An Investigation of Secretion and Metabolic Effects of Gastric Inhibitory Polypeptide in the Ruminant

McCarthy, John P (1993) An Investigation of Secretion and Metabolic Effects of Gastric Inhibitory Polypeptide in the Ruminant. PhD thesis, University of Glasgow.

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The studies presented in this thesis were designed to evaluate the secretion and metabolic actions of GIP in ruminants. The initial study determined whether GIP is responsive to glucose and fat absorption in pre-ruminant and young ruminant goat kids (Experiment 3.1). Goats were surgically prepared to facilitate intraduodenal administration of nutrients and portal blood sampling. It was clearly demonstrated in both pre-ruminant and young ruminant goats that, as in simple-stomached animals, fat is a potent GIP secretagogue. It appeared, however, that ruminants and non-ruminants differ regarding the ability of glucose to elicit GIP secretion. Glucose absorption had no effect on GIP secretion in the pre-ruminant or ruminant animal. Because GIP secretion may have been influenced by anaesthesia or surgical intervention it was decided to further investigate the GIP response to glucose, and to other nutrients, under more physiologically-normal conditions. In the pre-ruminant calf GIP secretion occurred within one hour of their normal milk feed (Experiment 3.2.a.). Postprandial GIP concentrations were also measured in pre-ruminant goat kids after ingestion of milk and milk constituents (Experiment 3.2.b). This study supported the observation made in anaesthetized goat kids that glucose absorption does not elicit GIP secretion, and that fat is a potent GIP secretagogue. The timing of the GIP response was also comparable to that observed in anaesthetized goat kids after intraduodenal injection of fat. Further studies in newborn goats (Experiment 3.2.c) demonstrated that suckling colostrum for the first time, immediately after birth, induced a GIP response of similar magnitude to that observed in older pre-ruminant goat kids after milk ingestion. Postprandial GIP concentrations were also determined in ruminant goat kids after ingestion of milk, milk constituents and cereal- based concentrates (Experiment 3.3). This study suggested that fat, and possibly protein, may induce GIP secretion in the ruminant. Plasma GIP concentrations were shown to increase postprandially after sheep ingested concentrates and hay (Experiment 4.1). The GIP response was delayed compared with the response to milk in pre-ruminant and ruminant goat kids, but about the same (approximately 2 hours after feeding) as that after concentrates in the ruminant kids. Postprandial plasma GIP concentrations in the adult animals appeared to be directly related to the level of dietary intake, as indicated by studies investigating GIP secretion during the development of obesity in sheep (Experiment 4.2). Comparisons were made between lactating and non-lactating sheep (Experiment 4.3). Basal GIP levels and the GIP response to feeding were increased during lactation. This appeared to reflect an increase in dietary intake. The metabolic effects of GIP on insulin secretion and adipose tissue metabolism were also evaluated in sheep. When GIP was intravenously injected with glucose, an insulinotrophic effect of GIP was not demonstrated (Experiment 5.1). However, this was consistent with the failure of glucose absorption to elicit GIP secretion in either sheep or goats. It seems unlikely that GIP is an incretin in ruminant species. When the biological activity of different GIP preparations were tested by measuring their lipogenic effect in rat adipose tissue, all were confirmed to be biologically active (Experiment 5.2.a). Further studies in ovine adipose tissue demonstrated a moderate lipogenic effect of GIP compared with that of insulin (Experiment 5.2.b). These observations are consistent with a role for GIP in lipid metabolism.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Paul Martin
Keywords: Physiology
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-74854
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 27 Sep 2019 15:49
Last Modified: 27 Sep 2019 15:49

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