Importance of peripheral insulin levels for insulin-induced suppression of glucose production in depancreatized dogs

It is generally believed that glucose production (GP) cannot be adequately suppressed in insulin-treated diabetes because the portal-peripheral insulin gradient is absent. To determine whether suppression of GP in diabetes depends on portal insulin levels, we performed 3-h glucose and specific activity clamps in moderately hyperglycemic (10 mM) depancreatized dogs, using three protocols: (a) 54 pmol · kg^-1 bolus + 5.4 pmol · kg^-1 · min^-1 portal insulin infusion (n = 7; peripheral insulin = 170±51 pM); (b) an equimolar peripheral infusion (n = 7; peripheral insulin = 294±28 pM, P < 0.001); and (c) a half-dose peripheral infusion (n = 7), which gave comparable (157±13 pM) insulinemia to that seen in protocol 1. Glucose production, use (GU) and cycling (GC) were measured using HPLC-purified 6-[³H]- and 2-[³H]glucose. Consistent with the higher peripheral insulinemia, peripheral infusion was more effective than equimolar portal infusion in increasing GU. Unexpectedly, it was also more potent in suppressing GP (73±7 vs. 55±7% suppression between 120 and 180 min, P < 0.001). At matched peripheral insulinemia (protocols 2 and 3), not only stimulation of GU, but also suppression of GP was the same (55±7 vs. 63±4%). In the diabetic dogs at 10 mM glucose, GC was threefold higher than normal but failed to decrease with insulin infusion by either route. Glycerol, alanine, FFA, and glucagon levels decreased proportionally to peripheral insulinemia. However, the decrease in glucagon was not significantly greater in protocol 2 than in 1 or 3. When we combined all protocols, we found a correlation between the decrements in glycerol and FFAs and the decrease in GP (r = 0.6, P < 0.01). In conclusion, when suprabasal insulin levels in the physiological postprandial range are provided to moderately hyperglycemic depancreatized dogs, suppression of GP appears to be more dependent on peripheral than portal insulin concentrations and may be mainly mediated by limitation of the flow of precursors and energy substrates for gluconeogenesis and by the suppressive effect of insulin on glucagon secretion. These results suggest that a portal-peripheral insulin gradient might not be necessary to effectively suppress postprandial GP in insulin-treated diabetics.