Novel type of ion channel activated by Pb2+, Cd2+, and Al3+ in cultured mouse neuroblastoma cells

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Abstract
Superfusion with Pb2+ induces a slow, noninactivating and reversible inward current in voltage-clamped N1E-115 neuroblastoma cells. The amplitude of this inward current increases in the range of 1–200 μm Pb2+. Single-channel patch-clamp experiments have revealed that this inward current is mediated by discrete ion channels. Reversal potentials from linearI–V relationships are close to 0 mV for whole-cell and single-channel currents and the single-channel conductance amounts to 24 pS. The Pb2+-induced membrane current is not mediated by various known types of ion channels, since it is not blocked by external tetrodotoxin, tetraethylammonium,d-tubocurarine, atropine, ICS 205-930 and by internal EGTA. In Na+-free solutions superfusion with Pb2+ neither evokes a whole-cell inward current, nor single-channel openings. At −80 mV the open-time distribution of the single channels activated by 1μm Pb2+ is dual exponential with time constants of 17 and 194 msec. When the Pb2+ concentration is increased from 1 to 20 μm these time constants decrease to 2 and 13 msec, but the amplitude of single-channel currents remains −1.9 nA. Cd2+ and Al3+ induce inward currents and single-channel openings similar to Pb2+. Time constants fitted to the open-time distribution of single channels are 14 and 135 msec in the presence of 1 μm Cd2+ and 15 and 99 msec in the presence of 50 μm Al3+. Conversely, Cu2+ induces an irreversible inward current in neuroblastoma cells. Single-channel openings are undetected in the presence of Cu2+ and in Na+-free solutions Cu2+ is still able to induce an inward current. It is concluded that Pb2+, Cd2+ and possibly Al3+ activate a novel type of metal ionactivated (MIA) channel in N1E-115 cells.