Phosphorylation of the I Ks Channel Complex Inhibits Drug Block

Abstract

Background— I Ks , an important repolarizing current in heart, is an antiarrhythmic drug target and is markedly increased by activation of protein kinase A (PKA; eg, by β-adrenergic stimulation). Because β-adrenergic stimulation is a frequent trigger of arrhythmias, we hypothesized that PKA stimulation inhibits drug block. Methods and Results— CHO cells were transfected with KCNQ1 cDNA (encoding the pore-forming subunit) with or without the ancillary subunit KCNE1. IC 50 for quinidine block of basal I Ks was 5.8±1.2 μmol/L, versus 19.9±3.2 μmol/L ( P <0.01) for PKA-stimulated current. A similar >3-fold shift was apparent in the absence of KCNE1 and with the I Ks -specific blocker chromanol 293B. The first current recorded after channels were held at rest and exposed to the drug was reduced ≈40%, and further depolarizations increased the block with a time constant (τ) of 181±27 seconds. By contrast, PKA-stimulated channels displayed a <5% first-pulse block and much slower block development (τ=405±85 seconds). Alanine substitution at 3 potential PKA target sites (S27, S468, and T470) generated an I Ks that did not increase with PKA stimulation; this mutant retained wild-type drug sensitivity that was unaffected by PKA. Conclusions— Activation of this key intracellular signaling pathway blunts drug block. The onset of block and the data with the PKA-resistant mutant support the concept that phosphorylation of the KCNQ1 subunit directly modulates drug access to a binding site on the channel. These data identify a novel mechanism for modulation of drug-channel interactions that may be especially important during β-adrenergic stimulation.