Separation of the components of the delayed rectifier potassium current using selective blockers of IKr and IKs in guinea-pig isolated ventricular myocytes.

Abstract

Delayed rectifier potassium current (IK) was investigated in guinea-pig isolated ventricular myocytes under voltage-clamp conditions ('switched' single electrode clamp), using selective blockers and/or different activation protocols to separate its rapid (IKr) and slow (IKs) components. The class III antiarrhythmic compound E4031 (5 microM) was used to block IKr and the anaesthetic drugs propofol (100 microM) or thiopentone (100 microM) to block IKs. In all experiments L-type calcium currents were blocked with nifedipine (2 microM). Complementary effects of E4031 and the anaesthetic drugs on the components of IK were observed. The E4031-sensitive current (IKr) resembled the current remaining in the presence of the anaesthetics and, likewise, the anaesthetic-sensitive current (IKs) resembled the current remaining in the presence of E4031. Under the conditions of these experiments, the relative contribution of the two components to total IK tail current was found to be approximately equal after a 400 ms depolarization to +40 mV. For example, IKr was 58 +/- 10% of total IK tail current when measured as the E4031-sensitive current, 41 +/- 6% as the propofol-insensitive current and 43 +/- 7% as the thiopentone-insensitive current. In the presence of both E4031 and propofol or thiopentone the IK tail current deactivating at -40 mV was completely eliminated, leaving a residual current during the pulse which reversed at -46 +/- 1 mV. To avoid complication of the 'envelope of tails' test with this residual current, the tail:pulse ratio was calculated for the anaesthetic-sensitive component and this was constant, consistent with block of a single component of IK. Forskolin (1 microM) enhanced the current most consistent with IKs. Propofol (300 microM) caused a 64 +/- 3% increase in action potential duration in the presence of both E4031 (5 microM) and nifedipine (2 microM), consistent with an important role for IKs in the repolarization of the action potential in the guinea-pig heart. The observations therefore provide further support for separate components of IK with different characteristics in the guinea-pig heart; it appears that E4031 and propofol or thiopentone are useful complementary tools for their separation.