Mechanism of block by tedisamil of transient outward current in human ventricular subepicardial myocytes

Abstract

1. Tedisamil is a new antiarrhythmic drug with predominant class III action. The aim of the present study was to investigate the blocking pattern of the compound on the transient outward current (I_to) in human subepicardial myocytes isolated from explanted left ventricles. Using the single electrode whole cell voltage clamp technique, I_to was analysed after appropriate voltage inactivation of sodium current and block of calcium current.

2. Tedisamil reduced the amplitude of peak I_to, but did not affect the amplitude of non-inactivating outward current. The drug accelerated the apparent rate of I_to inactivation. The reduction in time constant of I_to inactivation depended on drug concentration, the apparent IC₅₀ value was 4.4 μM.

3. Tedisamil affected I_to amplitude in a use-dependent manner. After 2 min at −80 mV, maximum block of I_to was reached after 4–5 clamp steps either at the frequency of 0.2 or 2 Hz, indicating that the block was not frequency-dependent in an experimentally relevant range. Recovery from block was very slow and proceeded with a time constant of 12.1±1.8 s. Also in the presence of drug, a fraction of channels recovered from inactivation with a similar time constant as in control myocytes (i.e. 81±40 ms and 51±8 ms, respectively, n.s.).

4. From the onset of fractional block of I_to by tedisamil during the initial 60 ms of a clamp step, we calculated k₁=9×10⁶ mol⁻¹ s⁻¹ for the association rate constant, and k₂=23 s⁻¹ for the dissociation rate constant. The resulting apparent K_D was 2.6 μM and is similar to the IC₅₀ value.

5. The effects of tedisamil on I_to could be simulated by assuming a four state channel model where the drug binds to the channel in an open (activated) conformation. It is concluded that in human subepicardial myocytes tedisamil is an open channel blocker of I_to and that this effect probably contributes to the antiarrhythmic potential of this drug.

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