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We used whole-cell patch clamp and fluorescent calcium imaging techniques to investigate the effects of adenosine 5'-triphosphate (ATP) on membrane currents and intracellular calcium concentration ([Ca2+]i)in rat retinal pigment epithelial (RPE) cells. In 62 % of RPE cells, application of 100 µM ATP elicited a fast inward current at negative membrane potentials. In 38 % of RPE cells recorded, a biphasic response to ATP was observed in which activation of the fast inward current was followed by activation of a delayed outward current.
The ATP-activated inward current was a non-selective cation (NSC) current that showed inward rectification, reversed at -1·5 ± 1 mV and was permeable to monovalent cations. The NSC current was insensitive to the P2 purinoceptor antagonists, suramin or PPADS but was activated by the purinoceptor agonists UTP, ADP and 2MeSATP.
The outward current activated by ATP reversed at -68 ± 3 mV (equilibrium potential for potassium (EK) = -84 mV) and was blocked by Ba2+ ions, consistent with the activation of a K+ conductance. The outward K+ conductance was also reduced by the maxi-KCa channel blocker iberiotoxin (IbTX; 10 nM), suggesting that ATP activated an outward Ca2+-activated K+ channel in rat RPE cells. The Ca2+-activated K+ current (IK(Ca)) was also activated by the purinoceptor agonists UTP, ADP and 2MeSATP.
In fluo-3 or fluo-4 loaded RPE cells, ATP and the pyrimidine agonist UTP elevated [Ca2+]i. The increase in Ca2+ was not dependent on extracellular Ca2+ influx, but was sensitive to the Ca2+-ATPase inhibitor thapsigargin, confirming the involvement of intracellular Ca2+ stores release.
These results suggest that rat RPE cells express both P2X purinoceptors that gate activation of a non-selective cation conductance and G protein-coupled P2Y purinoceptors that mediate Ca2+ release from intracellular stores and activation of a calcium-activated K+ current.
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