Role of muscarinic receptor subtypes in the regulation of migrating myoelectric complex in the dog.

The role played by muscarinic receptor subtypes in the regulation of the migrating myoelectric complex was investigated in 7 dogs chronically implanted with bipolar electrodes along the small intestine. Pirenzepine (3-300 micrograms/kg i.v.) and atropine (1-30 micrograms/kg i.v.) were used as selective and unselective antagonist, respectively. Atropine (30 micrograms/kg) significantly delayed the onset of the next complex. On the contrary, pirenzepine displayed a biphasic action: low doses (less than 100 micrograms/kg) shortened the cycle period, whereas at 300 micrograms/kg the drug behaved like atropine. Pirenzepine affected the cycle period in the low-dose range by reducing the length of phase I. Both atropine and pirenzepine impaired the migration of the ongoing complex, and significantly reduced the migration velocity of the following one. These findings suggest that the initiation of the migrating myoelectric complex in the dog is under an inhibitory influence mediated by the M1 muscarinic receptor subtype; on the other hand, M2 receptor activation is needed for the onset of the activity front. Finally, both receptor subtypes determine the normal migration of phase III.