Glucagon-like peptide 1 increases mass but not frequency or orderliness of pulsatile insulin secretion

Glucagon-like peptide 1 (GLP-1) is a peptide hormone that is released from the gut after luminal stimulation. The hormone is a potent insulin secretagogue and is a potential novel pharmaceutical adjuvant in the treatment of NIDDM. Insulin is secreted as a series of punctuated secretory bursts superimposed on a basal insulin release. Recently, the contribution of these secretory bursts to overall insulin secretion has been evaluated, and studies using catheterization across the pancreas in a canine model and studies using deconvolution in humans have revealed that the majority of insulin is released during these secretory bursts. Moreover, the main regulation of insulin secretion is through perturbation of mass and frequency of these secretory bursts. The mode of delivery of insulin into the circulation seems important for insulin action, and it is therefore important to know the impact of a potential therapeutic insulin secretagogue on the mode of insulin secretion. To assess the effects of GLP-1 on the mass, frequency, amplitude, and overall contribution of pulsatile insulin secretion, we used a recently validated deconvolution model to examine these variables before and during infusion of GLP-1 in eight healthy men (age 28 ± 2 years; BMI 24 ± 2 kg/m²). At a constant glucose infusion (2.5 mg · kg^{- 1} · min^-1), near-steady state was reached at 75 min, and sampling was performed every minute at t = 75-115 and 145-185 min. At t = 115 min, an infusion of saline or GLP-1 (50 pmol · kg^-1 · min^-1) was given. The regularity of insulin secretion was measured by approximate entropy, a recently developed mathematical statistic, applied herein to assess the regularity in a hormone concentration time series. After GLP-1 infusion, there was an abrupt increase in the peripheral concentrations of serum C- peptide (696 ± 65 vs. 1.538 ± 165 pmol/l) and insulin (49 ± 8 vs. 138 ± 21 pmol/l) concentrations. This increase was mainly due to an increase in the pulsatile component of insulin secretion that was achieved by a fourfold increase in secretory burst mass (28.2 ± 4.4 vs. 100.1 ± 15.8 pmol · 1^{- 1} · pulse^-1; P < 0.001), and amplitude (12.7 ± 2.2 vs. 4.3 ± 7.7 pmol · 1^-1 · min^-1; P < 0.002), whereas the secretory burst frequency was not affected by GLP-1 (11.5 ± 0.7 vs. 12.6 ± 0.6 pulses/h; P = 0.4). As a consequence, the detected contribution of pulsatile to overall insulin secretion was increased from 56 ± 4 to 77 ± 4% (P < 0.005). The orderliness of the insulin release process was not deteriorated by short-term GLP-1 infusion as assessed by approximate entropy (1.19 ± 0.04 vs. 1.18 ± 0.04; P = 0.7).