Adrenergic regulation of the rapid component of delayed rectifier K+ current: implications for arrhythmogenesis in LQT2 patients.
BACKGROUND
KCNH2 gene mutations disrupting rapid component of I(K) (I(Kr)) underlie type 2 congenital long QT syndrome (LQT2). Startled auditory stimuli are specific symptomatic triggers in LQT2, thus suggesting fast arrhythmogenic mechanism.


OBJECTIVE
We investigated acute alpha(1A)- and cyclic adenosine monophosphate (cAMP)-related beta-adrenergic modulation of I(Kr) in HL-1 cardiomyocytes, wild type (WT)- and 2 LQT2-associated mutant Kv11.1 channels (Y43D- and K595E-Kv11.1) reconstituted in Chinese hamster ovary (CHO) cells.


METHODS
I(Kr) and Kv11.1 currents were recorded using the whole-cell patch-clamp technique and confocal microscopy of HL-1 cardiomyocytes transfected with green fluorescent protein (GFP)-tagged pleckstrin homology domain of phospholipase C-delta(1) visualized fluctuations of membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) content.


RESULTS
In HL-1 cardiomyocytes expressing human alpha(1A)-adrenoceptor, superfusion with phenylephrine significantly reduced I(Kr) amplitude, shifted current activation to more positive potentials, and accelerated kinetics of deactivation. Confocal images showed a decline of membrane PIP(2) content during phenylephrine exposure. Simultaneous application of adenylyl cyclase activator forskolin and phosphodiesterase inhibitor 3-isobutyl-1-methylxantine (IBMX) shifted I(Kr) activation to more negative potentials and decreased tail current amplitudes after depolarizations between +10 and +50 mV. In CHO cells, alpha(1A)-adrenoceptor activation downregulated WT-Kv11.1 channels and forskolin/IBMX produced a dual effect. Expressed alone, the Y43D-Kv11.1 or K595E-Kv11.1 channel had no measurable function. However, co-expression of WT-Kv11.1 and each mutant protein evoked currents with loss-of-function alterations but identical to WT-Kv11.1 alpha(1A)- and forskolin/IBMX-induced regulation.


CONCLUSIONS
Acute adrenergic regulation of at least 2 Kv11.1 mutant channels is preserved as in WT-Kv11.1 and native I(Kr). Suppression of alpha(1A)-adrenoceptor-related transduction might have therapeutic implications in some cases of LQT2.
Department of Cardiovascular and Respiratory Medicine, Shiga University of Medical Science, Otsu, Shiga, Japan.
Dimitar P
DP
Zankov
Hidetada
H
Yoshida
Keiko
K
Tsuji
Futoshi
F
Toyoda
Wei-Guang
WG
Ding
Hiroshi
H
Matsuura
Minoru
M
Horie
D000328:Adult; D000818:Animals; D002478:Cells, Cultured; D006224:Cricetinae; D004195:Disease Models, Animal; D022062:Electrophysiologic Techniques, Cardiac; D005260:Female; D006801:Humans; D008133:Long QT Syndrome / Q000151:congenital / Q000235:genetics / Q000503:physiopathology*; D018613:Microscopy, Confocal; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D024642:Potassium Channels, Voltage-Gated / Q000235:genetics / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*
D024642:Potassium Channels, Voltage-Gated; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta