Relative Hypersecretion of Proinsulin in Rat Model of NIDDM

The plasma ratio of proinsulin to insulin is raised in individuals with non-insulin-dependent diabetes mellitus (NIDDM). Increased secretion of proinsulin relative to insulin is thought to be the cause, although differential changes in clearance have not been ruled out. This study was conducted in a rat model of NIDDM, 90% pancreatectomized (Px) rats, to investigate the pathophysiology of this observation. Proinsulin storage and secretion were assessed with high-performance liquid chromatography separation of the insulins and the proinsulins, followed by quantification of the peaks by insulin radioimmunoassay. In Px rats, the relative proportion of proinsulin in pancreas extracts was twice that of control (sham-operated) rats (15.6 ± 1.4 vs. 8.3 ± 1.4%, P < 0.01). Samples obtained from the portal vein during in vitro pancreas perfusion also had an elevated proinsulin fraction (Px, 10.3 ± 3.0; sham, 3.0 ± 0.6%; P < 0.006). In summary, 90% Px rats share many pathophysiological features with NIDDM, including loss of normal proinsulin homeostasis. Our results suggest that chronic hyperglycemia causes an intrinsic change in β-cells that is characterized by the increased storage and secretion of proinsulin.