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NEUROREGULATION AND MOTILITY

ICC pacing mechanisms in intact mouse intestine differ from those in cultured or dissected intestine

Department of Pharmacology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, T6G 2H7 Canada

Submitted 5 September 2003 ; accepted in final form 29 November 2003

Pacing of mouse intestine is driven by spontaneous activity of a network of interstitial cells of Cajal in the myenteric plexus (ICC-MP). So far, highly dissected circular muscle (CM) strips from control and mutant mice lacking ICC-MP and isolated, cultured ICC from newborn control mice were used to analyze its properties. Using intact circular and longitudinal segments of intestine, we recently reported that there were both significant similarities and differences between pacing studied in segments and from isolated, dissected tissues. Here, we report additional similarities and differences in our model from those in highly reduced systems. Similar to cultured or dissected intestine, blockade of sarcoplasmic-endoplasmic reticulum Ca²⁺ pumps with thapsigargin or cyclopiazonic acid reduced pacing frequency, but thapsigargin was less effective than in isolated, cultured ICC. Moreover, inhibition of inositol 1,4,5-trisphosphate (IP₃) receptors with xestospongin C, a putative inhibitor of IP₃ receptors, failed to affect pacing but successfully blocked increased pacing frequency by phorbol ester. 2-Aminoethoxy-diphenylborate, a putative blocker of IP₃-mediated calcium release, caused a significant decrease in the amplitude and frequency of contractions. The mitochondrial uncoupler carbonyl cyanide p-trifluormethoxyphenylhydrazone blocked pacing and KCl-induced contractions at a concentration of 1 µM. The cyclic nucleotide agonists sodium nitroprusside (SNP), forskolin, and 8-bromo-cGMP inhibited pacing in CM. In longitudinal muscle (LM), SNP and forskolin had little effect on pacing. Furthermore, dibutyryl-cAMP did not affect pacing in CM or LM. These results suggest that pacing in intact intestine is under partly similar regulatory control as in more reduced systems. However, pacing in intact intestine is not affected by xestospongin C, which abolishes pacing in isolated, cultured ICC and exhibits attenuated responses to thapsigargin. Also, major differences between LM and CM suggest a separate pacemaker may drive LM.

interstitial cells of Cajal; intestinal pacing; sarcoplasmic-endoplasmic reticulum Ca²⁺ pump block; xestospongin c; 2-aminoethoxy-diphenylborate

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