Fig. 3.

β₁ and β₄ produce differential effects on conductance-voltage (G-V) relationships that are dependent on Ca²⁺ concentration. A: G-V relationships of inside-out macropatches from oocytes injected with Slo alone, Slo with β₁, and, separately, Slo with β₄, in different concentrations of “intracellular” Ca²⁺. Both β-subunits cause a shift in G-V function in a depolarizing direction in nominally Ca²⁺-free solutions. In contrast to β₁, which causes a shift in a hyperpolarizing direction at all concentrations of Ca²⁺ (1 μM and greater), β₄ causes a shift in a depolarizing direction in G-V relationships except when the concentration of intracellular Ca²⁺ exceeds 25 μM (and above). cSlo, n = 11; cSlo + β₁, n = 12; cSlo + β₄, n = 15. B: β₄ increases Ca²⁺ sensitivity of BK channels at voltages that span a hair cell's operating voltage range (−50 mV ± 5 mV). The effects of increasing Ca²⁺ concentration on G/G_max at both −60 mV and −40 mV reveal β₄ to sensitize Slo to physiological concentrations of Ca²⁺ (5–50 μM). There is a dramatic rise in current in the presence of the β₄-subunit. In contrast, the β₁-subunit confers increased channel opening at <10 μM Ca²⁺, at these voltages (cSlo, n = 11; cSlo + β₁, n = 12; cSlo + β₄, n = 15).