Isolation of A-type K+ current in hair cells of the frog crista ampullaris

Different procedures to isolate the K+ A-type current (IA) from other membrane currents were tested on the complex inactivating outward K+ current generated in hair cells from the peripheral regions of the frog crista ampullaris. Experiments were performed in thin slices of epithelium using the whole-cell configuration of the patch-clamp technique. The conventional conditioning voltage protocol did not allow a satisfactory isolation of IA, due to the presence of other K+ currents showing overlapping steady-state inactivation properties. An attempt to block other K+ currents using calcium-free saline containing 50 mM TEA also failed to provide a satisfactory isolation of IA, due to contamination by a residual sustained current, probably consisting of a slow delayed outward K+ current (IK). Use of the selective A-channel blocker 4-aminopyridine (4-AP) at concentrations < 12 mM was also unsatisfactory because at these concentrations 4-AP produced a voltage-dependent blockade. Conversely, use of 4-AP at concentrations of 15-20 mM allowed a good separation of an uncontaminated IA. These results indicate that IA in hair cells of vestibular epithelium can be isolated most effectively by the 4-AP procedure, provided that sufficiently high concentrations of the A-channel blocker are used.