Clustering of Ca ²⁺ transients in interstitial cells of Cajal defines slow wave duration

Abstract

Electrical slow waves in the small intestine are generated by pacemaker cells called interstitial cells of Cajal. Drumm et al. record clusters of Ca ²⁺ transients in these cells that are entrained by voltage-dependent Ca ²⁺ entry and which define the duration of the electrical slow waves.

Abstract

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.