Type 2 Diabetes (T2DM) and prediabetes are characterized by β-cell and α-cell dysfunction which can occur independently. Defects in α-cell response to rising glucose can be evaluated with an oral or intravenous challenge: the former reflects normal physiology, the latter is unaffected by incretins or gut motility. We compared Glucagon Secretion Rate (GSR) in response to rising glucose during a Graded Glucose Infusion (GGI) and an Oral Glucose Tolerance Test (OGTT) . We studied 39 non-diabetic, weight-stable subjects (53 ± 2 yrs, 29 ± 1 Kg/M2) , with different fasting and glucose tolerance status. All subjects underwent a 75g OGTT and a GGI, with glucose infusion doubling every hour for 4 hours. GSR was calculated by deconvolution from plasma concentrations of glucagon on both study days (GSRGGI and GSROGTT) . The suppression of GSRGGI could be described with a single-exponential relationship between GSR and glucose concentrations. The slope of the log-transformed secretion line allows estimation of the change in glucose necessary to suppress GSR by 50% (G50) . This index of suppressibility is elevated in people with impaired fasting glucose compared to normal fasting glucose (2.7 ± 0.2 vs. 3.8 ± 0.4 mmol/L, p=0.01) . Intriguingly, GSROGTT, exhibited a delayed response to rising glucose (compared to GSRGGI) , during the early part of the OGTT. However, the relationship of GSROGTT to glucose did not differ from that of GSRGGI during the fasting state and at the time of peak postprandial suppression. This allowed the calculation of G50 during the OGTT, which was similar to that observed during the GGI (R2=0.66) and correlated with weight, age and fasting glucose (R2=0.49) . These data demonstrate the ability of a novel index (G50) , to reproducibly quantify α-cell suppression by oral or intravenous glucose in non-diabetic subjects. Future studies will be necessary to determine if this relationship also applies to T2DM and how gastrointestinal and other factors alter the rate of GSR suppression during an OGTT.
M.C.Laurenti: None. A.Manduca: None. C.Cobelli: None. C.Dalla man: Research Support; Becton, Dickinson and Company, Sanofi-Aventis Deutschland GmbH. A.Vella: Advisory Panel; Crinetics Pharmaceuticals, Inc., Rezolute, Inc., vTv Therapeutics, Zealand Pharma A/S, Other Relationship; Novo Nordisk.
National Institutes of Health (DK078646, DK116231)