Motilin secretion and the migrating myoelectric complex in the pig

The concentration of motilin in plasma from the abdominal aorta and the hepatic portal vein and the net portal motilin output varied with the phase of the migrating myoelectric complex (m.m.c.) in five of six pigs fasted for 17 h. Maximum concentrations and output occurred 9-12 min before phase III in the duodenum or upper jejunum. In fed pigs m.m.c.s occurred and the first phase III in the duodenum occurred within 90 min of feeding. Both portal and arterial motilin concentrations were reduced after feeding and no longer varied with the phase of the m.m.c. Altered secretion of motilin after feeding did not appear to be associated with absorption of glucose as infusion of glucose (50 g/l, 10 ml/min) into the duodenum raised arterial and portal plasma glucose concentrations to post-prandial levels yet motilin concentrations and output rates still varied with the phase of the m.m.c. Infusions of motilin (1 or 10 ng/kg X min) into the portal vein of 17 h fasted pigs did not induce an extra phase III or alter the duration of the m.m.c. Hydrochloric acid (100 mmol/l) infused into the duodenum of fasted pigs at 10-21 ml/min increased the concentration of motilin in the portal blood but was without effect at 5 ml/min. Rapid injections of 50 ml hydrochloric acid into the duodenum also increased the portal motilin concentration. Hydrochloric acid infusion or injection did not alter the interval between phase IIIs. It is concluded that motilin secretion is a consequence of the m.m.c. or shows the same periodicity as the m.m.c. but that motilin is not an important factor in the initiation and control of the m.m.c. in the pig.