A-Type potassium currents active at subthreshold potentials in mouse cerebellar purkinje cells

Voltage-dependent and calcium-independent K⁺ currents were whole-cell recorded from cerebellar Purkinje cells in slices. Tetraethylammonium (TEA, 4 m m ) application isolated an A-type K⁺ current ( I _{k ( a )}) with a peak amplitude, at +20 mV, of about one third of the total voltage-dependent and calcium-independent K⁺ current. The I _{k ( a )} activated at about ?60 mV, had a V _0.5 of activation of ?24.9 mV and a V _0.5 of inactivation of ?69.2 mV. The deactivation time constant at ?70 mV was 3.4 ± 0.4 ms, while the activation time constant at +20 mV was 0.9 ± 0.2 ms. The inactivation kinetics was weakly voltage dependent, with two time constants; those at +20 mV were 19.3 ± 3.1 and 97.6 ± 9.8 ms. The recovery from inactivation had two time constants of 60.8 ms (78.4%) and 962.3 ms (21.6%). The I _{k ( a )} was blocked by 4-aminopyridine with an IC₅₀ of 67.6 μM. Agitoxin-2 (2 n m ) blocked 17.4 ± 2.1% of the I _{k ( a )}. Flecainide completely blocked the I _{k ( a )} with a biphasic effect with IC₅₀ values of 4.4 and 183.2 μM. In current-clamp recordings the duration of evoked action potentials was affected neither by agitoxin-2 (2 n m ) nor by flecainide (3 μM), but action potentials that were already broadened by TEA were further prolonged by 4-aminopyridine (100 μM). The amplitude of the hyperpolarisation at the end of depolarising steps was reduced by all these blockers.