Calcium signalling through nucleotide receptor P2X1 in rat portal vein myocytes

1. ATP-mediated Ca2+ signalling was studied in freshly isolated rat portal vein myocytes by means of a laser confocal microscope and the patch-clamp t echnique. 2. In vascular myocytes held at -60 mV, ATP induced a large inward current that was supported mainly by activation of P2X1 receptors, although other P 2X receptor subtypes (P2X3, P2X4 and P2X5) were revealed by reverse transcr iption-polymerase chain reaction. 3. Confocal Ca2+ measurements revealed that ATP-mediated Ca2+ responses sta rted at initiation sites where spontaneous or triggered Ca2+ sparks were no t detected, whereas membrane depolarizations triggered Ca2+ waves by repeti tive activation of Ca2+ sparks from a single initiation site. mediated Ca2 release requires, at least, RYR2, but not RYR3. 4. ATP-mediated Ca2+ responses depended on Ca2+ influx through non-selectiv e cation channels that activated, in turn, Ca2+ release from the intracellu lar store via ryanodine receptors (RYRs). Using specific antibodies directe d against the RYR subtypes we show that ATP-mediated Ca2+ release requires, at least RYR2, but not RYR3. 5. Our results suggest that, in vascular myocytes, Ca2+ influx through P2X1 receptors may trigger Ca2+-induced Ca2+ release at intracellular sites whe re RYRs are not clustered.