Autocrine activation of P2Y1 receptors couples Ca2+ influx to Ca2+ release in human pancreatic beta cells

There is evidence that ATP acts as an autocrine signal in beta cells but the receptors and pathways involved are incompletely understood. Here we investigate the receptor subtype(s) and mechanism(s) mediating the effects of ATP on human beta cells. We examined the effects of purinergic agonists and antagonists on membrane potential, membrane currents, intracellular Ca2+ ([Ca2+](i)) and insulin secretion in human beta cells. Extracellular application of ATP evoked small inward currents (3.4 +/- 0.7 pA) accompanied by depolarisation of the membrane potential (by 14.4 +/- 2.4 mV) and stimulation of electrical activity at 6 mmol/l glucose. ATP increased [Ca2+](i) by stimulating Ca2+ influx and evoking Ca2+ release via InsP(3)-receptors in the endoplasmic reticulum (ER). ATP-evoked Ca2+ release was sufficient to trigger exocytosis in cells voltage-clamped at -70 mV. All effects of ATP were mimicked by the P2Y((1/12/13)) agonist ADP and the P2Y(1) agonist MRS-2365, whereas the P2X((1/3)) agonist alpha,beta-methyleneadenosine-5-triphosphate only had a small effect. The P2Y(1) antagonists MRS-2279 and MRS-2500 hyperpolarised glucose-stimulated beta cells and lowered [Ca2+](i) in the absence of exogenously added ATP and inhibited glucose-induced insulin secretion by 35%. In voltage-clamped cells subjected to action potential-like stimulation, MRS-2279 decreased [Ca2+](i) and exocytosis without affecting Ca2+ influx. These data demonstrate that ATP acts as a positive autocrine signal in human beta cells by activating P2Y(1) receptors, stimulating electrical activity and coupling Ca2+ influx to Ca2+ release from ER stores.