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The properties of spontaneous contractions and their modulation by transmural nerve stimulation (TNS) were investigated in circular smooth muscle preparations isolated from the "pacemaker area" in the flexure region of the guinea-pig colon. Four types of preparation were made; mucosal layer removed preparations, mucosal and submucosal layer removed preparations (submucosal layer removed preparations), longitudinal muscle removed preparations, and preparations with all layers attached (intact preparations). Intact and mucosal layer removed preparations periodically generated two types of phasic contractions; large contractions at a low frequency (about 2 times per min) and small contractions at a higher frequency (about 13 times per min). Submucosal layer removed preparations only produced large contractions, while longitudinal muscle removed preparations only produced small contractions. Both the amplitude and frequency of the large contractions were significantly inhibited by atropine, but were not markedly modulated by N(ω)-nitro-L-arginine (L-NA) in either intact, mucosal layer removed or submucosal layer removed preparations. Small contractions were not significantly modulated by atropine or L-NA in any type of preparation. In both intact and mucosal layer removed preparations, TNS (1 Hz frequency, for 2-3 min) increased the amplitude and/or frequency of large contractions, but caused no significant change in small contractions. TNS also increased the amplitude and/or frequency of large contractions in submucosal layer removed preparations, but did not significantly modulate small contractions in longitudinal muscle removed preparations. Exogenously applied acetylcholine (&gt;10(-7) M) enhanced, while sodium nitroprusside (&gt;10(-8) M) inhibited, the amplitude and frequency of both types of contraction, in a concentration-dependent manner. The results suggest that the large contractions were elicited by interstitial cells distributed in the myenteric layer (ICC-MP), while the small contractions were elicited by ICC distributed in the submuscular plexus layer (ICC-SMP). Intramural nerves mainly modulated the activity of ICC-MP, while neural regulation of ICC-SMP was weak.

journal of smooth muscle research

Properties of spontaneous contractions and their modulation by transmural nerve stimulation in circular smooth muscle isolated from the pacemaker area in the flexure region of the guinea-pig colon