Different mechanisms of the inhibition of the transient outward current in rat ventricular myocytes

Abstract

The mechanism of drug-induced inhibition of the transient outward current, I_to, has been investigated in rat ventricular myocytes using the whole cell patch clamp technique. I_to was activated by 300 ms depolarizing voltage clamp steps in 10 mV increments from −50 mV up to +40 mV. At +40 mV, I_to peaked after about 3 ms, and the time course of inactivation was appropriately described by two time constants, τ_fast = 17 ms and τ_slow = 203 ms. Verapamil, quinidine sulfate and nifedipine preferentially depressed I_to at the end of the 300 ms depolarizing voltage clamp step; the inactivation of I_to was accelerated by all drugs, whereas peak I_to was less affected. The time course of drug action at +40 mV was calculated by the fractional changes of I_to. Verapamil, quinidine sulfate and nifedipine exerted a block of I_to. increasing during the depolarizing voltage clamp step. The onset of block in response to verapamil, quinidine sulfate and nifedipine (30 μmol/each) was appropriately described by monoexponential functions with time constants τ_on = 9.3, 1.7 and 1.1 ms, respectively. Relief from block by verapamil, quinidine sulfate and nifedipine at −50 mV was assessed by comparison of the recovery process of peak I_to from inactivation with or without drugs. τ_off amounted to 695 ms in the case of quinidine sulfate; verapamil and nifedipine did not significantly affect the recovery process so that the determination of the time course of relief from block was not possible. 4-Aminopyridine preferentially depressed peak I_to in a concentration-dependent manner, whereas I_to at the end of the 300 ms depolarizing voltage step remained unaffected. The block of I_to by 4-aminopyridine (3 mmol/l) decreased during the voltage step from −50 mV to +40 mV. Relief from block was described by τ_off = 30.4 ms. The efficacy of 4-aminopyridine was diminished at short and enhanced at long pulse intervals (reverse use-dependence). The time course of 4-aminopyridine-induced block of Ito was described by τ_on = 1561 ms. Phenylephrine (30 μmol/l),papaverine (30 μmol/I) and tetraethylammonium chloride (5 mmol/l) reduced I_to at the peak and at the end of the 300 ms depolarizing voltage step in a time-independent manner. It is concluded that verapamil, quinidine sulfate and nifedipine bind to the I_to channel in the open state at positive membrane potentials. In contrast, 4-aminopyridine obviously binds to the channel in the closed state at negative membrane potentials. Phenylephrine, papaverine and tetraethylammonium chloride seem to block I_to independent of the channel state.