The signalling role of action potential depolarization in insulin secretion: Metabolism-dependent dissociation between action potential increase and secretion increase by TEA

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Graphical abstractWhen KATP channels are closed in the absence of glucose TEA still blocks Kv channels and increases cytosolic Ca2+, but inhibits insulin secretion. This cannot be overcome by Bay K8644.The K+ channel blocker, TEA is known to increase action potential amplitude and insulin secretion of mouse β-cells when added to a nutrient secretagogue. In the presence of a maximally effective sulfonylurea concentration (2.7 μM glipizide) the nutrient secretagogue α-ketoisocaproic acid (KIC, 10 mM) strongly increased insulin secretion (about elevenfold). Instead of enhancing the effect of KIC, TEA reduced the KIC-induced secretion by more than 50%. Also, the secretion rate produced by 2.7 μM glipizide alone was significantly reduced by TEA. In contrast, TEA enhanced the insulinotropic effect of glipizide when a basal glucose concentration (5 mM) was present. In the presence as well as in the absence of glucose glipizide produced a plateau depolarization with superimposed action potentials. Under both conditions, TEA increased the glipizide-induced action potential amplitude and further elevated the cytosolic free calcium concentration ([Ca2+]i) with an oscillatory characteristic. These effects depended on the activity of L-type Ca2+ channels, even though the effect of TEA differed from that of 1 μM of the Ca2+ channel opener, Bay K8644, which primarily increased action potential duration. TEA did not negatively affect parameters of β-cell energy metabolism (NAD(P)H fluorescence and ATP/ADP ratio), rather, it slightly increased NAD(P)H fluorescence. Apparently, TEA inhibits insulin secretion in the absence of glucose in spite of a persistent ability to block K+ ion conductance. Thus, the signalling role of action potential depolarization in insulin secretion may require reconsideration and ion conductance-independent actions of K+ channels may be involved in this paradox effect of TEA.

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