Potassium channel diversity within the muscular components of the feline lower esophageal sphincter

We hypothesized that regional differences in electrophysiological properties exist within the musculature of the feline lower esophageal sphincter (LES) and that they may potentially contribute to functional asymmetry within the LES. Freshly isolated esophageal smooth muscle cells (SMCs) from the circular muscle and sling regions within the LES were studied under a patch clamp. The resting membrane potential (RMP) of the circular SMCs was significantly more depolarized than was the RMP of the sling SMCs, resulting from a higher Na+ and Cl- permeability in circular muscle than in sling muscle. Large conductance Ca2+-activated K+ (BKCa) set the RMP at both levels, since specific BKCa inhibitors caused depolarization; however, BKCa density was greatest in the circular region. A significant portion of the outward current was due to non-BKCa, especially in sling muscle, and likely delayed rectifier K+ channels (KDR). There was a large reduction in outward current with 4-aminopyridine (4-AP) in sling muscle, while BKCa blockers had a limited effect on the voltage-activated outward current in sling muscle. Differences in BKCa:KDR channel ratios were also manifest by a leftward shift in the voltage-dependent activation curve in circular cells compared to sling cells. The electrophysiological differences seen between the circular and sling muscles provide a basis for their different contributions to LES activities such as resting tone and neurotransmitter responsiveness, and in turn could impart asymmetric drug responses and provide specific therapeutic targets.