Ca²⁺ signaling in myenteric interstitial cells of Cajal (ICC-MY) and their role as conditional pacemakers in the colon.

Interstitial cells of Cajal in the plane of the myenteric plexus (ICC-MY) serve as electrical pacemakers in the stomach and small intestine. A similar population of cells is found in the colon, but these cells do not appear to generate regular slow wave potentials, as characteristic in more proximal gut regions. Ca handling mechanisms in ICC-MY of the mouse proximal colon were studied using confocal imaging of muscles from animals expressing GCaMP6f exclusively in ICC. ICC-MY displayed stochastic, localized Ca transients that seldom propagated between cells. Colonic ICC express ANO1 channels, so Ca transients likely couple to activation of spontaneous transient inward currents (STICs) in these cells. The Ca transients were due to Ca release and blocked by cyclopiazonic acid (CPA), thapsigargin and caffeine, but unaffected by tetracaine. Antagonists of L- and T-type Ca channels and reduction in extracellular Ca had minimal effects on Ca transients. We reasoned that STICs may not activate regenerative Ca waves in ICC-MY because voltage-dependent Ca conductances are largely inactivated at the relatively depolarized potentials of colonic muscles. We tested the effects of hyperpolarization with pinacidil, a K agonist. Ca waves were initiated in some ICC-MY networks when muscles were hyperpolarized, and these events were blocked by a T-type Ca channel antagonist, NNC 55-0396. Ca waves activated by excitatory nerve stimulation were significantly enhanced by hyperpolarization. Our data suggest that colonic ICC-MY are conditional pacemaker cells that depend upon preparative hyperpolarization, produced physiologically by inputs from enteric inhibitory neurons and necessary for regenerative pacemaker activity.