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

J Gen Physiol. 2017 Jul 3;149(7):703-725. doi: 10.1085/jgp.201711771. Epub 2017 Jun 7.

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 Ca2+-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 Ca2+ regulation in ICC are fundamental to the motor patterns of the bowel. Here, we characterize the nature of Ca2+ transients in ICC-MY within intact muscles, using mice expressing a genetically encoded Ca2+ sensor, GCaMP3, in ICC. Ca2+ transients in ICC-MY display a complex firing pattern caused by localized Ca2+ release events arising from multiple sites in cell somata and processes. Ca2+ transients are clustered within the time course of slow waves but fire asynchronously during these clusters. The durations of Ca2+ 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 Ca2+ transients. Summation of CTCs results in relatively uniform Ca2+ responses from one slow wave to another. These Ca2+ transients are caused by Ca2+ release from intracellular stores and depend on ryanodine receptors as well as amplification from IP3 receptors. Reduced extracellular Ca2+ concentrations and T-type Ca2+ channel blockers decreased the number of firing sites and firing probability of Ca2+ transients. In summary, the fundamental electrical events of small intestinal muscles generated by ICC-MY depend on asynchronous firing of Ca2+ transients from multiple intracellular release sites. These events are organized into clusters by Ca2+ influx through T-type Ca2+ channels to sustain activation of ANO1 channels and generate the plateau phase of slow waves.

MeSH terms

  • Animals
  • Anoctamin-1 / metabolism
  • Calcium Channels, T-Type / metabolism
  • Calcium Signaling*
  • HEK293 Cells
  • Humans
  • Inositol 1,4,5-Trisphosphate Receptors / metabolism
  • Interstitial Cells of Cajal / metabolism*
  • Mice
  • Mice, Inbred C57BL

Substances

  • ANO1 protein, mouse
  • Anoctamin-1
  • Calcium Channels, T-Type
  • Inositol 1,4,5-Trisphosphate Receptors