The digestion of food (specifically carbohydrates) results in a surge in glucose in the
lumen of the gastrointestinal tract. Intestinal absorptive cells (enterocytes) absorb this
glucose and transfer it into circulation, where it is transported to different parts of the
body. Glucose is then absorbed by tissues (mainly muscle, adipose, liver and brain)
where it is used for the generation of energy (catabolism) or storage (anabolism). Two
counter regulatory hormones produced from the endocrine pancreas, insulin (lowers
blood glucose levels; mainly after feeding) and glucagon (increases blood glucose levels,
mainly during fasting) play a critical role in glucose homeostasis. In vertebrates,
including fish, the primary function of insulin is to maintain normoglycemia and promote
glucose anabolism. In order to keep the blood glucose level within the narrow
physiological range, in spite of ingestion and fasting periods, insulin increases the
absorption of glucose from the intestine, suppresses glycogen breakdown from the liver
and stimulates the tissue uptake. The resultant effect of these insulin actions is lowering
of the glucose present in the circulation of the organism.
In this lab, you will test how glucose is handled by the homeostatic system of a
representative vertebrate, the goldfish (Carassius auratus), which has a very highly
conserved glucoregulatory system in comparison to mammals. First you will inject
goldfish with a high concentration of glucose that closely mimics the glucose load after a
carbohydrate rich meal. This test is called the intraperitoneal glucose tolerance test
(IPGTT) as the solution is injected into the abdominal/peritoneal cavity (caution must be
taken not to inject into the internal organs). You will then monitor how the glucose level
changes in vivo in the goldfish during the post-injection phase. The injected glucose will
be absorbed by the capillary network/blood vessels in the peritoneal cavity and then
distributed to other parts of the body.

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