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 invest igated. 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 cha nnels was unchanged in Rb+ and Cs+, compared with K+, but biexponential ina ctivation over 10 s was reduced from similar to 100 % of peak current in Na + to similar to 65 % in K+, similar to 50 % in Rb+ and similar to 30 % in C s+. This occurred as a result of a decrease in both fast and slow component s 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 inactiva tion (tau(recovery)similar to 1 S). Mutation of residues at a putative regu latory site at R487 in the outer pore mouth did not affect slow inactivatio n or recovery from inactivation of hKv1.5, although sensitivity to extracel lular TEA was conferred. 4. Symmetrical reduction of both intra- and extracellular cation concentrat ions accelerated and augmented both components of inactivation of K+(K-d = 34.7 mM) and Cs+ (K-d = 20.5 mM) currents. These effects could be quantitat ively accounted for by unilateral reduction of intracellular K+ (K-i(+)) (K -d = 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 w ere 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 ac tions, although still of relatively low affinity. These results suggest the presence of ion binding sites capable of regulating inactivation located o n both intracellular and extracellular sides of the pore selectivity filter .