Modulation of the activity of two pacemakers by transmural nerve stimulation in circular smooth muscle preparations isolated from the rat proximal colon

Abstract

Circular smooth muscle preparations isolated from the rat proximal colon periodically generated two different amplitudes and frequencies of phasic contractions: large phasic contractions (LPC) with a frequency of about 1.5 times/min and small phasic contractions (SPC) with a frequency of about 9 times/min. Preparations with no attached longitudinal smooth muscle layer (and also myenteric layer) generated SPC alone, while those with no attached submucosal layer generated only LPC, indicating that the pacemakers of the LPC and SPC are distributed in the myenteric and submucosal layers, respectively. In intact preparations, transmural nerve stimulation (TNS) applied for 1-2 min with different frequencies (0.2-2 Hz) inhibited the phasic contractions. The amplitude of LPC was reduced at >0.25 Hz and abolished at >0.3 Hz, while the amplitude but not the frequency of SPC was reduced at >0.5 Hz (in a frequency-dependent way). The TNS-induced inhibitory responses were augmented by atropine and attenuated by N(omega)-nitro-L-arginine (L-NA). In the presence of L-NA and atropine, TNS elicited biphasic (inhibitory and following excitatory) responses. The former were not antagonized by apamin, guanethidine or suramin, while the latter were antagonized by capsaicin, suggesting an innervation by non-adrenergic non-cholinergic non-nitrergic (NANCNN) inhibitory and peptidergic excitatory nerves, respectively. In preparations with the longitudinal muscle layer removed, TNS inhibited only the amplitude of SPC, which was augmented by atropine and antagonized by L-NA. In intact preparations, muscarinic stimulation with acetylcholine increased the frequency of LPC, while nitrergic stimulation with sodium nitroprusside reduced the amplitude and frequency of LPC, and also the amplitude but not the frequency of SPC. These results indicate that the rat proximal colon has two types of pacemaker cells. Myenteric pacemaker cells which receive predominantly nitrergic, but also cholinergic, peptidergic and NANCNN innervation, and submucosal pacemaker cells that are not markedly influenced by intramural nerves.

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

“…The results also suggested that the small contractions were produced by activity generated by ICC-SMP distributed in the submucosal layer. These differential contributions by ICC-MY and ICC-SMP on the activity of circular smooth muscle were similar to those found in the proximal colon of both the rat (Plujà et al, 1999;Kato et al, 2009) and the mouse (Yoneda et al, 2004). Our results also confirmed the contribution of ICC-SMP on the generation of the high-frequency contraction of circular muscle in the pacemaker area of the guinea-pig colon (Kobayashi et al, 1996;Nahar et al, 1998).…”

“…Both the electrical (Plujà et al, 1995) and mechanical responses (Plujà et al, 1995;Kato et al, 2009) of circular smooth muscle cells have been shown to originate from both the ICC-MP and the ICC-SMP. The present experiments have indicated that in intact or mucosal layer removed preparations, both large and small contractions appeared together in the circular smooth muscle preparations, indicating that both ICC-MP and ICC-SMP may be electrically connected indirectly through circular smooth muscle cells.…”

“…The ICC-MP-mediated phasic contractions are strongly enhanced by cholinergic excitatory nerves and inhibited by nitrergic nerves, with only weak neural regulation of phasic contractions triggered by ICC-SMP (Kato et al, 2009). Thus, the neural regulatory mechanisms of the activity seem to differ between the different subtypes of ICC.…”

“…However in the colon, rhythmic activity of smooth muscle is paced by ICC-SMP in the dog (Smith et al, 1987;Kobayashi et al, 1995) and guinea-pig (Kobayashi et al, 1996;Nahar et al, 1998), while it is carried out by ICC-MP in the rat (Plujà et al, 2001;Kato et al, 2009) and mouse (Yoneda et al, 2004). In the guinea-pig, a group of ICC-SMP with a dense distribution in the flexure region at the boundary between proximal and distal divisions of colon produces regular periodic activity of 10-12 cycle per min in the circular smooth muscle (Kobayashi et al, 1995;Nahar et al, 1996).…”

“…In view of the heterogeneous contribution of different types of ICC to the rhythmic activity of the colon (Kobayashi et al, 1995;Nahar et al, 1998;Plujà et al, 2001;Kato et al, 2009), it was expected that individual nerve excitation would elicit different or differential responses in both smooth muscle cells and ICC. Experiments were thus carried out to investigate the properties of the mechanical responses elicited by transmural nerve stimulation (TNS) in circular smooth muscle preparations isolated from the pacemaker area.…”