The mouse β3b' subunit produces leftward gating shifts at all Ca²⁺ concentrations. (A) GV curves were generated from measurement of tail currents following activation protocols as in Fig. 6, either for patches expressing α subunits alone (n = 5; black symbols) or for patches with α+mβ3b' subunits (n = 5; red symbols) for cytosolic Ca²⁺ concentrations from 0 to 300 μM. (B) V_h estimated from a fit of a Boltzmann equation (Eq. 1) to GV curves at each Ca²⁺ is plotted as a function of applied [Ca²⁺]_i. (C) Time constants of activation and deactivation for α + mβ3b' channels are plotted as a function of voltage for four Ca²⁺ concentrations, highlighting a large change in activation rates for an increase in Ca²⁺ from 1 to 10 μM, and the more gradual slowing in tail current deactivation as Ca²⁺ is increased. (D) Time constants for activation and deactivation are compared for α alone (black symbols) and α+mβ3b' (red symbols), showing the similarity in activation rates at positive potentials, but marked changes in deactivation. (E) Red symbols plot single channel Po as a function of membrane potential for six single channel patches with Po approaching 0.8 at +200 mV. For comparison, the GV curve at 0 Ca²⁺ normalized to a maximum saturating fractional conductance of 0.95 is plotted. For comparison, the single channel Po was determined for five patches expressing single Slo1 α channels at +200 mV and 0 μM Ca²⁺ (solid black circle).