Activation and Two Modes of Blockade by Strontium of Ca(2+)-activated K(+) Channels in Goldfish Saccular Hair Cells

Effects of internal Sr(2+) on the activity of large-conductance Ca(2+)-activated K(+) channels were studied in inside-out membrane patches from goldfish saccular hair cells. Sr(2+) was approximately one-fourth as potent as Ca(2+) in activating these channels. Although the Hill coefficient for Sr(2+) was smaller than that for Ca(2+), maximum open-state probability, voltage dependence, steady state gating kinetics, and time courses of activation and deactivation of the channel were very similar under the presence of equipotent concentrations of Ca(2+) and Sr(2+). This suggests that voltage-dependent activation is partially independent of the ligand. Internal Sr(2+) at higher concentrations (>100 μM) produced fast and slow blockade both concentration and voltage dependently. The reduction in single-channel amplitude (fast blockade) could be fitted with a modified Woodhull equation that incorporated the Hill coefficient. The dissociation constant at 0 mV, the Hill coefficient, and zd (a product of the charge of the blocking ion and the fraction of the voltage difference at the binding site from the inside) in this equation were 58–209 mM, 0.69–0.75, 0.45–0.51, respectively (n = 4). Long shut events (slow blockade) produced by Sr(2+) lasted ∼10–200 ms and could be fitted with single-exponential curves (time constant, τ(l−s)) in shut-time histograms. Durations of burst events, periods intercalated by long shut events, could also be fitted with single exponentials (time constant, τ(b)). A significant decrease in τ(b) and no large changes in τ(l−s )were observed with increased Sr(2+) concentration and voltage. These findings on slow blockade could be approximated by a model in which single Sr(2+) ions bind to a blocking site within the channel pore beyond the energy barrier from the inside, as proposed for Ba(2+) blockade. The dissociation constant at 0 mV and zd in the Woodhull equation for this model were 36–150 mM and 1–1.8, respectively (n = 3).