Immature properties of large-conductance calcium-activated potassium channels in rat neuroepithelium.

The pharmacological and biophysical properties of large-conductance Ca-activated K (BK) channels from embryonic rat telencephalic neuroepithelium were investigated with in situ patch-clamp techniques. A fraction of these channels exhibited properties characteristic of BK channels recorded in well differentiated cells, including normal gating mode (BKN channels). The vast majority of BK channels expressed distinctive properties, the most conspicuous being their buzz gating mode (BKB channels). BKB channels were insensitive to a concentration of charybdotoxin that completely and reversibly blocked BKN channels. In contrast with the strict dependence of BKN channel activation on cytoplasmic Ca, BKB channels displayed substantially high open probability (Po) after inside-out patch excision in a Ca-free medium. Intracellular trypsin down-regulated the Po of BKB channels, which then exhibited a greater sensitivity to cytoplasmic Ca, mainly in the positive direction (increased Po with increased Ca). This suggested a modulatory role for Ca as opposed to its gating role in BKN channels. Ca ions also reduced current amplitude of both types of channels. BKB channels were less voltage sensitive than BKN channels, but this was not correlated with their lower Ca sensitivity. We speculate that BKB channels may represent immature forms in the developmental expression of BK channels.