Spontaneous autocrine release of protons activates ASIC-mediated currents in HEK293 cells

When examining HEK293 cells by whole-cell patch-clamp electrophysiology we found spontaneous currents, present in almost all cells. These currents were carried by Na(+) ions, were inhibited by amiloride and by cells exposure to acidic (pH 6.3) extracellular solutions. These properties (ion carrier, amiloride-sensitivity, and inactivation by constant lowering of extracellular pH) were similar to the properties of proton-activated currents measured from the same cells. Spontaneous currents required intracellular ATP, were completely inhibited by intracellular Ca(2+) buffering with BAPTA and were suppressed by intracellular administration of vesicular H(+)ATPase inhibitor bafilomycin. ATP-induced Ca(2+) influx through P2X receptors in HEK293 cells stably transfected with P2X(2), P2X(2/3) or P2X(4) purinoreceptor subunits transiently potentiated amplitude and frequency of spontaneous currents; this effect was antagonized by bafilomycin. We concluded that spontaneous currents represent activation of acid-sensitive ion channels (ASICs) by autocrine vesicular release of protons from HEK cells.