Single channel Ca2+ currents in Helix pomatia neurons.

Unitary Ca2+ currents of TEA injected Helix neurons were recorded in the "Giga seal" situation (6,7) from microscopic membrane patches exposed to 50 mM [Ca2+]o, O [Na+]o, 20 mM [TEA+]o and 2.5 microM [TTX]o. Constant field assumptions yield a channel permeability of 2.9 +/- 1.0 x 10(-14) cm3s-1 corresponding to slope conductances of 5 to 15 pS between 0 and -30 mV. Frequency of occurrence of the units strongly increased with depolarization. Mean open time of the Ca2+ channels was about 3 ms without obvious dependence on voltage. A similar open time was seen with [Ba2+]o, yielding about double the current strength when compared with [Ca2+]o.