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Evidence for a Novel K⁺ Channel Modulated by _1A-Adrenoceptors in Cardiac Myocytes

Department of Physiology and Cardiovascular Research Laboratories, School of Medical Sciences, University of Bristol, Bristol, United Kingdom (S.C.M.C., J.C.H., L.A.A., A.F.J.); Department of Renal Medicine, King's College School of Medicine & Dentistry, London, United Kingdom (A.M.R.); and Cardiac Physiology, Centre for Cardiovascular Biology & Medicine, St Thomas' Hospital, London, United Kingdom (M.J.S.)

Accumulating evidence suggests that steady-state K⁺ currents modulate excitability and action potential duration, particularly in cardiac cell types with relatively abbreviated action potential plateau phases. Despite representing potential drug targets, at present these currents and their modulation are comparatively poorly characterized. Therefore, we investigated the effects of phenylephrine [PE; an ₁-adrenoceptor (₁-AR) agonist] on a sustained outward K⁺ current in rat ventricular myocytes. Under K⁺ current-selective conditions at 35°C and whole-cell patch clamp, membrane depolarization elicited transient (I_t) and steady-state (I_ss) outward current components. PE (10 µM) significantly decreased I_ss amplitude, without significant effect on I_t. Preferential modulation of I_ss by PE was confirmed by intracellular application of the voltage-gated K⁺ channel blocker tetraethylammonium, which largely inhibited I_t without affecting the PE-sensitive current (I_ss,PE). I_ss,PE had the properties of an outwardly rectifying steady-state K⁺-selective conductance. Acidification of the external solution or externally applied BaCl₂ or quinidine strongly inhibited I_ss,PE. However, I_ss,PE was not abolished by anandamide, ruthenium red, or zinc, inhibitors of TASK acid-sensitive background K⁺ channels. Furthermore, the PE-sensitive current was partially inhibited by external administration of high concentrations of tetraethylammonium and 4-aminopyridine, which are voltage-gated K⁺ channel-blockers. Power spectrum analysis of I_ss,PE yielded a large unitary conductance of 78 pS. I_ss,PE resulted from PE activation of the _1A-AR subtype, involved a pertussis toxin-insensitive G-protein, and was independent of cytosolic Ca²⁺. These results collectively demonstrate that _1A-AR activation results in the inhibition of an outwardly rectifying steady-state K⁺ current with properties distinct from previously characterized cardiac K⁺ channels.

Address correspondence to: Dr. Andrew F. James, Department of Physiology and Cardiovascular Research Laboratories, School of Medical Sciences, University of Bristol, University Walk, Bristol, BS8 1TD, United Kingdom. E-mail: a.james{at}bristol.ac.uk

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