Inhibition of Ca2+signaling and glucagon secretion in mouse pancreatic α-cells by extracellular ATP and purinergic receptors

Abstract

Glucagon secreted from pancreatic α-cells plays a critical role in glycemia, mainly by hepatic glucose mobilization. In diabetic patients, an impaired control of glucagon release can worsen glucose homeostasis. Despite its importance, the mechanisms that regulate its secretion are still poorly understood. Since α-cells are particularly sensitive to neural and paracrine factors, in this report we studied the role of purinergic receptors and extracellular ATP, which can be released from nerve terminals and β-cell secretory granules. Using immunocytochemistry, we identified in α-cells the P2 receptor subtype P2Y1, as well as the P1 receptors A1and A2A. In contrast, only P2Y1and A1receptors were localized in β-cells. To analyze the role of purinergic receptors in α-cell function, we studied their participation in Ca2+signaling. At low glucose concentrations, mouse α-cells exhibited the characteristic oscillatory Ca2+signals that lead to secretion. Application of ATP (1–10 μM) abolished these oscillations or reduced their frequency in α-cells within intact islets and isolated in culture. ATPγS, a nonhydrolyzable ATP derivative, indicated that the ATP effect was mainly direct rather than through ATP-hydrolytic products. Additionally, adenosine (1–10 μM) was also found to reduce Ca2+signals. ATP-mediated inhibition of Ca2+signaling was accompanied by a decrease in glucagon release from intact islets in contrast to the adenosine effect. Using pharmacological agonists, we found that only P2Y1and A2Awere likely involved in the inhibitory effect on Ca2+signaling. All these findings indicate that extracellular ATP and purinergic stimulation are effective regulators of the α-cell function.