Identification of rhythmically active cells in guinea-pig stomach
Abstract
When intracellular recordings were made from the antral region of guinea-pig stomach, cells with different patterns of electrical activity were detected.
One group of cells, slow-wave cells, generated slow waves which consisted of initial and secondary components. When filled with either Lucifer Yellow or neurobiotin, the cells identified as smooth muscle cells lying in the circular muscle layer.
A second group of cells, driving cells, generated large, rapidly rising, potential changes, driving potentials. They had small cell bodies with several processes. With neurobiotin, a network of cells was visualized that resembled c-kit positive interstitial cells of the myenteric region.
A third group of cells generated sequences of potential changes which resembled driving potentials but had smaller amplitudes and slow rates of rise. These cells resembled smooth muscle cells lying in the longitudinal muscle layer.
When simultaneous recordings were made from the driving and slow-wave cells, driving potentials and slow waves occurred synchronously. Current injections indicated that both cell types were part of a common electrical syncytium.
The initial component of slow waves persisted in low concentrations of caffeine, but the secondary component was abolished; higher concentrations shortened the duration of the residual initial component. Driving potentials continued in the presence of low concentrations of caffeine; moderate concentrations of caffeine shortened their duration.
Hence three different types of cells were distinguished on the basis of their electrical activity, their responses to caffeine and their structure. These were smooth muscle cells, lying in the longitudinal and circular layers, and interstitial cells in the myenteric region. The observations suggest that interstitial cells initiate slow waves.
Many organs containing smooth muscle are myogenically active. This was assumed to originate from activity within the individual smooth muscle cells. Some smooth muscle cells have low resting membrane potentials and generate myogenic activity, in much the same way as cardiac pacemaker cells, through the sequential activation of voltage-dependent ion channels (see for example Anderson, 1993). In others, myogenic activity originates from the cyclic release of calcium ions (Ca) from stores inside the smooth muscle cells (Van Helden, 1993; Hashitani et al. 1996). Many regions of the gastrointestinal tract generate slow waves and contract rhythmically at low frequencies in the absence of stimulation (Tomita, 1981; Sanders, 1992). Again it was initially thought that the generation of slow waves reflected some properties of gastrointestinal smooth muscle cells (Connor et al. 1974; El-Sharkaway & Daniel, 1975; Tomita, 1981). More recently it has been suggested that slow waves result from the interaction between two distinct groups of cells: one group acts as pacemaking cells and activates a second group which generates slow waves. Several observations suggest that activity originates in interstitial cells of Cajal (ICC), and that smooth muscle cells, rather than initiating activity, act as follower cells. ICC form diffuse networks of cells which are thought to be linked together as electrical syncytia (Thuneberg, 1982). When ICC lying near the submucous border of the circular muscle layer of dog colon are dissected away, nearby smooth muscles stop generating slow waves (Smith et al. 1987). Intestinal preparations taken from mice devoid of ICC fail to generate normal slow waves (Ward et al. 1994; Huizinga et al. 1995).
However recordings have rarely been obtained from ICC. When this was done (Barajas-Lopez et al. 1989) the electrical activity was found to be similar to that recorded in nearby smooth muscle cells (see Sanders & Smith, 1989). Thus recordings from a group of cells which show distinct pacemaker-like activity and which are in continuity with intestinal muscle cells have not been obtained. The hypothesis that ICC are pacemaker cells, or indeed that separate pacemaker cells exist, has not been tested directly (Sanders & Ward, 1996). In this report we describe electrophysiological and histological observations on cells lying in the antral region of the guinea-pig stomach which show that three different groups of cells are present. Most cells generated slow waves with initial and secondary components; these cells were found to be smooth muscle cells lying in the circular muscle layer. Two other groups of cells generated different sequences of membrane potential changes. Cells of one of these groups resembled ICC which were identified by their reaction with an antibody to c-kit (Torihashi et al. 1995). Cells of the other group were found to be smooth muscle cells lying in the longitudinal muscle layer.
Acknowledgments
We wish to thank Drs Narelle Bramich and Frank Edwards for their helpful comments on the manuscript. We are also very grateful to Dr Heather Young for her advice on histological techniques. This project was supported by a grant from the Australian National Health & Medical Research Council.