Spontaneous electrical rhythmicity in cultured interstitial cells of Cajal from the murine small intestine

Phasic gastrointestinal muscles are characterized by continuous, rhythmic electrical activity known as slow waves (e.g. Szurszewski, 1987). These events consist of a rapid upstroke depolarization, partial repolarization, and then a sustained, ‘plateau’ potential that can last several seconds. Slow waves occur at frequencies ranging from a few cycles per minute to more than 30 cycles min−1, depending upon the species. These events have been referred to as myogenic because they occur spontaneously without input from the enteric nervous system (Szurszewski, 1987). Recent studies have shown that the term ‘myogenic’ is misleading because slow waves are generated by interstitial cells of Cajal (ICC) and propagate into electrically coupled smooth muscle cells (Langton et al. 1989; Ward et al. 1994; Huizinga et al. 1995; Torihashi et al. 1995). The mouse small intestine has become a model for studies of slow wave origin because of the facility with which ICC populations can be manipulated in mutant animals (see Sanders, 1996, for review). Despite attempts for many years to understand the mechanisms for slow wave activity in the small bowel, the ionic mechanisms for these events still remain unclear. In previous studies we isolated pacemaker ICC from the submucosal border of the circular muscle layer in the canine colon (Langton et al. 1989). Patch clamp studies of these cells revealed a unique population of ionic conductances that might contribute to rhythmicity (Lee & Sanders, 1993), and a mechanism for slow waves in the colon was proposed (see Sanders, 1992). Although this mechanism seemed to fit the canine colon, and others have applied similar ideas to the small intestine (Malysz et al. 1995; Cayabyab et al. 1996), certain features of slow wave activity in the small bowel and stomach do not agree with the mechanism proposed for colonic slow waves. For example, low Ca2+ inhibits slow waves in the small bowel (Malysz et al. 1995; Cayabyab et al. 1996), but the sensitivity of slow waves in this organ is much less than reported for the colon (E. Flynn & K. Sanders, unpublished results). These observations suggest that other ionic mechanisms may be responsible for slow waves in the small intestine. In the present study we have returned to the approach of studying ICC isolated from the musculature to investigate the pharmacology and ionic dependence of slow waves of the small intestine. In contrast to ICC from the canine colon, ICC from the small bowel are difficult to identify in cell suspensions. Therefore, we isolated and cultured small intestinal ICC. These cells grow into well-defined networks within 1–3 days that are morphologically similar to the myenteric plexus network of ICC in the intact small intestine. After formation of ICC networks, we identified the cells as ICC with c-Kit immunoreactivity (see Torihashi et al. 1995) and recorded electrical activity. We have compared the spontaneous rhythmicity of cultured ICC with the activity of intact muscle strips from the small intestine. There are significant similarities in the activities of the two preparations, suggesting that the events retained in culture provide a new model to study the mechanism and pharmacology of slow waves.