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

Abstract

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 (>10 -7 M) enhanced, while sodium nitroprusside (>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.

Carbon dioxide-induced inhibition of mechanical activity in gastrointestinal smooth muscle preparations isolated from the guinea-pig

“…The inhibitory responses of the colonic smooth muscle to the stimulation with CO2-gas solution were reversible, and similar responses could be elicited several times with repeated stimulation (data not shown). Intact preparations of the proximal colon periodically generated bursts of irregular contractions, as reported previously (Fujimoto et al, 2010), and stimulation with CO2-gas solution elicited responses similar to those observed in the submucosal layer removed preparation (data not shown). Attempts were made to investigate some properties of the inhibition of spontaneous contraction by CO2-gas solution.…”

“…In these preparations, TNS (10 pulses at 10 Hz frequency) elicited an enhanced amplitude of phasic contraction which was abolished by 10 -7 M atropine, confirming that this muscle received a cholinergic excitatory nerve (Fujimoto et al, 2010). Figure 6A shows the mechanical responses produced by repeated application of TNS every 5 min, while stimulation of muscle with CO2-gas…”

“…Intestinal smooth muscle is innervated mainly by cholinergic excitatory and nitrergic inhibitory nerves (Hoyle and Burnstock, 1989;Furness, 2000), as has been shown in the colon of the guineapig (Fujimoto et al, 2010). Experiments were carried out to observe the effects of N M atropine, as reported previously (Fujimoto et al, 2010).…”

“…These results suggest that an exposure of a preparation to CO2-gas solution for 1 min is sufficient to alter the state of the intracellular mechanism so as to block the generation of spontaneous activity. In the proximal colon, the rhythmic activity of the phasic contractions has been suggested to be paced by ICC-MP distributed in the myenteric layer (Plujà et al, 2001;Kato et al, 2009;Fujimoto et al, 2010). Therefore, it is possible to suggest that the CO2-gas solution inhibits the pacemaker activity of the ICC-MP.…”

“…Circular smooth muscle preparations isolated from the proximal colon of guinea-pigs were spontaneously active, with intact muscle preparations generating periodic bursts of phasic contractions, while submucosal layer removed preparations periodically generated large amplitude phasic contractions (Fujimoto et al, 2010). Figure 1A shows the mechanical responses of a submucosal layer removed colonic preparation during stimulation with CO2-gas solution for 3 min.…”