Adrenaline Stimulates Glucagon Secretion by Tpc2-Dependent Ca²⁺ Mobilization From Acidic Stores in Pancreatic α-Cells

Adrenaline is a powerful stimulus of glucagon secretion. It acts by activation of β-adrenergic receptors, but the downstream mechanisms have only been partially elucidated. Here, we have examined the effects of adrenaline in mouse and human α-cells by a combination of electrophysiology, imaging of Ca²⁺ and PKA activity, and hormone release measurements. We found that stimulation of glucagon secretion correlated with a PKA- and EPAC2-dependent (inhibited by PKI and ESI-05, respectively) elevation of [Ca²⁺]_i in α-cells, which occurred without stimulation of electrical activity and persisted in the absence of extracellular Ca²⁺ but was sensitive to ryanodine, bafilomycin, and thapsigargin. Adrenaline also increased [Ca²⁺]_i in α-cells in human islets. Genetic or pharmacological inhibition of the Tpc2 channel (that mediates Ca²⁺ release from acidic intracellular stores) abolished the stimulatory effect of adrenaline on glucagon secretion and reduced the elevation of [Ca²⁺]_i Furthermore, in Tpc2-deficient islets, ryanodine exerted no additive inhibitory effect. These data suggest that β-adrenergic stimulation of glucagon secretion is controlled by a hierarchy of [Ca²⁺]_i signaling in the α-cell that is initiated by cAMP-induced Tpc2-dependent Ca²⁺ release from the acidic stores and further amplified by Ca²⁺-induced Ca²⁺ release from the sarco/endoplasmic reticulum.