Shal-type channels contribute to the Ca2+-independent transient outward K+ current in rat ventricle

1. The hypothesis that Kv4.2 and Kv4.3 are two of the essential K+ channel isoforms underlying the Ca2+-independent transient outward K+ current (It) in rat ventricle has been tested using a combination of electrophysiological measurements and antisense technology in both native myocytes and a stably transfected mammalian cell line, mouse Ltk- cells (L-cells). 2. The transient outward currents generated by Kv4.2 channels in L-cells exhibit rapid activation and inactivation properties similar to those produced by It in rat ventricular cells. The current-voltage relationships and the voltage dependence of steady-state inactivation are also very similar in these two preparations. However, the recovery from inactivation of Kv4.2 is much slower (time constant, 378 ms) than that of It in rat ventricular cells (58 ms). 3. The K+ current due to Kv4.2 can be blocked by millimolar concentrations of 4-aminopyridine in L-cells; a similar pharmacological response has been observed in rat ventricular myocytes. 4. Quinidine inhibits Kv4.2 in L-cells and It in rat ventricular cells in a similar fashion. In L-cells quinidine reduced the amplitude of Kv4.2 and accelerated its time course of inactivation, suggesting that quinidine may act as an open channel blocker of Kv4.2, as has been described for It in rat ventricle. 5. To provide further independent evidence that Kv4.2 and Kv4.3 channel isoforms contribute to It in rat ventricular cells, the effects of 20-mer antisense phosphorothioate oligodeoxynucleotides directed against Kv4.2 and Kv4.3 mRNAs were examined in ventricular myocytes isolated from 14- and 20-day-old rats, and in L-cells. In both preparations, Kv4.2 antisense pretreatment significantly reduced the transient outward K+ current (by approximately 55-60%). Similar reduction of It was produced by the Kv4.3 antisense oligonucleotide on the 14-day-old rat myocytes. 6. In 14-day rat ventricular cells, combination of Kv4.2 and Kv4.3 antisense oligonucleotides did not produce a significantly larger reduction of It than that observed after pretreatment with either antisense oligonucleotide alone. 7. L-cells stably transfected with Kv4.2 were treated with Kv4.3 antisense oligonucleotide to evaluate the possibility of cross-reactivity between Kv4.3 antisense and Kv4.2 mRNA. This antisense treatment produced no change in It, verifying the lack of cross-reactivity. 8. These biophysical and pharmacological results together with the antisense data show that Kv4.2 and Kv4.3 are essential components of the Ca2+-independent transient outward K+ current, It, in rat ventricular myocytes.