Modulation of small conductance calcium-activated potassium channels in C6 glioma cells

Using the patch clamp technique, we have characterized a small conductance, calcium-activated potassium (SK) channel in the C6 glioma cell line. Elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_i) by applications of serotonin or ionomycin induced bursts of channel openings recorded in the cell-attached configuration. These channels underlie the serotonin-induced, [Ca²⁺]_i-activated whole-cell K⁺ conductance described previously. [Ca²⁺]_i directly activated SK channels in inside-out patches with a biphasic concentration dependence. Submicromolar [Ca²⁺]_i induced bursts of channel openings with a unitary conductance of about 25 pS, similar to that of the serotonin-induced channels. Supramicromolar [Ca²⁺]_i caused prolonged openings with a unitary conductance of about 35 pS, resulting in a pronounced increase of the average current in patches exposed to [Ca²⁺]_i above 100 μm. The two modes of opening reflect the activity of the same SK channel. The channel conductance depended on external K⁺ concentration with K_Dof 5 μm. The channel was slightly permeable to cations other than K⁺, with a permeability ratio for K⁺:Ca²⁺:Na⁺ of 1:0.040:0.030, respectively. ATP was required to maintain channel activity in outside-out patches but was not essential in inside-out patches. The modulation of SK channels in C6 cells by components in their microenvironment may be related to the role of glial cells in controlling the extracellular milieu in the CNS.