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

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.