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Background and Purpose: Cardiac I(Ks) is enhanced by protein kinase A (PKA) stimulation. And PKA-stimulated I(Ks) is about threefold less sensitive to quinidine block than basal current. In this study, we further tested two competing hypotheses: I(Ks) phosphorylation either (i) modulates access of blocking drugs to a binding site; or (ii) destabilizes the drug-channel interaction.Experimental Approach: To distinguish between these hypotheses, we studied quinidine block of I(Ks) channels in which three PKA site residues of the alpha-subunit KCNQ1 were mutated with a bulky negative charged aspartic acid (D). To study alleviation of I(Ks) block by quinidine, we compared activating current at +60 mV, either with or without 5 s hyperpolarizing prepulses to -120 mV.Key Results: Without PKA stimulation, quinidine (100 microM) blocked wild-type current to a similar extent with and without the prepulse (93 +/- 2% of pre-drug current at +60 mV vs. 95 +/- 1%). With PKA-stimulated wild-type channels, however, there was less block with the hyperpolarization to -120 mV: at +60 mV, block was 71 +/- 2% (-prepulse) versus 58 +/- 3% (+prepulse). Individual D-mutations and the triple-D mutant were resistant to quinidine block similar to that seen with PKA-stimulated wild-type I(Ks).Conclusions and Implications: We conclude that phosphorylation-induced insertion of bulky negative charges alleviates quinidine block and that PKA-induced stimulation, by conferring negative charges to the channels, blunts I(Ks) block as the interaction between the channels and blockers becomes destabilized. These effects would be of clinical significance in providing protective mechanisms against pro-arrhythmias caused by drug-induced inhibition of I(Ks) and I(Kr). [ABSTRACT FROM AUTHOR]

Introduction: Sympathetic activation is a trigger for life-threatening arrhythmias in many patients with the congenital long QT syndrome (LQTS), and an increase in heart rate has been reported just prior to torsades de pointes in patients with drug-associated (acquired) LQTS (aLQTS). We compared the frequencies of five recognized nonsynonymous coding region polymorphisms in genes encoding the β1-adrenergic and β2-adrenergic receptors (AR) in 93 patients with aLQTS and 3 control groups: an ethically diverse set of individuals from middle Tennessee (n = 71), a subset of the Polymorphism Discovery Resource obtained from National Human Genome Research Institute (n = 89), and patients who tolerated QT-prolonging drugs without aLQTS (non-aLQTS group; n = 66). Methods and Results: Polymerase chain reaction-restriction fragment length polymorphism was used to screen for Ser49Gly and Gly389Arg (β1-AR) and Thr164Ile (β2-AR). For Argl6Gly and Gln27Glu, polymorphic sites 33 nucleotides apart in the β2-AR, single-stranded conformational polymorphism was used to distinguish among the 4 possible haplotypes and 10 possible genotypes. Allele frequencies were similar among the 4 groups at the 2 β1-AR sites. The uncommon Ile164 variant in β2-AR slightly more frequent in patients (3.2%) than in any of the 3 control groups (0.6% to 2.3%). At the 16-27 neighboring sites in the β2-AR, one haplotype (Arg16/Glu27) was not detected, as in previous studies; hence, only 6 genotypes were present. There were fewer Gly16/Gln27 homozygotes in the non-aLQTS group (1.5%) than in two other control groups or the aLQTS group (8.5% to 10%). Conclusion: None of the five common nonsynonymous coding region polymorphisms in the β-AR genes predict drug-associated torsades de pointes, although the Gly16/Gln27 haplotype may be a risk factor. [ABSTRACT FROM AUTHOR]