Gastric acid secretory capacity falls after repeated within-day pentagastrin testing in fed subjects

Between-day pentagastrin testing yields highly reproducible stimulated gastric acid output values, but little is known of the reproducibility of repeated within-day pentagastrin tests. We have performed three pentagastrin tests within the 1 day in nine healthy subjects. Within-day tests were 6 hours apart; the first followed an overnight fast and the second and third were both 4 hours after a substantial meal. A further test was performed the following morning, again after an overnight fast, which allowed comparison of within-day and between-day testing. In the second and third within-day tests there was a marked decrease of stimulated gastric acid output, with both maximal and peak acid output decreased to approximately half of the value of the first test (P less than 0.01). By contrast there were no significant differences in the acid output values obtained in between-day tests (both following an overnight fast). Possible mechanisms for the decreased output on repeated within-day testing include alterations in the sensitivity of the gastrin receptor, or some neurohumoral influence secondary to the preceding meal. Future studies of the duration of action of drugs affecting acid secretion may need to take account of these findings.