Mechanism of [Ca2+](i) increase by extracellular ATP in isolated rabbit renal proximal tubules

The effects of extracellular ATP on cytosolic Ca2+ concentrations ([Ca2+](i)) and cell membrane potentials (V-b) of rabbit renal proximal tubules were investigated using fura 2 and microelectrodes. ATP transiently increased [Ca2+](i) without an apparent sustained phase, and the maximum effect was obtained at 10 mu M. ADP, adenosine 5'-O-(3-thiotriphosphate), and 2-methylthioadenosine 5'-triphosphate were equally effective as ATP, whereas UTP, adenosine, and alpha beta-methyleneadenosine 5'-triphosphate were far less effective. The [Ca2+](i) responses to ATP were strongly inhibited by reactive blue 2, a P-2-purinergic receptor antagonist. The removal of extracellular Ca2+ as well as the addition of thapsigargin also markedly attenuated the responses to ATP. In addition, ATP had virtually no effect on V-b, except for the occasional small depolarization by 300 mu M ATP. These results indicate that extracellular ATP increases [Ca2+](i) through a P-2-purinergic receptor and that this effect of ATP is dependent on both extracellular and intracellular Ca2+.