Nutrient stimuli of insulin secretion depolarize single beta-cells but not single alpha-cells

Objective: Like beta-cells, pancreatic alpha-cells express glucokinase and KATP channels, however, the increase of glucose inhibits glucagon secretion. It is unclear whether the secretion is regulated by alpha-cell intrinsic mechanisms and whether signal recognition by mitochondria plays a role in it. Recently, we observed that in contrast to beta-cells, alpha-cells show only a modest increase in the ATP/ADP ratio in response to glucose and no increase at all in response to alpha-ketoisocaproic acid (KIC) or glutamine plus BCH.

Methodology: Islets from NMRI mice were isolated and dispersed into single beta- and alpha-cells. The membrane potential of the single cells was measured using the perforated patch technique. Fluo4 AM was employed to assess the cytosolic calcium concentration of single cells while they were perfused on the stage of a patch-clamp microscope. In addition to the cellular capacitance the beta- and alpha-cell identity was checked by immunofluorescence after the experiment.

Results: In the presence of 1 mM glucose single beta-cells had a membrane potential of -61mV and reacted within 10 min to stimulation with either 30 mM glucose, 10 mM alpha-ketoisocaproic acid (KIC) or glutamine plus BCH (10 mM each) with a depolarization by 28 mV, 21 mV or 14 mV, respectively. The depolarization by glutamine plus BCH was much slower than the depolarization by glucose or KIC, so the maximal value was only attained during wash-out and may thus be underestimated. The reaction of the alpha-cells was much more heterogeneous. When only those alpha-cells which responded to arginine and stained positive for glucagon were included in the evaluation, the following picture emerges: The membrane potential at 1 mM glucose was very noisy and was about -49 mV. The reaction to glucose, KIC or glutamine plus BCH led to a depolarisation of 3 mV, 8 mV and 4 mV, respectively. The modest depolarizing effect of KIC on the alpha-cells, which was the only significant effect, was virtually complete after 2 min, the same time as required by the KATP-blocking sulfonylurea tolbutamide to depolarize the alpha-cells by ca. 15 mV. Thus, the depolarization by KIC is likely due to its direct effect on the KATP channels rather than on its effect on the mitochondrial ATP synthesis. The modest depolarizing effect of KIC on alpha-cells was reflected by a much lower increase of the cytosolic Ca2+concentration than in beta-cells.

Conclusion: Insulinotropic nutrient secretagogues depolarize beta-cells with kinetics which reflect their effect on the mitochondrial oxidative phosphorylation. The at best modest effect of these compounds on the membrane potential of alpha-cells fits to their virtual inefficiency to raise the ATP/ADP ratio in these cells. Alpha-cell-intrinsic recognition of nutrients, if existent at all, does not rely on the oxidative phosphorylation.

Publication History

Article published online:
28 May 2025