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

1. The hypothesis that K(v)4.2 and K(v)4.3 are two of the essential K+ channel isoforms underlying the Ca2+-independent transient outward K+ current (I-t) 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 K(v)4.2 channels in L-cells exhibit rapid activation and inactivation properties similar to those produced by I-t 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 K(v)4.2 is much slower (time constant, 378 ms) than that of I-t in rat ventricular cells (58 ms). 3. The K+ current due to K(v)4.2 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 K(v)4.2 in L-cells and I-t in rat ventricular cells in a similar fashion. In L-cells quinidine reduced the amplitude of K(v)4.2 and accelerated its time course of inactivation, suggesting that quinidine may act as an open channel blocker of K(v)4.2, as has been described for I-t in rat ventricle. 5. To provide further independent evidence that K(v)4.2 and K(v)4.3 channel isoforms contribute to I,in rat ventricular cells, the effects of 20-mer antisense phosphorothioate oligodeoxynucleotides directed against K(v)4.2 and K(v)4.3 mRNAs were examined in ventricular myocytes isolated from 14- and 20-day-old rats, and in L-cells. In both preparations, K(v)4.2 antisense pretreatment significantly reduced the transient outward K+ current (by approximately 55-60%). Similar reduction of I-t was produced by the K(v)4.3 antisense oligonucleotide on the 14-day-old rat myocytes. 6. In 14-day rat ventricular cells, combination of K(v)4.2 and K(v)4.3 antisense oligonucleotides did not produce a significantly larger reduction of I-t than that observed after pretreatment with either antisense oligonucleotide alone. 7. L-cells stably transfected with K(v)4.2 were treated with K(v)4.3 antisense oligonucleotide to evaluate the possibility of cross-reactivity between K(v)4.3 antisense and K(v)4.2 mRNA. This antisense treatment produced no change in I-t, verifying the lack of cross-reactivity. 8. These biophysical and pharmacological results together with the antisense data show that K(v)4.2 and K(v)4.3 are essential components of the Ca2+-independent transient outward K+ current, I-t, in rat ventricular myocytes.