Fasting Glucagon Concentrations Predict Longitudinal Changes in ß-Cell Function of Nondiabetic Humans

The diabetes-associated allele at rs7903146 in the TCF7L2 locus predisposes to type 2 diabetes (T2DM). Recent work has shown that this variant is associated with impaired postprandial glucagon suppression. Since defects in α-cell function occur independently of impaired β-cell function, we hypothesized that impaired glucagon suppression could accelerate progression to T2DM. Therefore, we recruited 73 nondiabetic subjects with prior (6.7 ± 0.3 years) measurement of β-cell function using the oral minimal model (Baseline) and compared these indices to those obtained during a repeat Experiment (Present). Weight increased slightly, but significantly (2.8 ± 0.7 Kg, p < 0.01) over the period of study. Overall Disposition Index (DI) which expresses β-cell responsivity (ϕ) as a function of insulin action (S_i) did not change (1234±160 vs. 1143±116 10^-14, dl/kg/min² per pmol/l, DI_Baseline vs. DI_Present, p=0.55). However, there was considerable heterogeneity with a decline in DI associated with impaired S_i Baseline after adjusting for age and weight changes (R² = 0.22, p = 1.2 x 10^-3). Of note, fasting glucagon concentrations at the time of initial study ([Glucagon]_Baseline) were also significantly and inversely related to DI_Present (R² = 0.13, p = 8 x 10^-3). Baseline nadir and integrated (Area Under Basal-AUB) glucagon concentrations did not predict (p > 0.05) changes in DI. Fasting [Glucagon]_Baseline, adjusted for age and weight changes interacts with S_i Baseline to predict the longitudinal decline in DI (DI_Baseline-DI_Present: (R² = 0.29, p < 1 x 10^-4). Taken together, these data suggest that defects in α-cell function, manifest as elevated fasting glucagon, drive subsequent decline in β-cell function. It remains to be ascertained if defects in postprandial-glucagon suppression attributable to or independent of, diabetes-associated genetic variation in TCF7L2 also interact with defects in fasting glucagon secretion to drive decline in β-cell function in nondiabetic humans.