6533b858fe1ef96bd12b634a

RESEARCH PRODUCT

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

subject

description

The mechanism of drug-induced inhibition of the transient outward current, Ito, has been investigated in rat ventricular myocytes using the whole cell patch clamp technique. Ito was activated by 300 ms depolarizing voltage clamp steps in 10 mV increments from −50 mV up to +40 mV. At +40 mV, Ito peaked after about 3 ms, and the time course of inactivation was appropriately described by two time constants, τfast = 17 ms and τslow = 203 ms. Verapamil, quinidine sulfate and nifedipine preferentially depressed Ito at the end of the 300 ms depolarizing voltage clamp step; the inactivation of Ito was accelerated by all drugs, whereas peak Ito was less affected. The time course of drug action at +40 mV was calculated by the fractional changes of Ito. Verapamil, quinidine sulfate and nifedipine exerted a block of Ito. 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 τon = 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 Ito from inactivation with or without drugs. τoff 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 Ito in a concentration-dependent manner, whereas Ito at the end of the 300 ms depolarizing voltage step remained unaffected. The block of Ito by 4-aminopyridine (3 mmol/l) decreased during the voltage step from −50 mV to +40 mV. Relief from block was described by τoff = 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 τon = 1561 ms. Phenylephrine (30 μmol/l),papaverine (30 μmol/I) and tetraethylammonium chloride (5 mmol/l) reduced Ito 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 Ito 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 Ito independent of the channel state.