Propagation and neural regulation of calcium waves in longitudinal and circular muscle layers of guinea pig small intestine

Background & Aims: The relative movements of longitudinal muscle (LM) and c ircular muscle (CM) and the role that nerves play in coordinating their act ivities has been a subject of controversy. We used fluorescent video imagin g techniques to study the origin and propagation of excitability simultaneo usly in LM and CM of the small intestine. Methods: Opened segments of guine a pig ileum were loaded with the Ca2+ indicator fluo-3, Mucosal reflexes we re elicited by lightly depressing the mucosa with a sponge. Results: Sponta neous Ca2+ waves occurred frequently in LM (1.2 s(-1)) and less frequently in CM (3.2 min(-1)). They originated from discrete pacing sites and propaga ted at rates 8-9 times faster parallel (LM, 87 mm/s; CM, 77 mm/s) compared with transverse to the long axis of muscle fibers. The presence of Ca2+ wav es in one muscle layer did not affect the origin, rate of conduction, or ra nge of propagation in the other layer, The extent of propagation was limite d by collisions with neighboring waves or recently excited regions. Simulta neous excitation of both muscle layers could be elicited by mucosal stimula tion of either ascending or descending reflex pathways. Neural excitation r esulted in an increase in the frequency of Ca2+ waves and induction of new pacing sites without eliciting direct coupling between layers. Conclusions: Localized, spontaneous Ca2+ waves occur independently in both muscle layer s, promoting mixing (pendular or segmental) movements, whereas activation o f neural reflexes stimulates Ca2+ waves synchronously in both layers, resul ting in strong peristaltic or propulsive movements.