The Endocrine System

This post will cover the sixth topic in our Veterinary Physiology 1 unit - the Endocrine System. We'll discuss what a hormone is and the basic mechanisms of hormone action at the cellular level. We'll look at the factors influencing hormone-receptor binding and the concentration of hormones in the blood. In addition, I'll discuss the association between homeostasis and hormone secretion. 

 What is a hormone?

A hormone is a substance that is manufactured and secreted in very small quantities into the bloodstream by an endocrine gland or specialised nerve cell and regulates the growth or functioning of a specific tissue or organ in a distant part of the body. The hormones bind to the receptors on the target site which triggers a response. It is important to remember that the same hormone can cause different responses in different types of cells. The endocrine system consists of endocrine glands which have a rich vascular and lymphatic drainage for the rapid distribution of hormones around the body. Hormones control the long-term physiological responses of an animal, eg growth, metabolism and reproduction.

The concentration of a hormone in the blood at a particular time is determined by:

• The rate of hormone secretion
• The amount of hormone bound to carrier/binding proteins
• The rate of metabolism (the degradation) by the liver or kidney.

The concentration in the blood is not fixed and hormone secretion may be continuous, episodic, pulsatile or follow a daily rhythm.

Mechanisms of Hormone Action

In order for a hormone to affect a tissue or organ it must first bind to a specific receptor. The receptor binds to the hormone and then translates the 'message' of the hormone into a cellular response. Target cell receptors can be found on the external surface of the cell membrane, in the cytosol or in the nucleus.

Lipophilic hormones diffuses across the cell membrane and bind to receptors inside the cell or in the nucleus. The receptors bind to a part of the DNA known as the 'steroid response element', the receptors have a 'finger-like' projection which fit the shape of the DNA helix. Lipophobic hormones bind to the cell membrane receptors. After the hormone binds to the membrane receptor adenyl cyclase is stimulated. Adenyl cyclase converts ATP to cAMP, which is referred to as the 'second messenger' because it propagates the effect of the hormone.

The interaction between the receptor and the hormone depends on:

• the amount of hormone present in the blood
• the number of receptors present
• the affinity (strength) of the bond

There are three types of hormone interaction:

1. Synergism: the effect is greater than the effects of the two hormones added together.
2. Permissiveness: a hormone cannot fully exert its effects without another hormone
3. Antagonism: a hormone opposes the action of another. 

Homeostasis and Hormones

Hormone secretion is triggered by an internal or external stimulus and the synthesis and release of most hormones is regulated by a negative feedback system. As the hormone levels rise, they cause the target organ to inhibit the release of more hormone. Therefore, many hormones exist within a narrow homoestatic range.

That's it for this post, see you next time :)