Disproportionately elevated proinsulin levels reflect the degree of impaired B cell secretory capacity in patients with noninsulin-dependent diabetes mellitus.

An increased proportion of fasting proinsulin (PI) relative to immunoreactive insulin (IRI; increased PI/IRI) occurs in noninsulin-dependent diabetes mellitus (NIDDM). To determine whether the magnitude of the increase in PI/IRI is an indicator of the degree of reduced B cell secretory capacity, we measured fasting plasma glucose, PI, IRI, and PI/IRI and related them to maximal B cell secretory capacity (AIRmax) in 9 subjects with NIDDM [age, 61 +/- 3 yr; body mass index (BMI), 27.5 +/- 1.3 kg/m2; duration of NIDDM, 10.8 +/- 1.8 yr; mean +/- SEM] and in 10 healthy subjects matched for age and BMI (age, 61 +/- 6 yr; BMI, 27.9 +/- 1.5 kg/m2). AIRmax was quantified as the incremental insulin response to i.v. arginine at maximal glycemic potentiation (plasma glucose > 25 mmol/L). Mean fasting plasma glucose was 13.7 +/- 1.4 mmol/L (range, 7.5-18.3 mmol/L) in NIDDM subjects and 5.0 +/- 0.1 mmol/L in the controls. Fasting PI was higher in NIDDM (33.1 +/- 5.2) than in controls (9.4 +/- 2.5 pmol/L; P < 0.01), but IRI levels were similar (93.4 +/- 10.9 vs. 82.8 +/- 23.4 pmol/L; P = NS). The PI/IRI ratio was significantly elevated in NIDDM compared to control subjects (35.9 +/- 4.1% vs. 12.8 +/- 0.8%; P < 0.01). After elevation of the glucose level to 30.3 +/- 0.4 mmol/L (NIDDM) and 30.3 +/- 0.5 mmol/L (controls), AIRmax was quantified as 622 +/- 71 pmol/L in NIDDM and 1997 +/- 315 pmol/L in controls, (P < 0.001). The PI/IRI ratio correlated inversely with AIRmax in the NIDDM patients (r = -0.76; P < 0.01). We conclude that the magnitude of the elevation in fasting PI/IRI is related to the reduction in AIRmax. Thus, the fasting PI/IRI ratio appears to be a marker of the degree of reduced AIRmax in NIDDM.