Adrenergic regulation of the rapid component of delayed rectifier K+ current: Implications for arrhythmogenesis in LQT2 patients

BackgroundKCNH2 gene mutations disrupting rapid component of IK (IKr) underlie type 2 congenital long QT syndrome (LQT2). Startled auditory stimuli are specific symptomatic triggers in LQT2, thus suggesting fast arrhythmogenic mechanism.ObjectiveWe investigated acute α1A- and cyclic adenosine monophosphate (cAMP)–related β-adrenergic modulation of IKr 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.MethodsIKr 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-δ1 visualized fluctuations of membrane phosphatidylinositol 4,5-bisphosphate (PIP2) content.ResultsIn HL-1 cardiomyocytes expressing human α1A-adrenoceptor, superfusion with phenylephrine significantly reduced IKr amplitude, shifted current activation to more positive potentials, and accelerated kinetics of deactivation. Confocal images showed a decline of membrane PIP2 content during phenylephrine exposure. Simultaneous application of adenylyl cyclase activator forskolin and phosphodiesterase inhibitor 3-isobutyl-1-methylxantine (IBMX) shifted IKr activation to more negative potentials and decreased tail current amplitudes after depolarizations between +10 and +50 mV. In CHO cells, α1A-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 α1A- and forskolin/IBMX-induced regulation.ConclusionAcute adrenergic regulation of at least 2 Kv11.1 mutant channels is preserved as in WT-Kv11.1 and native IKr. Suppression of α1A-adrenoceptor-related transduction might have therapeutic implications in some cases of LQT2.