Search

We previously demonstrated that a balance of K+ and Ca2+-activated Cl- channel activity maintained the basal tone of circular smooth muscle of opossum lower esophageal sphincter (LES). In the current studies, the contribution of major K+ channels to the LES basal tone was investigated in circular smooth muscle of opossum LES in vitro. K+ channel activity was recorded in dispersed single cells at room temperature using patch-clamp recordings. Whole-cell patch-clamp recordings displayed an outward current beginning to activate at -60 mV by step test pulses lasting 400 ms (-120 mV to +100 mV) with increments of 20 mV from holding potential of -80 mV ([K+]I = 150 mM, [K+]o = 2.5 mM). However, no inward rectification was observed. The outward current peaked within 50 ms and showed little or no inactivation. It was significantly decreased by bath application of nifedipine, tetraethylammonium (TEA), 4-aminopyridine (4-AP), and iberiotoxin (IBTN). Further combination of TEA with 4-AP, nifedipine with 4-AP, and IBTN with TEA, or vice versa, blocked more than 90% of the outward current. Ca2+-sensitive single channels were recorded at asymetrical K+ gradients in cell-attached patch-clamp configurations (100.8+/-3.2 pS, n = 8). Open probability of the single channels recorded in inside-out patch-clamp configurations were greatly decreased by bath application of IBTN (100 nM) (Vh = -14.4+/-4.8 mV in control vs. 27.3+/-0.1 mV, n = 3, P < 0.05). These data suggest that large conductance Ca2+-activated K+ and delayed rectifier K+ channels contribute to the membrane potential, and thereby regulate the basal tone of opossum LES circular smooth muscle.

The opossum esophagus, like that of the human, is composed of striated muscle fibres proximally and smooth muscle fibres distally. Because of this similarity the opossum has been used extensively as an animal model for esophageal studies, but to date no data on esophageal blood flow have been reported in this species. The purpose of this study was to establish the basal blood flow characteristics of different regions of the opossum gastrointestinal tract with particular reference to the esophagus. Intracardiac injection of 15-microns microspheres was used to provide an estimate of blood flow (mL.min-1.g-1 dry tissue) to the whole wall, the combined layer of mucosa plus submucosa, and the muscularis propria. Basal blood flow in the whole tissue and mucosa-submucosa was significantly higher in the lower esophageal sphincter than in the proximal or distal esophagus. The muscularis propria blood flow displayed an aborally increasing gradient with flow to proximal esophagus (striated muscle) less than distal esophagus (smooth muscle) less than lower esophageal sphincter. Regional differences in blood flow to other regions of the gastrointestinal tract were similar to that described in other species. In addition, no changes in basal blood flow occurred despite repeated microsphere injections, suggesting that this species provides a good animal model for the study of gastrointestinal blood flow.