Characterisation of Kv4.3 in HEK293 cells:: comparison with the rat ventricular transient outward potassium current

Objective: The Shal (or Kv4) gene family has been proposed to be responsible for primary subunits of the transient outward potassium current (I-to). More precisely, Kv4.2 and Kv4.3 have been suggested to be the most likely molecular correlates for I-to in rat cells. The purpose of the present study was to compare the properties of the rat Kv4.3 gene product when expressed in a human cell line (HEK293 cells) with that of I-to recorded from rat ventricular cells. Methods: The cDNA encoding the rat Kv4.3 potassium channel was cloned into the pHook2 mammalian expression vector and expressed into HEK293. Patch clamp experiments using the whole cell configuration were used to characterise the electrophysiological parameters of the current induced by Kv4.3 in comparison with the rat ventricular myocyte I-to current. Results: The transfection of HEK293 cells with rat Kv4.3 resulted in the expression of a time- and voltage-dependent outward potassium current. The current activated for potentials positive to -40 mV and the steady-state inactivation curve had a midpoint of -47.4+/-0.3 mV and a slope of 5.9+/-0.2 mV. Rat ventricular I-to current was activated at potentials positive to -20 mV and inactivated with a half-inactivation potential and a Boltzmann factor of -29.1+/-0.7 mV and 4.5+/-0.5 mV, respectively. The time course of recovery from inactivation of rat Kv4.3 expressed in HEK293 cells and of I-to recorded from native rat ventricular cells were exponentials with time constants of 213.2+/-4.1 msec and 23.+/-1.5 msec, respectively. Pharmacologically, I-to of rat myocytes showed a greater sensitivity to 3-aminopyridine than Kv4.3 since half-maximal effects were obtained with 1.54+/-0.13 mM and 0.14+/-0.02 mM on Kv4.3 and I-to, respectively. In both Kv4.3 and I-to, 4-aminopyridine appears to bind to the closed state of the channel. Finally, although a higher level of expression was observed in the atria compared to the ventricle, the distribution of the Kv4.3 gene across the ventricles appeared to be homogenous. Conclusion: The results of the present study show that Kv4.3 channel may play a major role in the molecular structure of the rat cardiac I-to current. Furthermore, because the distribution of Kv4.3 across the ventricle is homogenous, the blockade of this channel by specific drugs may not alter the normal heterogeneity of I-to current. (C) 1999 Elsevier Science B.V. All rights reserved.