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Interstitial cells of Cajal (ICC) in the myenteric plexus region (ICC-MY) of the small intestine are pacemakers that generate rhythmic depolarizations known as slow waves. Slow waves depend on activation of Ca 2+ -activated Cl - channels (ANO1) in ICC, propagate actively within networks of ICC-MY, and conduct to smooth muscle cells where they generate action potentials and phasic contractions. Thus, mechanisms of Ca 2+ regulation in ICC are fundamental to the motor patterns of the bowel. Here, we characterize the nature of Ca 2+ transients in ICC-MY within intact muscles, using mice expressing a genetically encoded Ca 2+ sensor, GCaMP3, in ICC. Ca 2+ transients in ICC-MY display a complex firing pattern caused by localized Ca 2+ release events arising from multiple sites in cell somata and processes. Ca 2+ transients are clustered within the time course of slow waves but fire asynchronously during these clusters. The durations of Ca 2+ transient clusters (CTCs) correspond to slow wave durations (plateau phase). Simultaneous imaging and intracellular electrical recordings revealed that the upstroke depolarization of slow waves precedes clusters of Ca 2+ transients. Summation of CTCs results in relatively uniform Ca 2+ responses from one slow wave to another. These Ca 2+ transients are caused by Ca 2+ release from intracellular stores and depend on ryanodine receptors as well as amplification from IP 3 receptors. Reduced extracellular Ca 2+ concentrations and T-type Ca 2+ channel blockers decreased the number of firing sites and firing probability of Ca 2+ transients. In summary, the fundamental electrical events of small intestinal muscles generated by ICC-MY depend on asynchronous firing of Ca 2+ transients from multiple intracellular release sites. These events are organized into clusters by Ca 2+ influx through T-type Ca 2+ channels to sustain activation of ANO1 channels and generate the plateau phase of slow waves.

Clustering of Ca2+ transients in interstitial cells of Cajal defines slow wave duration