Different inhibition patterns of tedisamil for fast and slowly inactivating transient outward current in rat...

Tedisamil has been described as a selective inhibitor of a fast inactivating transient outward current (i(to,f)) in rat ventricular myocytes. Because recent reports demonstrated the existence of a second slowly inactivating transient component (i(to,s)) we investigated i(to,s) and differentiated the effects of tedisamil on both transient outward current components and their influence on action potential duration. Standard electrophysiological techniques were used for whole cell recordings at 24-26 degrees C from enzymatically isolated myocytes. Inhibition of i(to,f) by tedisamil was the result of an acceleration of inactivation at positive test potentials with a concentration for half-maximal inhibition (EC50) of 4-7 micromol/l, which is confirmatory to reports from other investigators. Our new results show that i(to,s) is more sensitive to tedisamil with an EC50 of 0.5 micromol/l. Furthermore the pattern of i(to,s) inhibition is different compared with i(to,f), because inactivation of i(to,s) is not accelerated by tedisamil. Instead the amplitude of the steady state inactivation curve of i(to,s) is attenuated which indicates a reduction of maximally available current. I(to,s) was evaluated by three different methods as time-dependently inactivating current (7.5 s test pulse duration), voltage-dependently inactivated current and tedisamil-sensitive current. All approaches yield similar inactivation curves. The potential for halfmaximal inactivation of i(to,s) lies about 35 mV more negative than that for i(to,f) and the slope factor (K = -23 mV) is different to that of i(to,f) (K = -3 mV). Effectiveness of tedisamil-induced modulation of i(to,f) and i(to,s) on action potential repolarization was tested. Action potentials stimulated at 0.5 Hz were not prolonged by 1 micromol/l tedisamil (dominant i(to,s) block) at a repolarization level of 0 mV but prolonged to about 120% of control at -70 mV. This indicates that i(to,f) was sufficient to guarantee a regular early repolarization whereas decrease of i(to,s) delayed the final repolarization. In conclusion, the observation that tedisamil inhibits i(to,f) and i(to,s) differently supports the hypothesis that the two i(to)-components are related to two different channel populations expressed in rat ventricular myocytes.