Proceedings of The Physiological Society

Mitochondria: Form and function (London, UK) (2017) Proc Physiol Soc 38, PC07

Poster Communications

Defective metabolism provokes delayed hormone secretion in pancreatic islets

A. I. Tarasov1, J. Knudsen1, J. Adam1, M. Chibalina1, P. Rorsman1

1. OCDEM, University of Oxford, Oxford, United Kingdom.

Aim. Oxidative metabolism of pancreatic β-cell couples increases in peripheral blood glucose to secretion of hyperglycaemic hormone insulin. In prediabetic condition, insulin secretion is elevated and, interestingly, the hormone is secreted even after the removal of glucose stimulus, i.e. under the de facto basal conditions. Higher rates of basal, non-stimulated insulin secretion are reported in pancreatic islets isolated from type 2 diabetic (T2D) individuals, which can be modelled in islets from healthy donors by chronic hyperglycaemia. We aimed to elucidate the mechanism of the delayed response and the role of mitochondrial metabolism in this phenomenon. Materials and Methods. Experiments were performed on isolated mouse and human pancreatic islets. The dynamics of intracellular concentrations of ATP, Ca2+, cAMP, NAD(P)H, ψm and DAG was monitored in the islets cells in response to metabolic stimuli using laser scanning confocal or zoom microscopy. Insulin secretion was assayed using ELISA. Results. Chronic hyperglycaemia resulted in an extended acute metabolic response. Intracellular Ca2+ dynamics was observable in islets pre-cultured at high glucose for up to 1h after the return into basal glucose, which was the result of a rapid and prolonged increase of intracellular ATP/ADP ratio in response to the removal of the glucose clamp. This ATP "off-response" depended on the duration of the acute exposure to high glucose; it was observable in the case when methyl-succinate was used as a metabolic stimulus and was reflected in insulin secretion kinetics. Chronic hypoglycaemia, introduction of creatine in the culture medium, or genetic knock-out of fumarate hydratase (Fh1βKO) significantly attenuated the effect. The off-response was absent in the kinetics of DAG, which is synthesised de novo from glucose/glycerol-3-phosphate in β-cells, in a malonyl-CoA-dependent manner. Conclusions. The mechanism responsible for the elevated basal insulin secretion in prediabetic pancreatic islets relies on the prolonged/inert transduction of the metabolic stimulus in β-cells. The phenomenon has mostly a mitochondrial nature. The lack of the effect in the DAG kinetics argues against the anaplerotic origin and the ability of methylated succinate to induce the off-response excludes any glycolytic/glycogenolytic options too. The source of the signal can be restricted to the set of the TCA cycle reactions from aconitase to α-ketoglutarate as fumarate abundant in the Fh1βKO animals is likely to inhibit the former enzyme.

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