Distribution of pacemaker function through the tunica muscularis of the canine gastric antrum

Interstitial cells of Cajal (ICC) are found at specific locations within the tunica muscularis of the gastrointestinal (GI) tract. Studies performed on tissues of the mouse and guinea-pig have suggested that ICC in different anatomical locations have discrete physiological roles. Studies in the mouse have been aided by the fact that c-kit and stem cell factor mutant animals fail to develop certain types of ICC, and specific functional losses have been observed in these animals (Ward et al. 1994, 1995; Huizinga et al. 1995; Burns et al. 1996). For example, when ICC in the myenteric region of the small intestine (IC-MY) are lost, slow wave activity is not present, suggesting that IC-MY are pacemaker cells (Ward et al. 1994; Huizinga et al. 1995). When intramuscular ICC of the stomach and lower oesophageal and pyloric sphincters are lost, neural inputs from the enteric nervous system are greatly reduced, suggesting these cells are important mediators of neurotransmission (Burns et al. 1996; Ward et al. 1998, 2000a). Studies with neutralizing antibodies to Kit protein have supported the idea that IC-MY are pacemaker cells (Torihashi et al. 1995) and demonstrated that these cells are also needed for active propagation of slow waves in the small bowel and stomach (Ordog et al. 1999). Thus, a picture has emerged regarding the functional significance of ICC in the GI tract, and these studies have suggested that a ‘division of labour’ exists between pacemaker ICC (IC-MY) and ICC involved in neurotransmission (IC-IM in the stomach and IC-DMP in the small intestine: see Sanders et al. 1999). The concept that electrical slow waves originate in ICC, actively propagate in ICC, and passively spread into electrically coupled smooth muscle cells is supported by studies showing that pacemaker activity can be recorded from isolated ICC but not from isolated smooth muscle cells (see Horowitz et al. 1999). Furthermore, when regions of the muscle with pacemaker ICC are removed by dissection, slow waves decay in a manner suggestive of electrotonic (i.e. passive) conduction (Sanders et al. 1990). These observations suggested that ICC have the ability to generate and regenerate slow waves, but smooth muscle cells do not share these mechanisms (Horowitz et al. 1999). This organization seems plausible for thin-walled organs as in the mouse, but it is unclear how ICC distributed solely within a thin surface of pacemaker cells (e.g. the myenteric region between circular and longitudinal muscle layers or the submucosal surface of the circular muscle layer in the colon) could generate enough current to activate voltage-dependent responses in the smooth muscle syncytium of thicker-walled organs of humans and large animals. Previous studies have suggested that propagation of pacemaker activity is active in thicker-walled gastric muscles. Bauer and co-workers (1985a, b) recorded with intracellular electrodes from cells within the myenteric half and submucosal half of the circular muscle layer of canine antrum. These studies demonstrated slow wave propagation in both regions of the muscle. In studies of the canine small intestine Jimenez and co-workers (1996) observed that separation of the region near the deep muscular plexus from the myenteric half of the circular muscle layer did not block rhythmic electrical activity. These data suggest that cells other than IC-MY can provide pacemaker activity. Finally, we have previously shown that functional pacemaker-like ICC in the canine colon infiltrate septa between muscle layers well into the thickness of the circular muscle layer (see Ward & Sanders, 1990). ‘Septal’ ICC might provide a pathway for active propagation of slow waves deep within the muscularis and thereby increase transmission of depolarization to the smooth muscle cells. In the present study we used the canine gastric antrum to investigate the populations of ICC within a thick-walled stomach and to characterize the relationships between ICC, smooth muscle cells and enteric neurons. Four basic classes of ICC were identified, and septal ICC (i.e. ICC between bundles of muscle fibres) were abundant. Intracellular microelectrode recordings were used to determine whether sites other than the myenteric region were capable of generating and propagating slow waves in the antral wall. The data suggest that while cells in the myenteric region provide the dominant pacemaker activity in intact antral muscles, cells within the interior of the circular muscle layer are also capable of pacemaker activity. Cells within the interior of the circular muscle layer can also be paced by extrinsic stimuli and slow waves of normal amplitude were recorded many millimetres from the stimulus. These observations suggest that there are pathways for active propagation throughout the antral tunica muscularis.