Mechanisms of insulin resistance after insulin-induced hypoglycemia in humans: the role of lipolysis

Abstract

Objective: Changes in glucose metabolism occurring during counterregulation are, in part, mediated by increased plasma free fatty acids (FFAs), as a result of hypoglycemia-activated lipolysis. However, it is not known whether FFA plays a role in the development of posthypoglycemic insulin resistance as well.

Research design and methods: We conducted a series of studies in eight healthy volunteers using acipimox, an inhibitor of lipolysis. Insulin action was measured during a 2-h hyperinsulinemic-euglycemic clamp (plasma glucose [PG] 5.1 mmo/l) from 5:00 p.m. to 7:00 p.m. or after a 3-h morning hyperinsulinemic-glucose clamp (from 10 a.m. to 1:00 p.m.), either euglycemic (study 1) or hypoglycemic (PG 3.2 mmol/l, studies 2-4), during which FFA levels were allowed to increase (study 2), were suppressed by acipimox (study 3), or were replaced by infusing lipids (study 4). [6,6-(2)H(2)]-Glucose was infused to measure glucose fluxes.

Results: Plasma adrenaline, norepinephrine, growth hormone, and cortisol levels were unchanged (P > 0.2). Glucose infusion rates (GIRs) during the euglycemic clamp were reduced by morning hypoglycemia in study 2 versus study 1 (16.8 +/- 2.3 vs. 34.1 +/- 2.2 micromol/kg/min, respectively, P < 0.001). The effect was largely removed by blockade of lipolysis during hypoglycemia in study 3 (28.9 +/- 2.6 micromol/kg/min, P > 0.2 vs. study 1) and largely reproduced by replacement of FFA in study 4 (22.3 +/- 2.8 micromol/kg/min, P < 0.03 vs. study 1). Compared with study 2, blockade of lipolysis in study 3 decreased endogenous glucose production (2 +/- 0.3 vs. 0.85 +/- 0.1 micromol/kg/min, P < 0.05) and increased glucose utilization (16.9 +/- 1.85 vs. 28.5 +/- 2.7 micromol/kg/min, P < 0.05). In study 4, GIR fell by approximately 23% (22.3 +/- 2.8 micromol/kg/min, vs. study 3, P = 0.058), indicating a role of acipimox per se on insulin action.

Conclusion: Lipolysis induced by hypoglycemia counterregulation largely mediates posthypoglycemic insulin resistance in healthy subjects, with an estimated overall contribution of approximately 39%.

FIG. 1.

Plasma glucose, insulin concentrations, and rates of glucose infusion in study 1 (euglycemia), study 2 (hypoglycemia), study 3 (hypoglycemia + acipimox), and study 4 (hypoglycemia + acipimox + heparin + intralipid). The diagonal area depicts t₁ (0–180 min, euglycemia or hypoglycemia), the white area depicts t₂ (180–420 min, euglycemia or recovery to hypoglycemia), and the dotted area depicts t₃ (420–540 min, euglycemic clamp) of each study.

FIG. 2.

Glucose utilization, endogenous glucose production, glucose oxidation, and lipid oxidation in study 1 (euglycemia), study 2 (hypoglycemia), study 3 (hypoglycemia + acipimox), and study 4 (hypoglycemia + acipimox + heparin + intralipid). The diagonal area depicts t₁ (0–180 min, euglycemia or hypoglycemia), the white area depicts t₂ (180–420 min, euglycemia or recovery to hypoglycemia), and the dotted area depicts t₃ (420–540 min, euglycemic clamp) of each study.

FIG. 3.

Plasma counterregulatory hormones glucagon, adrenaline, norepinephrine, cortisol, and growth hormone in study 1 (euglycemia), study 2 (hypoglycemia), study 3 (hypoglycemia + acipimox), and study 4 (hypoglycemia + acipimox + heparin + intralipid). The diagonal area depicts t₁ (0–180 min, euglycemia or hypoglycemia), the white area depicts t₂ (180–420 min, euglycemia or recovery to hypoglycemia), and the dotted area depicts t₃ (420–540 min, euglycemic clamp) of each study.

FIG. 4.

Plasma nonglucose substrates free fatty acids, glycerol, β-OH-butyrate, lactate, and alanine in study 1 (euglycemia), study 2 (hypoglycemia), study 3 (hypoglycemia + acipimox), and study 4 (hypoglycemia + acipimox + heparin + intralipid). The diagonal area depicts t₁ (0–180 min, euglycemia or hypoglycemia), the white area depicts t₂ (180–420 min, euglycemia or recovery to hypoglycemia), and the dotted area depicts t₃ (420–540 min, euglycemic clamp) of each study.