Characterization of a calcium-activated potassium channel from rabbit intestinal smooth muscle incorporated into planar bilayers.

Interaction of vesicles from a microsomal fraction of rabbit intestinal smooth muscle with planar bilayers promotes the incorporation of a large conductance potassium-selective channel. The channel conductance fluctuates between two states: closed and open and the fraction of time the channel dwells in the open state is a function of the electric potential difference and the calcium concentrations. This channel seems to correspond to a Ca-activated K channel described by other authors in smooth muscle cells with the patch-clamp technique. Single-channel conductance is a saturating function of the potassium concentration. The relationship between conductance and concentration cannot be described by a hyperbolic function, suggesting multiple occupancy of the channel. The single-channel conductance is 230 pS in symmetrical 0.1 M KCl. Current is a linear function of the applied voltage in the range between -100 and +100 mV, at concentrations of 0.1 M KCl or higher. At lower concentrations, current-to-voltage curves bend symmetrically to the voltage axis. Sodium, lithium and cesium ions do not pass through the channel and the permeability for Rb is 66% that of potassium. All these alkali cations and Ca2+ block the channel in a voltage-dependent manner. A two-site three-barrier model on Eyring absolute reaction rate theory can account for the conduction and blocking characteristics.