Block of cloned BKCa channels (rSlo)expressed in HEK 293 cells by N-methyl d -glucamine

We have investigated the conductance properties of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels formed by stable expression of the rSlo gene in HEK 293 cells. Single-channel recordings were obtained from inside-out patches excised into solution containing 100 μM Ca²⁺ to ensure a relatively high open probability over the range of membrane potentials studied (–120 to +100 mV). The unitary conductance of these channels at +80 mV was 221.6±5.4 pS in symmetrical 140 mM K⁺. Decreasing the K⁺ concentration on either side of the membrane, while maintaining ionic strength by adding N-methyl d-glucamine (NMDG⁺), reduced the unitary conductance. The reduction in conductance was greater when internal K⁺ was lowered by replacement with NMDG⁺. However, if sucrose was used as the internal K⁺ substitute instead of NMDG⁺ the reduction in unitary conductance was similar to that seen on reducing external K⁺. A rate-theory model whereby NMDG⁺ produces a very rapid block of the BK_Ca channel from the inside, but not the outside, is able to describe our results. Received:18 May 1998 / Received after revision: 17 June 1998 / Accepted: 2 July 1998