Voltage Dependence of Slow Inactivation in Shaker Potassium Channels Results from Changes in Relative K⁺ and Na⁺ Permeabilities

Time constants of slow inactivation were investigated in NH₂-terminal deleted Shaker potassium channels using macro-patch recordings from Xenopus oocytes. Slow inactivation is voltage insensitive in physiological solutions or in simple experimental solutions such as K⁺_o//K⁺_i or Na⁺_o//K⁺_i. However, when [Na⁺]_i is increased while [K⁺]_i is reduced, voltage sensitivity appears in the slow inactivation rates at positive potentials. In such solutions, the I-V curves show a region of negative slope conductance between ∼0 and +60 mV, with strongly increased outward current at more positive voltages, yielding an N-shaped curvature. These changes in peak outward currents are associated with marked changes in the dominant slow inactivation time constant from ∼1.5 s at potentials less than approximately +60 mV to ∼30 ms at more than +150 mV. Since slow inactivation in Shaker channels is extremely sensitive to the concentrations and species of permeant ions, more rapid entry into slow inactivated state(s) might indicate decreased K⁺ permeation and increased Na⁺ permeation at positive potentials. However, the N-shaped I-V curve becomes fully developed before the onset of significant slow inactivation, indicating that this N-shaped I-V does not arise from permeability changes associated with entry into slow inactivated states. Thus, changes in the relative contributions of K⁺ and Na⁺ ions to outward currents could arise either: (a) from depletions of [K⁺]_i sufficient to permit increased Na⁺ permeation, or (b) from voltage-dependent changes in K⁺ and Na⁺ permeabilities. Our results rule out the first of these mechanisms. Furthermore, effects of changing [K⁺]_i and [K⁺]_o on ramp I-V waveforms suggest that applied potential directly affects relative permeation by K⁺ and Na⁺ ions. Therefore, we conclude that the voltage sensitivity of slow inactivation rates arises indirectly as a result of voltage-dependent changes in the ion occupancy of these channels, and demonstrate that simple barrier models can predict such voltage-dependent changes in relative permeabilities.