Excitatory Neuronal Responses of Ca(2+) Transients in Interstitial Cells of Cajal in the Small Intestine

Interstitial cells of Cajal (ICC) regulate smooth muscle excitability and motility in the gastrointestinal (GI) tract. ICC in the deep muscular plexus (ICC-DMP) of the small intestine are aligned closely with varicosities of enteric motor neurons and thought to transduce neural responses. ICC-DMP generate Ca(2+) transients that activate Ca(2+) activated Cl(-) channels and generate electrophysiological responses. We tested the hypothesis that excitatory neurotransmitters regulate Ca(2+) transients in ICC-DMP as a means of regulating intestinal muscles. High-resolution confocal microscopy was used to image Ca(2+) transients in ICC-DMP within murine small intestinal muscles with cell-specific expression of GCaMP3. Intrinsic nerves were stimulated by electrical field stimulation (EFS). ICC-DMP exhibited ongoing Ca(2+) transients before stimuli were applied. EFS caused initial suppression of Ca(2+) transients, followed by escape during sustained stimulation, and large increases in Ca(2+) transients after cessation of stimulation. Basal Ca(2+) activity and the excitatory phases of Ca(2+) responses to EFS were inhibited by atropine and neurokinin 1 receptor (NK1) antagonists, but not by NK2 receptor antagonists. Exogenous ACh and substance P (SP) increased Ca(2+) transients, atropine and NK1 antagonists decreased Ca(2+) transients. Neurokinins appear to be released spontaneously (tonic excitation) in small intestinal muscles and are the dominant excitatory neurotransmitters. Subcellular regulation of Ca(2+) release events in ICC-DMP may be a means by which excitatory neurotransmission organizes intestinal motility patterns.