Dependence of the acid-sensitive ion channel, ASIC1a, on extracellular Ca2+ ions

The early H+-induced current in the embryonic spinal cord neurone depends on extracellular Ca2+ for its function. We have studied the Ca2+-dependence of homo- and heteromultimeric acid-sensing ion channels (ASICs) expressed in Cos cells. It was found that single-channel conductance: of both the ASIC2a and the ASIC1a channel is reduced at membrane potentials more negative than -40 mV by elevated extracellular Ca2+. Due to this effect on unitary currents, the macroscopic ASIC2a peak current at -60 mV decreases gradually with rising extracellular Ca2+ concentration. In addition, the macroscopic ASIC1a current is very small at low extracellular Ca2+ and increases with rising Ca2+ up to 5 mM before decreasing again at still higher concentrations of extracellular Ca2+. (C) 2001 Elsevier Science B.V. All rights reserved.