We have previously reported that during the interdigestive state repeated episodes of high-amplitude contractions interrupted by long-lasting motor quiescence were seen to occur regularly until the next meal in the dog. However, none of the dogs used in our laboratory showed such regular changes in gastric motor pattern all the time during the interdigestive state. In the present study, we measured plasma motilin concentrations and intraduodenal pH together with gastric motor activity in conscious dogs. It was found that as long as intraduodenal pH remained between 7.0 and 8.5 the regular occurrence of the interdigestive contractions was closely associated with increases in the plasma immunoreactive motilin (IRM) concentration. However, when intraduodenal pH became acid, there were no typical interdigestive contractions even though plasma IRM concentration was elevated. We consider that high-amplitude contractions like the interdigestive contractions could not be evoked in the acid-secreting stomach even if the plasma IRM concentration was elevated during the interdigestive state. Acid secretion during the fasted state, the cause of which is not known, seems to be one of the main factors in disturbing the regularity of the interdigestive motor pattern.

Continuous measurements of intraduodenal pH changes were made in five healthy conscious dogs by means of a miniature pH electrode inserted into the duodenum through a chronically implanted duodenal cannula. Gastric motor activity was also recorded simultaneously by means of a chronically implanted force transducer. When dogs were fed once a day with a measured volume of dog food, it was found that 24-h pH changes in the duodenal contents could be divided into three major periods: the weak acid period, the strong acid period, and alkaline period. The weak acid period lasted for 9.2 +/- 0.43 h after feeding with the duodenal pH ranging from 7.0 to 4.0. The weak acid period was gradually changed to the strong acid period in which the duodenal pH fluctuated widely from 8.0 to 1.0. This period lasted for 5.2 +/- 0.26 h. After the strong acid period, pH of the duodenum abruptly turned alkaline and remained between 7.0 and 8.3 until the next meal. These three periods of pH change are closely related to those of three different gastric motor patterns; the digestive, intermediate, and interdigestive patterns, respectively.

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