10.02 Endocrine system feedback/systems

Secondly, we will consider a couple of important feedback/systems systems in the endocrine system. Firstly, we will consider the link between plasma glucose levels and insulin secretion. Plasma glucose levelshave to be kept in a small range. If the levels of glucose are too low hypoglycaemia develops, which is characterised by sweating, hunger, palpitations, tremor, anxiety, convulsions, and coma.

When we have a meal, the food is broken down, and the levels of plasma glucose rise. Glucose is transported into the pancreatic β-cells and this stimulates the release of insulin from these cells (Figure 10.3).Insulin then stimulates the conversion of glucose to glycogen in the liver, the conversion of glucose to triglycerides in adipose tissue, and the conversion of amino acids to protein in muscle. These biochemical changeslead to a decrease in the plasma glucose, which removes the stimulus for insulin secretion (Figure 10.3). This feedback prevents excessive insulin secretion, which would lead to hypoglycaemia.

Figure 10.3 Plasma glucose and control of insulin release (Copyright Sheila Doggrell, QUT)

The second example of endocrine control is the negative feedback of circulating corticosteroids on the release of corticotrophin releasing factor and adrenocorticotrophic hormone, and subsequently on thecorticosteroids themselves (Figure 10.4). In times of stress, the hypothalamus releases corticotrophin releasing factor, which in turn stimulates the anterior pituitary to release adrenocorticotrophic hormone(ACTH, corticotrophin hormone), and the adrenal cortex to release the corticosteroids. The corticosteroids have major roles in metabolism, and less prominent effects as anti-inflammatory and immunosuppressants. Once the corticosteroids are released they feedback on the hypothalamus to inhibit the release of corticotrophin releasing factor, and on the anterior pituitary to inhibit the release of ACTH (Figure 10.4).

Figure 10.4 Feedback inhibition of corticosteroid release (Copyright Sheila Doggrell, QUT)

In pharmacology, the properties of the corticosteroids have been manipulated to produce synthetic drugs that are potent as anti-inflammatory agents and as immunosuppressants (e.g. for use after hearttransplants). Unfortunately, if these agents are used long term they cause long term suppression of the axis in Figure 10.4 by mimicking the physiological feedback, and the axis can take a long time to recover. Thus,if synthetic corticosteroids are abruptly withdrawn after a long period of use, the natural axis can be slow to recover, leaving the body unable to cope with stress, infection, and immune challenges.