Abstract
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(2+) 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(2+) transients that seldom propagated between cells. Colonic ICC express ANO1 channels, so Ca(2+) transients likely couple to activation of spontaneous transient inward currents (STICs) in these cells. The Ca(2+) transients were due to Ca(2+) release and blocked by cyclopiazonic acid (CPA), thapsigargin and caffeine, but unaffected by tetracaine. Antagonists of L- and T-type Ca(2+) channels and reduction in extracellular Ca(2+) had minimal effects on Ca(2+) transients. We reasoned that STICs may not activate regenerative Ca(2+) waves in ICC-MY because voltage-dependent Ca(2+) conductances are largely inactivated at the relatively depolarized potentials of colonic muscles. We tested the effects of hyperpolarization with pinacidil, a K(ATP) agonist. Ca(2+) waves were initiated in some ICC-MY networks when muscles were hyperpolarized, and these events were blocked by a T-type Ca(2+) channel antagonist, NNC 55-0396. Ca(2+) 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.