Single channel analysis reveals different modes of Kv1.5 gating behavior regulated by changes of external pH

In the voltage-gated potassium channel Kv1.5, extracellular acidification decreases the peak macroscopic conductance and accelerates slow inactivation. To better understand the mechanistic basis for these two effects, we recorded unitary currents of Kv1.5 expressed in a mouse cell line (ltk-) using the voltage clamp technique both in cell-attached and excised outside-out patches. Single channel current amplitude at 100 mV (1.7 +/- 0.2 pA at pH 7.4, 1.7 +/- 0.2 pA at pH 6.4) and the single channel conductance between 0 and 100 mV (11.8 +/- 0.6 pS at pH 7.4 and 11.3 +/- 0.8 pS at pH 6.4) did not change significantly with pH. External acidification significantly decreased the number of active sweeps, and this reduction in channel availability accounted for most of the reduction of the peak macroscopic current. The results of runs analyses suggested the null sweeps occur in clusters, and the rate constants for the transition between clusters of null and active sweeps at pH 6.4 were slow (0.12 and 0.18 s(-1), to and from the active clusters, respectively). We propose that low pH facilitates a shift from an available mode (mode A) into an unavailable mode of gating (mode U). In addition to promoting mode U gating, external acidification accelerates depolarization-induced inactivation, which is manifest at the single channel level as a reduction of the mean burst length and an apparent increase of the interburst interval. These effects of external acidification, which are thought to reflect the protonation of a histidine residue in the turret (H-463), point to an important role for the turret in the regulation of channel availability and inactivation.