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Modeling of the adrenergic response of the human I_Ks current (hKCNQ1/hKCNE1) stably expressed in HEK-293 cells

Safety and Exploratory Pharmacology, Safety Assessment, Merck Research Laboratories, West Point, Pennsylvania

Submitted 24 April 2008 ; accepted in final form 28 July 2008

Stable coexpression of human (h)KCNQ1 and hKCNE1 in human embryonic kidney (HEK)-293 cells reconstitutes a nativelike slowly activating delayed rectifier K⁺ current (HEK-I_Ks), allowing β-adrenergic modulation of the current by stimulation of endogenous receptors in the host cell line. HEK-I_Ks was enhanced two- to fourfold by isoproterenol (EC₅₀ = 13 nM), forskolin (10 µM), or 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (50 µM), indicating an intact cAMP-dependent ion channel-regulating pathway analogous to the PKA-dependent regulation observed in native cardiac myocytes. Activation kinetics of HEK-I_Ks were accurately fit with a novel modified second-order Hodgkin-Huxley (H-H) gating model incorporating a fast and a slow gate, each independent of each other in scale and adrenergic response, or a "heterodimer" model. Macroscopically, β-adrenergic enhancement shifted the current activation threshold to more negative potentials and accelerated activation kinetics while leaving deactivation kinetics relatively unaffected. Modeling of the current response using the H-H model indicated that observed changes in gating could be explained by modulation of the opening rate of the fast gate. Under control conditions at nearly physiological temperatures (35°C), rate-dependent accumulation of HEK-I_Ks was observed only at pulse frequencies exceeding 3 Hz. Rate-dependent accumulation of I_Ks at high pulsing rate had two phases, an initial staircaselike effect followed by a slower, incremental accumulation phase. These phases are readily interpreted in the context of a heterodimeric H-H model with two independent gates with differing closing rates. In the presence of isoproterenol after normalizing for its tonic effects, rate-dependent accumulation of HEK-I_Ks appeared at lower pulse frequencies and was slightly enhanced (25%) over control.

slowly activating cardiac delayed rectifer potassium current; human embryonic kidney-293 cells; β-adrenergic modulation; KCNQ1; KCNE1