Shinsuke Nakayama
Department of Cell Physiology, Nagoya University Graduate School of Medicine, Nagoya, Japan

https://doi.org/10.36866/pn.67.32

After swallowing, peristaltic waves flowing throughout the gastrointestinal (GI) tract automatically transport food towards the rectum. It is well known that the neural circuit of the enteric nervous system is responsible for this activity: namely, the mechanical pressure of the food in the lumen simultaneously stimulates excitatory motor neurones of the anterior gut and inhibitory motor neurones of the posterior gut via interneurones, thereby co-ordinating proximal contraction and distal relaxation.

The existence of special pacemaker cells for GI motility has been recently recognised. Interstitial cells expressing c-Kit (Maeda et al. 1992), a receptor tyrosine kinase, are distributed throughout GI tract and are referred to as interstitial cells of Cajal (ICCs) due to histological resemblance. The mechanisms underlying spontaneous electrical activities in the GI tract have remained unclear for a long time, but, with the progress of ICC studies, are now beginning to be identified.

Network-forming ICCs in the myenteric region (ICC-MyP) are thought to generate primary pacemaker potentials (Dickens et al. 1999). It is hypothsised that spontaneous Ca2+ oscillations in ICCs (Torihashi et al. 2002), produced by co-ordinated actions of intracellular Ca2+ release channels and Ca2+-permeable channels in the plasmalemma (Aoyama et al. 2004; Liu et al. 2005; Nakayama et al. 2005), periodically activate plasmalemmal Ca2+-dependent ion channels, e.g. Ca2+-activated Cl-channels and non-selective cation channels, thereby activating smooth muscle cells connected electrically (Nakayama & Torihashi, 2002). Furthermore, distinct types of ICCs (intramuscular ICCs: ICC-IM) act as intermediates between ICC-MyP and GI smooth muscle, amplifying ICC-MyP pacemaker activity (Fig. 1).

Similarly to the essential co-operation of enteric neural activities, it is important to understand how network-forming interstitial cells produce total pacemaker electrical activity along the GI tract. However, electrophysiological recordings generally require expert techniques, particularly for simultaneous membrane potential recordings from multiple sites. It may take years for such a research project to achieve reliable results.

In our paper, a convenient method to enable spatio-temporal analyses of ICC pacemaker electrical activity was introduced (Nakayama et al. 2006). The top panel of Fig. 2 displays a recording chamber with 64 planar electrodes (8 x 8 grid with a polar distance of 300 µm, corresponding to a ∼2 x 2 mm recording area) on the bottom plane, where the smooth muscle tissue isolated from the stomach of guinea-pig is mounted. The field potentials were simultaneously measured over the recording area via a multi-channel amplifier. This system has been applied to record spike-like fast electrical activities in brain slices. To record slowly oscillating GI pacemaker activity, it is important to apply an appropriate high-pass filter (Brock & Cunnane, 1987). In this recording, we normally applied 0.1 Hz, because a high-pass filter of 1 Hz or higher greatly reduces the signal intensity. Moreover, to differentiate ICC pacemaker electrical activity from smooth muscle activity, it is necessary to add dihydropyridine (DHP) Ca2+ antagonists to the medium. This drug and analogs selectively block L-type Ca2+ channels, which make a major contribution to phasic smooth muscle contraction accompanied by depolarisation (Huang et al. 1999; Dickens et al. 1999).

A typical ICC pacemaker electrical activity (field potential) recorded from the gastric antrum consisted of an initial, fast negative potential (red arrow in the middle panel of Fig. 2) followed by a slowly decaying component (green double arrow).

Potential images reconstructed from the multi-channel recordings revealed a phase shift of ICC pacemaker activity clearly resolvable in the small recording region. Spontaneous electrical activity frequently propagated from the oral to the anal end in the longitudinal direction. Furthermore, a phase shift in the circular direction was observed in many preparations. TTX had little effect in the phase shift, suggesting that the ICC network itself works through phase-shifting Ca2+ oscillations. Presumably, this mechanism plays an important role in producing smooth peristaltic waves to squeeze the luminal content and complete emptying of the GI tract.

Approximately 20 years ago, an attempt to measure the distribution of GI pacemaker activity using multiple glass electrodes was made in our lab under the supervision of Emeritus professor Tadao Tomita. We aimed to elucidate the interaction of GI pacemaker activity, and we got several interesting results. For example, some of the results suggested that the slow component of the field potential is produced by the interaction of electrical activities in a relatively wide area rather than by the local pacemaking cells just beneath the recording electrode. However, the data obtained at that time were not published because field potentials acquired from a small number of recording sites were not considered to be reliable enough.

Later the arrayed 8 x 8 planar electrode system became available. Thus, some of our speculations become a reality, which was a satisfactory result. It was demonstrated that the slow component also involves the influence of an initial fast component generated from a region with rather long distance (>1000 µm) as well as the plateau and repolarising components of spontaneous electrical activity produced by the nearest pacemaker cells to the recording electrode (Nakayama et al. 2006).

Numerous diseases are known to impair GI movement (Sanders, 2006). For example, diabetes mellitus, a very common disease in western countries and now Japan, as well, is frequently complicated with GI dismotility, which makes it difficult to control the postprandial blood-glucose concentration.

Measurements of the interaction and coupling of pacemaker activity are likely to provide a useful index in such diseases. As a model of GI dismotility, the pacemaker electrical activity in GI tracts of W/Wv mice, in which ICCs reduce in number, are now being measured in our lab. Some of the recordings and analyses are expected to be communicated in the near future.

References

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Huang S-M, Nakayama S, Iino S & Tomita T (1999). Voltage sensitivity of Slow wave frequency in isolated circular muscle strips from guinea pig gastric antrum. Am J Physiol 276, G518-528.

Liu H-N, Ohya S, Furuzono S, Wang J, Imaizumi Y & Nakayama S (2005). Co-contribution of IP3R and Ca2+ influx pathways to pacemaker Ca2+ activity in stomach ICC. J Biol Rhythm 20, 15-26.

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Nakayama, S., Shimono, K., Liu, H.-N., Jiko, H., Katayama, N., Tomita, T & Goto, K (2006). Pacemaker phase shift in the absence of neural activity in guinea-pig stomach: a microelectrode array study. J Physiol 576, 727-738.

Nakayama S & Torihashi S. (2002). Spontaneous rhythmicity in cultured cell clusters isolated from mouse small intestine. Jpn J Physiol 52, 217-227.

Sanders KM (2006). Interstitial cells of Cajal at the clinical and scientific interface. J Physiol 576, 683-687.

Torihashi S, Fujimoto T, Trost C & Nakayama S. (2002). Calcium oscillation linked to pacemaking of interstitial cells of Cajal; Requirement of calcium influx and localisation of TRP4 in caveolae. J Biol Chem 277, 19191-19197