Visualization of Origins and Propagation of Excitation in Canine Gastric Smooth Muscle

The origin and spread of excitation were visualized with fluo 3 fluorescence in tissues isolated from canine gastric antrum. Sheets of circular muscle (5 x 6 mm) had at least 1 (30%) and up to 3 discrete slow-wave pacing sites located near the longitudinal-circular muscle boundary, whereas similarly sized longitudinal sheets had an average of 5 sites (range 3-12 sites) that initiated Ca2+ waves. Superimposed fluorescent oscillations (circular muscle) and spikes (longitudinal muscle) were seen to initiate and propagate as distinct events, separate from their underlying activities. Average propagation velocities transverse (6-7 mm/s) and parallel (39-45 mm/s) to the long axis of muscle fibers were similar for each type of event in circular and longitudinal tissues; however, distinct regions where velocities of some (but not all) events decreased by up to an order of magnitude were present. The distance propagated by individual events was limited by collisions with concurrent excitable events or recently activated regions. Complex patterns of excitation in gastrointestinal smooth muscle arise as a result of interactions between multiple pacing sites, heterogeneous conduction velocities, and the interplay of adjacent pacemaker domains.