Endocrinology

The next topic in our Veterinary Biochemistry unit deals with endocrinology - the study of the structure and function of the endocrine glands and the hormones they produce. In this post I'll define the terms endocrine, paracrine, autocrine and hormone. I'll also discuss the hormones produced by the anterior lobe of the pituitary gland and the adrenal glands and I'll describe the actions of posterior pituitary hormones. In addition, I''ll talk about the thyroid hormones, blood glucose regulation and melatonin.

Major Glands of the Endocrine System

Firstly, there are some terms which we need to know. A hormone is any substance that carries a signal to generate some sort of alteration at the cellular level. Hormones are secreted into body fluids (ie. blood), they bind to specific receptors in or on the target cells. Hormones work to initiate changes in cellular activity and the primary function of the endocrine system is to maintain homeostasis. Hormones are degraded by enzymes in the target cell, liver or kidneys. 

In autocrine signalling the cell which produces the signal is the same as which responds to the signal. This is the least common form of signalling by cells. In paracrine signalling one cell "talks" nearby cells by releasing a chemical, eg. the release of a neurotransmitter. In endocrine signalling, the most important type, hormones are secreted into the blood and carried by blood and tissue fluids to the cells they act upon.

Hormones Produced by the Pituitary Gland

The anterior lobe of the pituitary gland releases the following:

• Growth Hormone (GH): causes the growth of the cell to which the hormone is bound. GH also causes the growth of tissues, the uptake of amino acids into tissues, stimulates protein synthesis, conserves plasma glucose and opposes the effects of insulin and increases the synthesis of glucose and its release into the blood
• Thyroid Stimulating Hormone (TSH): promotes the production of thyroid hormones, regulates metabolic rates and body temperatures and releases calcitonin.
• when not enough thyroid hormones are produces, hypothyroidism results. Symptoms include a reduced metabolism, cold intolerance, slow heart rate, weight gain and lethargy
• when too much thyroid hormones are produced hyperthyroidism may result. Symptoms include: increased metabolism, nervousness and anxiety, insomnia and fatigue, as well as protrusion of the eyeballs.
• Leutinising Hormone (LH): stimulates the gonads. In males the production of testosterone and sperm development is promoted. In females estrogen production and egg development are promoted, and ovulation is stimulated. 
• Follicle Stimulating Hormone (FSH): works with LH to stimulate the gonads
• Adrenocorticotrophic Hormone (ACTH): acts on the adrenal gland and triggers the production of adrenaline and noradrenaline.
• Prolactin (PRL)

Posterior pituitary hormones include:

• Oxytocin: which initiates labour and causes milk ejection
• Antidiuretic Hormone (ADH): its key role is to maintain fluid homeostasis. It stimulates the kidney to conserve body water by reducing the output of urine. 

Hormones Produced by the Adrenal Glands: 

The adrenal medulla (the inside of the adrenal gland) secretes adrenaline and noradrenaline which increase heart rate, and increase the amount of glucose released from the liver to prepare the body for a "fight or flight" situation. 

The adrenal cortex (the outside of the adrenal gland) secretes:

• Mineralocorticoids:  this includes aldosterone and regulates electrolyte balance.
• Glucocorticoids: which includes cortisol and helps regulate glucose metabolism.
• Sex Steroids: which supplement the sex steroid secretion by the gonads.    

 Glucose Regulation

When the amount of glucose in the blood becomes too high, the beta cells, located in the Islets of Langerhan in the pancreas, secrete insulin. Insulin removes glucose from the blood by causing the liver to make glycogen, prevent gluconeogenesis and increase the glucose transport to cells. 

When the amount of glucose in the blood becomes too low, the alpha cells, located in the Islets of Langerhans in the pancreas secrete glucagon. Glucagon works on the liver to increase the amount of glucose in the blood via gycogenolysis and gluconeogenesis. 

Melatonin 

The pineal gland secretes melatonin which determines circadian rhythms. Melatonin decreases the time it takes to fall asleep by decreasing motor activity and inducing fatigue. Melatonin is inhibited by light and is associated with jet lag and "winter depression".


That's it for this post, if you have any questions please feel free to ask in the comments section below :)