Heterogeneities in ICC Ca2+ Activity Within Canine Large Intestine

Background & Aims: In 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. Methods: Segments 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 degrees C using an electron multiplying charge coupled device (EMCCD). Results: Rhythmic, 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 (similar to 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. N omega-nitro-L-arginine (100 mu mol/L; nitric oxide antagonist) blocked the reductions in Ca2+ and increased the frequency (similar to 19/min) of intracellular Ca2+ waves within ICC-MY. Conclusions: ICC-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.