The hyperglycaemic hormone glucagon, which is released from pancreatic islet alpha-cells, plays a critical role in the maintenance of blood glucose homeostasis. Several factors including nutrients, circulating hormones and neurotransmitters control glucagon secretion. The secretory process in these endocrine cells is triggered by a rise in intracellular calcium concentration ([Ca²⁺]_i). In the present study, we have investigated the effects of the neurotransmitter and beta cell secretory product ATP on the Ca²⁺ signalling system of mouse alpha-cells within intact islets using confocal microscopy (1). Data were expressed as means ± SEM. Statistical significance was analysed using a Student’s t test (p<0.05). All cases were found significant. Hypoglycaemic conditions (0.5 mM glucose), which stimulate glucagon secretion, induced oscillatory Ca²⁺ signals in alpha cells with a frequency of about 0.9 min^-1. Addition of 100 μM of ATP markedly inhibited these low glucose-induced Ca²⁺ signals (62.3 ± 7.5% blockade, n=19). These results were reproduced in the presence of clonidine (70.7 ± 9.4%, n=11), a potent inhibitor of insulin release, suggesting that the effects on alpha-cells were not due to paracrine stimulation from neighbouring beta-cells. The inhibitory effects were also generated by both ADP and adenosine (68.5 ± 7.5%, n=24; 75.7 ± 5.8%, n=23, respectively), products resulting from ATP hydrolysis by ectonucleotidases, and by the non-hydrolysable ATPγS and ADPβS (29.7 ± 5.3%, n=8; 49.9 ± 6.1%, n=22, respectively), indicating thus, the potential involvement of multiple P1 and P2 receptors (2). Because no [Ca²⁺]_i elevations derived from Ca²⁺ influx were monitored during ATP stimulation, we ruled out the presence of functional P2X receptors. While ATP stimulation in the beta-cell triggers Ca²⁺ release from intracellular stores via phospholipase C activation, we discarded this possibility in alpha-cells because any [Ca²⁺]_i rise was detected. We also explored the involvement of adenylyl cyclase since the effect of multiple purine receptors is mainly mediated by changes in the cAMP levels, and this signalling molecule has an important effect on alpha-cell Ca²⁺ signalling. The addition of the adenylyl cyclase inhibitor 2’,5’-dideoxyadenosine mimicked the effects of ATP on [Ca²⁺]_i signals (63.2 ± 7.7%, n=19). All these results suggest that ATP is a potent inhibitor of Ca²⁺ signalling in pancreatic alpha-cells, implicating the activation of multiple P1 and P2Y receptors coupled to G proteins. Although a further characterisation of the potential mechanism is required, the signalling pathway probably involves changes in cytoplasmic cAMP levels.