Metabolic Regulation of Hormone Secretion in Beta-Cells and Alpha-Cells of Female Mice: Fundamental Differences

Abstract

It is unclear whether the secretion of glucagon is regulated by an alpha-cell-intrinsic mechanism and whether signal recognition by the mitochondrial metabolism plays a role in it. To measure changes of the cytosolic ATP/ADP ratio, single alpha-cells and beta-cells from NMRI mice were adenovirally transduced with the fluorescent indicator PercevalHR. The cytosolic Ca 2+ concentration ([Ca 2+]i) was measured by use of Fura2 and the mitochondrial membrane potential by use of TMRE. Perifused islets were used to measure the secretion of glucagon and insulin. At 5mM glucose, the PercevalHR ratio in beta-cells was significantly lower than in alpha-cells. Lowering glucose to 1mM decreased the ratio to 69% within 10 minutes in beta-cells, but only to 94% in alpha-cells. In this situation 30mM glucose, 10mM alpha-ketoisocaproic acid and 10mM glutamine plus 10mM BCH (a non-metabolizable leucine analogue) markedly increased the PercevalHR ratio in beta-cells. In alpha-cells only glucose was slightly effective. However, none of the nutrients increased the mitochondrial membrane potential in alpha-cells, whereas all did so in beta-cells. The kinetics of the PercevalHR increase were reflected by the kinetics of [Ca 2+]i. increase in the beta-cells and insulin secretion. Glucagon secretion was markedly increased by washing out the nutrients with 1mM glucose, but not by reducing glucose from 5mM to 1mM. This pattern was still recognizable when the insulin secretion was strongly inhibited by clonidine. It is concluded that mitochondrial energy metabolism is a signal generator in pancreatic beta-cells, but not in alpha-cells.