Depolarization in the absence of exogenous nutrients inhibits the metabolic amplification by glucose but not KIC

Background and aims: Metabolic amplification of insulin secretion is the signaling process whereby nutrient secretagogues enhance secretion beyond the extent established by the triggering pathway. Separating triggering from amplifying pathway should facilitate investigation of amplification. Here, we characterized the role of mitochondrial respiration during amplification.

Methods: All parameters were measured by perifusion of freshly isolated mouse islets. Oxygen consumption rate (OCR) kinetics were measured by fluorescence quench technique and insulin secretion by ELISA of fractionated perifusate.

Results: A 60 min pre-perifusion with 500µM tolbutamide in absence of nutrients abolished the insulinotropic effect of 30 mM glucose but left that of 10 mM (α-Ketoisocaproic acid) KIC unchanged as compared to control (no tolbutamide in the 60 min pre-perifusion). The same dissociation was seen when tolbutamide was replaced by 15 mM KCl. Under control condition 30 mM glucose raised the OCR quickly from 6.5 to 10.4 pmol O₂ x min^-1 x islet^-1, thereafter a plateau existed. In the presence of 500µM tolbutamide 30 mM glucose was unable to raise the OCR beyond the prestimulatory level. The measurement of the insulin secretion by these islets confirmed that the lack of OCR increase was associated with a lack of secretory response. In contrast to glucose, KIC raised the OCR even in the presence of 500µM tolbutamide. Here, the kinetics of OCR closely paralleled those of insulin secretion.

Conclusion: In freshly isolated islets, the inhibition of metabolic amplification by prolonged depolarization in absence of exogenous nutrients points to a decisive role of mitochondrial metabolism.