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. Ca²⁺ 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 Ca²⁺ transients that seldom propagated between cells. Colonic ICC express ANO1 channels, so Ca²⁺ transients likely couple to activation of spontaneous transient inward currents (STICs) in these cells. The Ca²⁺ transients were due to Ca²⁺ release and blocked by cyclopiazonic acid (CPA), thapsigargin and caffeine, but unaffected by tetracaine. Antagonists of L- and T-type Ca²⁺ channels and reduction in extracellular Ca²⁺ had minimal effects on Ca²⁺ transients. We reasoned that STICs may not activate regenerative Ca²⁺ waves in ICC-MY because voltage-dependent Ca²⁺ conductances are largely inactivated at the relatively depolarized potentials of colonic muscles. We tested the effects of hyperpolarization with pinacidil, a K_ATP agonist. Ca²⁺ waves were initiated in some ICC-MY networks when muscles were hyperpolarized, and these events were blocked by a T-type Ca²⁺ channel antagonist, NNC 55–0396. Ca²⁺ 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.