body mass index and sex matched 6 non-diabetic controls respectively. Insulin clearance, estimated by the level of insulin and C-peptide before and during a euglycemic clamp study did not differ between the two groups. These results indicate that the postload hyperinsulinemia observed in MyD is not solely resulted from insulin resistance. Other factors, such as intrinsic beta-cell disorder may be involved.