Morphine versus motilin in the initiation of migrating myoelectric complexes.

The roles of morphine and motilin in the initiation of migrating myoelectric complex (MMC) cycles were studied in four conscious dogs. Morphine or motilin boluses and morphine or motilin infusions were administered to healthy conscious dogs, each implanted with a set of 12 electrodes on the small intestine. The durations of premature phase IIIs initiated by morphine and motilin were not significantly different from each other or from control values. The premature phase IIIs initiated by morphine boluses propagated significantly faster over the 1st half of small intestine than the control or motilin bolus-initiated phase IIIs. The latent period for the onset of phase IIIs after morphine was not significantly different from that after motilin. Morphine infusion (50 micrograms X kg-1 X h-1) generally initiated one or two premature MMC cycles and then disrupted further MMC cycling. Motilin infusion (0.3 microgram X kg-1 X h-1) initiated one premature MMC cycle, and then further cycling continued at the normal rate. We conclude that morphine acts on both the MMC-initiating and MMC-propagating mechanisms, whereas motilin acts only on the MMC-initiating mechanism. Morphine has both excitatory and disruptive effects on MMC cycling, while motilin has only excitatory effects.