16 Pancreas

Endocrine Pancreas:

The pancreas as described in the previous chapter has several exocrine functions to aid in digestion. However, the pancreas also has tremendous endocrine function through its pancreatic islets or islets of Langerhans which comprise <1% of the cells in the pancreas.

Have you ever wondered how your body can function for hours after a meal? How is it you can still have energy despite refuelling hours ago? This is due to the combination of hormones produced from your pancreas.

Each pancreatic islets are composed of 4 hormone secreting cells:

• Alpha or A cells: about 17% of pancreatic islets cells which produce glucagon
• Beta or B cells: about 70% of pancreatic islet cells that produce insulin
• Delta or D cells constitute ~7% of the cells and secrete somatostatin (identical to growth-hormone-inhibiting hormone in the hypothalamus)
• F cells constitute the remaining cells which secrete pancreatic polypeptide

Insulin and Glucagon Regulation:

Insulin and glucagon – both peptide hormones – play an integral role in maintaining blood glucose levels, and both operate within 2 negative-feedback loops.

1. Low blood sugar(hypoglycemia) stimulates secretion of glucagon from pancreatic alpha cells.
2. Glucagon acts on hepatocytes to accelerate conversion of glycogen to glucose and promotes gluconeogenesis (conversion of glucose from lipids or amino acids)
3. As a result blood glucose levels rise, restoring homeostasis.
4. If blood glucose continues to rise(hyperglycemia) (as seen just after eating) this may signal release of insulin from pancreatic beta cells.
5. Insulin can act in several facets, but it also prompts hepatocytes to reduce gluconeogenesis, and instead promote glycogenesis (creation of glycogen from free glucose). Also, insulin promotes the synthesis of fatty acids(lipogenesis). Further, insulin promotes the uptake of blood borne glucose into cells.
6. As a result blood glucose falls to normal levels restoring homeostasis.

As you can see the effects of hypo or hyperglycemia promote the expression of either glucagon or insulin and consequently inhibit the expression of the opposing hormone (negative feedback). These two integral hormones play key roles in maintaining homeostatic concentrations of blood glucose.