Triggering and amplification of insulin secretion by dimethyl α-ketoglutarate, a membrane permeable α-ketoglutarate analogue

Introduction: It is estimated that about 70 percent of the insulin secretion stimulated by glucose and other fuels are due to the K_ATP channel-independent amplifying pathway which transduces signals from the energy metabolism to the exocytotic machinery of the beta-cell. Recently, cytosolic α-ketoglutarate has been suggested as a potential signalling compound in this context. This hypothesis was mainly based on the insulin-releasing effect of the membrane permeable ester dimethyl α-ketoglutarate (DMKG) which enters the beta-cell and is cleaved to produce cytosolic monomethyl α-ketoglutarate and eventually α-ketoglutarate.

Methods: By measuring insulin release, K_ATP channel currents, membrane potential, ATP/ADP ratio and fluorescence of NAD(P)H (reduced pyridine nucleotides) this hypothesis was tested using mouse pancreatic islets and primary beta-cells.

Results: At a substimulatory glucose concentration (5 mM), 15 mM DMKG produced a sustained insulin release, but no change of the islet ATP/ADP ratio and NAD(P)H fluorescence. In the absence of glucose, however, 15 mM DMKG did not stimulate insulin release although it significantly increased the ATP/ADP ratio and the NAD(P)H fluorescence. However, these increases attained only values comparable to those established by 5 mM glucose alone. Suprisingly, DMKG depolarized the plasma membrane of intact beta-cells and generated action potentials in the absence of glucose. This was apparently due to a direct inhibitory effect of DMKG on K_ATP channels as became evident from the use of inside-out membrane patches from β-cells. Finally, DMKG strongly amplified the insulin secretion induced by a maximally effective concentration of the K_ATP channel-blocking sulfonylurea glipizide in the presence of 5 mM glucose and moderately but significantly amplified the glipizide-induced secretion in the absence of glucose.

Conclusion: The insulinotropic effect of DMKG is not sufficient to demonstrate a role of cytosolic a-ketoglutarate in the amplifying pathway. Rather, DMKG produces a complex pattern of effects by influencing several signalling pathways in the B-cell. The metabolism of α-ketoglutarate generated intracellularly by ester cleavage contributes to the stimulatory effect both by indirect K_ATP channel inhibition (via activation of ATP production) and by a K_ATP channel-independent amplifying effect. In the presence of basal glucose, the direct K_ATP channel inhibition is of critical importance for the insulin-releasing property of DMKG.