A Hypothetical Energy-Dissipating Mechanism Regulated by Glucose in β-Cells Preceding Sustained Insulin Secretion

In this review we propose the hypothesis that an energy-dissipating process precedes the continuous stimulation of insulin secretion by glucose. This process is mediated by connexin 36 hemichannels (Cx36H), or Cx36 connexons. Cx36H oligomers are expressed at the plasma membrane, and their gating activity (opening) is activated by plasma membrane depolarization after the closure of K⁺ATP channels by glucose (>5 mM) metabolism. This initial depolarization (1st step) might be responsible for the first phase of insulin secretion, with the subsequent opening of Cx36H increasing β-cell plasma membrane permeability, allowing for the efflux of metabolites (less than 1KD) (GABA, adenine nucleotides) and K⁺ (2nd step). This provokes a breakdown of oxidative glucose metabolism and the repolarization of the plasma membrane. As the extracellular glucose concentration increases further (>>5 mM), it exerts a progressive inhibition effect on Cx36H opening, allowing for the continuous stimulation of insulin secretion (3d step, second phase,). The glucose feature of regulating Cx36H closing with sigmoidal kinetics (8 mM IC₅₀ and around 20 mM at maximum) has been confirmed in mouse Cx36 connexin expression in Xenopus oocytes and in mouse islets stimulated by a range of glucose concentrations in the presence of 70 mM KCl. This gating activity was also inhibited by some non-metabolized glucose analogs. Glucose inhibition of Cx3H opening might not only contribute to making the insulin secretory response more specific for glucose but might also play a role in the pulsatility of sustained insulin secretion. Cx36H opening also offers the opportunity to potentiate the secretory effect in vivo by, permeant or not, metabolic stimuli. Confirmation of this novel physiological role for Cx36H in β-cells would place them as new susceptibility locus for type 1 and type 2 diabetes, whose physiological implication in the mechanism of insulin secretion regulation should be evaluated by in vivo studies in diabetic patients.