Modulation of slow inactivation in human cardiac Kv1.5 channels by extra- and intracellular permeant cations.

1. The properties and regulation of slow inactivation by intracellular and extracellular cations in the human heart K+ channel hKv1.5 have been investigated. Extensive NH2- and COOH-terminal deletions outside the central core of transmembrane domains did not affect the degree of inactivation. 2. The voltage dependence of steady-state inactivation curves of hKv1.5 channels was unchanged in Rb+ and Cs+, compared with K+, but biexponential inactivation over 10 s was reduced from approximately 100 % of peak current in Na+ to approximately 65 % in K+, approximately 50 % in Rb+ and approximately 30 % in Cs+. This occurred as a result of a decrease in both fast and slow components of inactivation, with little change in inactivation time constants. 3. Changes in extracellular cation species and concentration (5-300 mM) had only small effects on the rates of inactivation and recovery from inactivation (tau recovery approximately 1 s). Mutation of residues at a putative regulatory site at R487 in the outer pore mouth did not affect slow inactivation or recovery from inactivation of hKv1.5, although sensitivity to extracellular TEA was conferred. 4. Symmetrical reduction of both intra- and extracellular cation concentrations accelerated and augmented both components of inactivation of K+ (Kd = 34.7 mM) and Cs+ (Kd = 20.5 mM) currents. These effects could be quantitatively accounted for by unilateral reduction of intracellular K+ (K+i) (Kd = 43.4 mM) or Cs+i with constant 135 mM external ion concentrations. 5. We conclude that inactivation and recovery from inactivation in hKv1.5 were not typically C-type in nature. However, the ion species dependence of inactivation was still closely coupled to ion permeation through the pore. Intracellular ion modulatory actions were more potent than extracellular actions, although still of relatively low affinity. These results suggest the presence of ion binding sites capable of regulating inactivation located on both intracellular and extracellular sides of the pore selectivity filter.