Hyperproinsulinemia Is Associated with Increased b Cell Demand after Hemipancreatectomy in Humans

The cause of disproportionate hyperproinsulinemia in patients with type II diabetes is controversial. To examine whether increased b cell demand might contribute, we measured proinsulin and insulin concentrations in clinically healthy humans who had undergone hemipancreatectomy for the purpose of organ donation, a procedure previously demonstrated to increase b cell demand and diminish insulin secretory reserve capacity. Subjects were studied at least 1 yr after hemipancreatectomy. Seven donors were followed prospectively and served as their own controls. Nine additional donors were matched with normal controls (crosssectional group). Fasting serum concentrations of intact proinsulin and conversion intermediates (total) were measured by a two-step radioimmunoassay; independent determinations of intact proinsulin and 32,33 split proinsulin were performed using an immunoradiometric assay. Serum total proinsulin values were significantly greater in hemipancreatectomized groups than controls (prospective group: predonation 5 6.24 6 1.14 pM, postdonation 5 34.63 6 17.47 pM, P , 0.005; cross-sectional group: controls 5 5.78 6 1.12 pM, donors 5 15.22 6 5.20 pM, P , 0.025). The ratio of total proinsulin to immunoreactive insulin was directly correlated with fasting plasma glucose and showed a significant inverse relationship to secretory reserve capacity. Both absolute and relative hyperproinsulinemia is found in hemipancreatectomized donors. These data demonstrate that partial pancreatectomy with its associated increase in b cell demand raises measures of proinsulin in humans. ( J. Clin. Invest. 1996. 97:455–460.)