A sustained TTX-sensitive current contributes to spontaneous activity of neonatal rabbit SA-node myocytes

Hyperpolarization-elicited potassium currents in GH3/B6 cells bathed in high-potassium external solution were recorded to assess effects of the class III antiarrhythmic agent E-4031 on the inactivating inward-rectifying potassium current (I(K,IR)). E-4031 potently blocked I(K,IR) With an IC50 value of 10 nM. The complete block of I(K,IR) achieved with concentrations ≤1 μM revealed the presence of a non-inactivating outward-rectifying current which contributed to the membrane currents recorded under control conditions. The time dependence of the I(K,IR) block depended on the concentration of E-4031. Two other methanesulfonanilides were investigated: WAY-123,398 (10 μM) also totally blocked I(K,IR), while sotalol (100 μM) was almost ineffective, Also lanthanum (100 μM) had only a very small effect on I(K,IR). E-4031 did not affect sodium, calcium and voltage-dependent outward-rectifying potassium currents, suggesting a selective block of I(K,IR) in GH3/B6 cells. In an external solution containing 16 mM potassium, the E-4031-sensitive current was present: as a steady outward current within a broad potential range positive to the potassium equilibrium potential, E(K). In many, but not all, cells E-4031 induced an increase in the frequency of action potentials suggesting an important role of I(K,IR) in controlling cell excitability. Our experiments show that E-4031 is a valuable tool in characterizing I(K,IR) and its physiological function.