Earlier studies had indicated that H441 cells express intermediate conductance K⁺ channels (K_Ca3.1) and so, in the present study, this sequence was isolated from these cells by RT-PCR, cloned into pCDNA3 and expressed in CHO cells by co-transfection with a pEGFP (Clontech). Tranfected cells were identified by GFP fluorescence and membrane currents recorded under voltage clamp in the whole cell recording configuration. The pipette and bath solutions were designed to maintain quasi-physiological ionic gradients and [Ca²⁺]_i was buffered using 5 mM EGTA. When [Ca²⁺]_i was ~0.5 mM, large outward currents flowed at physiologically relevant membrane potentials and replacing external Na⁺ with K⁺ depolarized the cells in a manner essentially identical to that predicted by the Nernst Equation. The conductive properties of the transfected cells are thus dominated by a K⁺ conductance which was shown to be clotrimazolesensitive. Membrane conductance was low when [Ca²⁺]_i was

0.05 mM or 0.2 mM but, under these conditions, EBIO, an activator of K_Ca3.1 caused a rapid increase in conductance. These data thus confirm expression but suggest that the activity of these channels will be minimal at the values of [Ca²⁺]_i found in resting cells.