AJP - Gastrointestinal and Liver Physiology, Vol 256, Issue 1 72-G77, Copyright © 1989 by American Physiological Society
Opioid receptors and the initiation of migrating myoelectric complexes in dogs
G. L. Telford, R. E. Condon and J. H. Szurszewski
Department of Surgery, Clement J. Zablocki Veterans Administration Medical Center, Milwaukee 53295.
The role of endogenous opioids and opioid receptors in the control of
migrating myoelectric complexes (MMCs) was studied in conscious dogs
implanted with silver-silver chloride electrodes. In normal fasted dogs,
MMC cycle times were 103 +/- 7 min in the duodenum. During naloxone
infusion (1-2 mg/kg iv, then 0.2-1.0 mg.kg-1.h-1 iv) cycle times increased
to 219 +/- 29 min (P less than 0.01). Naloxone (2 mg/kg iv, then 1
mg.kg-1.h-1 iv) had no effect on the response of the small intestine to
bethanecol (5 mg sc) or to feeding. Pretreatment with naloxone (2 mg/kg iv)
5 min before the administration of motilin (400-500 micrograms/kg iv) did
not block the initiation of MMCs by motilin. In separate experiments,
animals were pretreated with the positive or negative isomer of the opioid
receptor antagonist WIN-44,441 (0.2 mg/kg iv) 5 min before morphine
administration. The negative isomer binds to opioid receptors whereas the
positive isomer does not. The negative but not the positive isomer
antagonized all effects of morphine on intestinal myoelectric activity.
These studies suggest that endogenous opioids and opioid receptors may play
a role in control of the initiation of MMCs and that motilin and exogenous
opioids act via different mechanisms to initiate MMCs.
