The cardiac transient outward current I(to) is regulated by thyroid hormone 
(T3). However, it remains unclear whether T3 directly modulates underlying gene 
transcription and which thyroid receptor (TR) isoform might be responsible for 
gene transactivation. To clarify this situation, we analysed the role of T3 and 
its receptors alpha1 (TRalpha1) and beta1 (TRbeta1) in regulation of KCNA4, 
KCND2, KCND3 and KCNIP2 transcription in rat cardiomyocytes. Initial results 
demonstrated a T3-mediated increase of I(to) current density. T3 stimulation 
enhanced KCND2 and KCND3 expression and decreased KCNA4 transcription, while 
KCNIP2 remained unaffected. To dissect the role of TRalpha1 and TRbeta1 in 
T3-dependent I(to) modulation, TRalpha1 and TRbeta1 were overexpressed in 
cardiomyocytes by adenovirus-mediated gene transfer. TRalpha1 increased I(to), 
while TRbeta1 significantly reduced I(to) in size, which was associated with 
TRalpha1-mediated increase and TRbeta1-mediated reduction of KCND2/3 
transcription. To further evaluate a possible direct interaction of TRalpha1 and 
TRbeta1 with the KCND3 promoter, TR expression vectors were cotransfected with a 
construct containing 2335 bp of the KCND3 5'-flanking sequence linked to a 
luciferase reporter into ventricular myocytes. While the TRalpha1 aporeceptor 
enhanced KCND3 transcription, the TRbeta1 aporeceptor suppressed KCND3 
expression, with both effects exhibiting ligand-dependent amplification upon T3 
stimulation. Deletion of the KCND3 5'-flanking region localized the suppressible 
promoter sequence for TRbeta1 to within -293 bp and the activating promoter 
sequence for TRalpha1 to within -2335 to -1654 bp of the transcription start 
site. Disruption of putative TR binding sites by mutagenesis abolished the 
TRalpha1- (G-1651T) and TRbeta1- (G-73T) mediated effects, indicating that 
TRalpha1 and TRbeta1 response elements map to different regions of the KCND3 
promoter. Thus, I(to) is modulated by diverse T3-dependent regulation of 
underlying gene transcription. TRalpha1 and TRbeta1 exhibit distinct effects on 
KCND3 transactivation with TRalpha1 enhancing and TRbeta1 suppressing KCND3 
transcription.