Heterogeneities in ICC Ca2+ Activity Within Canine Large Intestine

Background & AimsIn human and canine colon, both slow (slow waves, 2–8/min) and fast (myenteric potential oscillations [MPOs]; 16–20/min) electrical rhythms in the smooth muscle originate at the submucosal and myenteric borders, respectively. We used Ca2+ imaging to investigate whether interstitial cells of Cajal (ICCs) at these borders generated distinct rhythms.MethodsSegments of canine colon were pinned with submucosal or myenteric surface uppermost or cut in cross section. Tissues were loaded with a Ca2+ indicator (fluo-4), and activity was monitored at 36.5 ± 0.5°C using an electron multiplying charge coupled device (EMCCD).ResultsRhythmic, biphasic Ca2+ transients (5–8/min), similar in waveform to electrical slow waves, propagated without decrement as a wave front (2–5 mm/s) through the ICC-SM network lying along the submucosal surface of the circular muscle (CM). In contrast, rhythmic intracellular Ca2+ waves (∼16/min) and spontaneous reductions in Ca2+ were observed in ICCs at the myenteric border (ICC-MY). Normally, intracellular Ca2+ waves were unsynchronized between adjacent ICC-MY, although excitatory nerve activity synchronized activity. In addition, spontaneous reductions in Ca2+ were observed that inhibited Ca2+ waves. Nω-nitro-L-arginine (100 μmol/L; nitric oxide antagonist) blocked the reductions in Ca2+ and increased the frequency (∼19/min) of intracellular Ca2+ waves within ICC-MY.ConclusionsICC-SMs form a tightly coupled network that is able to generate and propagate slow waves. In contrast, Ca2+ transients in ICC-MYs, which are normally not synchronized, have a similar duration and frequency as MPOs. Like MPOs, their activity is inhibited by nitrergic nerves and synchronized by excitatory nerves.