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1 Physiology and Cell Biology, University of Nevada School of Medicine, Reno, NV, USA
* To whom correspondence should be addressed. E-mail: kent{at}unr.edu.
Spontaneous electrical pacemaker activity occurs in tunica muscularis of the gastrointestinal (GI) tract and drives phasic contractions. Interstitial cells of Cajal (ICC) are the pacemaker cells that generate and propagate electrical slow waves. We used Ca2+ imaging to visualize spontaneous rhythmicity in ICC in the myenteric region (ICC-MY) of the murine small intestine. ICC-MY, verified by co-labeling with Kit antibody, displayed regular Ca2+ transients that occurred after electrical slow waves. ICC-MY formed networks, and Ca2+ transient wavefronts propagated through the ICC-MY networks at approximately 2 mm.s-1 and activated attached longitudinal muscle (LM) fibers. Nicardipine blocked Ca2+ transients in LM, but had no visible effect on the transients in ICC-MY. 
-Glycyrrhetinic acid (-GA) reduced the coherence of propagation, causing single cells to pace independently. Thus, virtually all ICC-MY are spontaneously active, but normal activity is organized into propagating wavefronts. Inhibitors of dihydropyridine-resistant Ca2+ entry (Ni2+ and mibefradil) and elevated external K+ reduced the coherence and velocity of propagation, eventually blocking all activity. The mitochondrial uncouplers, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and antimycin and IP3 receptor inhibitory drug, 2-Aminoetoxydiphenyl borate (2-APB) abolished rhythmic Ca2+ transients in ICC-MY. These data show that global Ca2+ transients in ICC-MY are a reporter of electrical slow waves in GI muscles. Imaging of ICC networks provides a unique multicellular view of pacemaker activity. The activity of ICC-MY is driven by intracellular Ca2+ handling mechanisms and entrained by voltage-dependent Ca2+ entry and coupling of cells via gap junctions.
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