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NEUROREGULATION AND MOTILITY

Functional evidence for Na⁺-activated K⁺ channels in circular smooth muscle of the opossum lower esophageal sphincter

Gastrointestinal Diseases Research Unit, Queen's University, Kingston, Ontario, Canada

Submitted 13 December 2005 ; accepted in final form 28 February 2007

Na⁺ reduction induces contraction of opossum lower esophageal sphincter (LES) circular smooth muscle strips in vitro; however, the mechanism(s) by which this occurs is unknown. The purpose of the present study was to investigate the electrophysiological effects of low Na⁺ on opossum LES circular smooth muscle. In the presence of atropine, quanethidine, nifedipine, and substance P, conventional intracellular electrodes recorded a resting membrane potential (RMP) of –37.5 ± 0.9 mV (n = 4). Decreasing [Na⁺] from 144.1 to 26.1 mM by substitution of equimolar NaCl with choline Cl depolarized the RMP by 7.1 ± 1.1 mV. Whole cell patch-clamp recordings revealed outward K⁺ currents that began to activate at –60 mV using 400-ms stepped test pulses (–120 to +100 mV) with increments of 20 mV from holding potential of –80 mV. Reduction of [Na⁺] in the bath solution inhibited K⁺ currents in a concentration-dependent manner. Single channels with conductance of 49–60 pS were recorded using cell-attached patch-clamp configurations. The channel open probability was significantly decreased by substitution of bath Na⁺ with equimolar choline. A 10-fold increase of [K⁺] in the pipette shifted the reversal potential of the single channels to the positive by –50 mV. These data suggest that Na⁺-activated K⁺ channels exist in the circular smooth muscle of the opossum LES.

sodium-activated potassium channels; intracellular recording; patch-clamp recording