An investigation of the distribution of gastrin-like activity, and its nature, in the gastrointestinal tract of the sheep
Using a modification of the extraction procedure of Gregory and Tracy (1964), and a perfused rat stomach bioassay, gastrin-like activity has been located in the ovine rumen, reticulum, omasum, abomasal body, antrum, duodenum and caecum. The antrum showed the highest concentration of gastrin-like activity, but substantial concentrations were found in the rumen, reticulum, omasum and caecum and proximal duodenum declining progressively with distance from the pylorus. A lesser concentration occurred in the abomasal body. Consideration of the relative total content of gastrin-like activity showed that the amount of extra-antral activity was substantial. The active principle was unlikely to have been cholecystokinin as the extracts had isoelectric points of a more acidic nature than that of cholecystokinin. Similarly, bioassay using an isolated guinea-pig ileum showed histamine to be absent from the extracts. Extraction of rumenal, reticular, omasal, abomasal body and antral mucosa and muscle separately showed that the gastrin-like activity resided almost completely in the mucosa. An investigation of the gastrin-like activity of air dried and freeze dried extracts showed that the freeze dried extracts were approximately 50% more active than the air dried extracts. Bioassay of a freeze dried antral extract over a twenty four week period showed that the activity was maintained for the initial six weeks, and that 80% of the activity was then lost over the following eighteen weeks. Studies using columns of Sephadex have shown that extracts from all regions of the ovine gastro-intestinal tract investigated were similar and consisted of several components of differing molecular weight. By a comparison of their respective molecular weights, to those reported for components separated from extracts of non-ruminant origin, it seems likely that components corresponding to MG, HLG, BG, BBG and component I were present in varying amounts. The response to infusions of gastrin extract in fed and fasted sheep were different. In fasted sheep infusions of extract, after a relatively long latent period, caused a small, but prolonged increase in acid secretion. In contrast pentagastrin caused an immediate and pronounced response. In fed sheep the responses were variable. Experiments on a single dolphin (Delphinus delpis) have shown there to be gastrin-like activity present in the forestomach, mainstomach, pyloric stomach, duodenal ampulla and intestine of this mammal. The distribution of gastrin-like activity reported in the present work has been compared to previous reports concerning the distribution of gastrin activity in animals with a more simple stomach, and the possible function of gastrin in the ruminant has been discussed.
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