Although glucagon released from pancreatic alpha-cells plays an essential role in the control of glycaemia, little is known about the mechanisms that regulate its secretion. In the present study, we studied in intact islets of Langerhans the effects of the neurotransmitter and beta-cell secretory product ATP, and its derivatives, on the Ca²⁺ signalling system and secretion of alpha-cells. To analyze Ca²⁺ signals in intact islets at the single-cell level, we monitored fluorescence changes in thin optical sections by confocal microscopy. Hypoglycaemic conditions (0.5 mM glucose), which stimulate glucagon secretion, induced oscillatory Ca²⁺ signals with a frequency of about 0.9 min-1. Addition of 1-100 micromolar of ATP, inhibited these Ca²⁺ signals (n=19). These blocking effects were also produced by both ADP and adenosine (n=24, n=23, respectively), products resulting from ATP hydrolysis, and by the non-hydrolyzable ATP[gamma]S and ADP[beta]S (n=8, n=22, respectively), indicating the potential involvement of multiple P1 and P2 receptors. Using immunocytochemistry, we detected in glucagon-containing cells the presence of the P2 receptor subtype P2Y1, and the P1 receptor subtypes A1 and A2A. Agonists of these receptors blocked Ca²⁺ signals as well, indicating their involvement in the inhibitory effects of ATP and adenosine. Measurements of glucagon release after static incubation of intact islets revealed that purinergic stimulation leads to a decrease in glucagon secretion.

These results demonstrate that purinergic receptors are important regulators of the pancreatic alpha-cell function. This work was supported by grants (BFU2004-07283 and PCI2005-A7-0131) from Ministerio de Educación y Ciencia to IQ.