Inhibition of purinoceptors amplifies glucose-stimulated insulin release with removal of its pulsatility

External ATP has been proposed to be an autocrine regulator of glucose-stimulated insulin secretion and responsible for the synchronization of the Ca2+ rhythmicity in the P-cells required for a pulsatile release of insulin from the pancreas. The importance of external ATP for glucose-stimulated insulin release was evaluated in rats with the aid of 2-deoxy-N-methyladenosine3,5-bisphosphate (MRS 2179), an inhibitor of the purinoceptors known to affect the Ca2+ signaling in R-cells. The concentration of cytoplasmic Ca2+ was measured in single P-cells and small aggregates with ratiometric fura-2 technique and the release of insulin recorded from isolated islets and the perfused pancreas. Addition of 1 mu mol/l ATP induced premature cytoplasmic Ca2+ concentration ([Ca2+](i)) oscillations similar to those found in P-cells exposed to 20 mmol/l glucose. In most experiments, the presence of 10 mu mol/l MRS 2179 did not remove the glucose-induced [Ca2+] rhythmicity in single R-cells or the synchronization seen in coupled cells. Nevertheless, the same concentration of MRS 2179 promptly interrupted the pulsatility (frequency 0.22 +/- 0.01/min) of insulin secretion, raising the total amounts released from the pancreas. Prolonged exposure of islets to 1 and 10 mu molA MRS 2179 enhanced insulin secretion at 20 mmol/l glucose 33% (P < 0.05) and 63% (P < 0.01), respectively, without affecting the release at 3 mmol/l glucose. The results support the idea that neural ATP signals entrain the islets into a common rhythm resulting in pulsatile release of insulin and that glucose stimulation of the secretory activity is counteracted by accumulation of inhibitory ATP around the beta-cells.