The Site of Arachidonic Acid Release Drives Calcium Dynamics in β-Cells

Abstract

Insulin is the blood glucose-lowering hormone essential for glucose homeostasis. β-Cells secrete insulin in pulses in response to glucose. Loss of the oscillatory nature of insulin secretion is associated with the development of type II diabetes. Calcium is the most important trigger of insulin release even though the exact mechanism of insulin secretion regulation by calcium is elusive. Arachidonic acid (AA) is an essential signalling molecule involved in regulation of physiological functions of many cell types. In insulin secreting pancreatic β-cells AA was shown to modulate calcium levels and as result to trigger insulin secretion. However, the exact interplay between AA signalling and insulin secretion is still a matter of speculation. In the present work we investigated the difference of AA action at the internal membranes (IMs) and the plasma membrane (PM) of β-cells. To perform this study, we developed a caging group which permits localization, visualization and quantitative photorelease of AA exclusively on the PM of living cells. We applied it in combination with a previously reported caging group used to release AA on the IMs. We found that the release of AA on the PM and the IMs leads to a significantly different modulation of intracellular calcium dynamics. Uncaging of AA on the PM induces calcium oscillations in non-oscillating cells and increases the duration of calcium transients in oscillating cells, leading to overall higher calcium levels. Release of AA on the IM results in transiently or permanently diminished calcium oscillations in β-cells and lower average calcium levels. We attribute the observed effects to direct action of AA on Arachidonic acid Regulated Calcium (ARC) channels localized on the PM and potentially a negative feedback mechanism triggered by higher levels of AA at the internal membranes.