Quantal release of ATP from clusters of PC12 cells

Although ATP is important for intercellular communication, little is known about the mechanism of endogenous ATP release due to a dearth of suitable models. Using PC12 cells known to express the P2X(2) subtype of ATP receptors and to store ATP with catecholamines inside dense-core vesicles, we found that clusters of PC12 cells cultured for 3-7 days generated small transient inward currents (STICs) after an inward current elicited by exogenous ATP. The amplitude of STICs in individual cells correlated with the peak amplitude of ATP-induced currents. STICs appeared as asynchronous responses (approximately 20 pA average amplitude) for 1-20 s and were investigated with a combination of patch damping, Ca2+ imaging, biochemistry and electron microscopy. Comparable STICs were produced by focal KCI pulses and were dependent on extraceflular Ca2+. STICs were abolished by the P2X antagonist PPADS and potentiated by Zn2+, suggesting they were mediated by P2X(2) receptor activation. The highest probability of observing STICs was after the peak of intracellular Ca2+ increase caused by KCI. Biochemical measurements indicated that KCI application induced a significant release of ATP from PC12cells. Electron microscopy studies showed narrow clefts without 'synaptic-like' densities between clustered cells. Our data suggest that STICs were caused by quantal release of endogenous ATP by depolarized PC12 cells in close juxtaposition to the recorded cell. Thus, STICs maybe a new experimental model to characterize the physiology of vesicular release of ATP and to study the kinetics and pharmacology of P2X(2) receptor-mediated quantal currents.