Delivering a message

The horse, hormones, and endocrine systemThere are two major organ systems within the bodies of horses and people that permit communication between cells, organs, and the various body systems. These two systems are the nervous system and the endocrine system. The nervous system usually carries messages that are delivered quickly and produce immediate and relatively short-acting responses. Electrical, electrochemical, and chemical messengers are the means by which cells and tissues communicate via the nervous system. The nervous system has been described in earlier articles.
The endocrine system usually carries and delivers messages more slowly; however, they tend to have longer-lasting effects. The endocrine system utilizes chemical messengers called hormones. These hormones are deposited by an endocrine gland directly into the bloodstream and are carried by the circulation to their specific target cells or tissue. They exert either a stimulating or inhibitory effect upon their target cells. They produce these effects by stimulating a messenger chemical within their target cells, or by activating genes which then control target cell function.
Based upon their chemical structure, hormones fall into one of four major groups. The endocrine glands that make up the endocrine system control the function of many vital organs. Growth and reproduction are also largely regulated by the endocrine system.
There are several organs that have a strictly endocrine hormone-producing function. There are other organs, however, which produce hormones but also have other important functions (kidney, pancreas). Functions of the nervous and endocrine systems overlap a great deal. Their activities, interactions, and regulation are complex. The following brief description of the major endocrine glands, their hormones, and their biological effects provide a starting point for an understanding of and appreciation for the potential manipulation of a horse's endocrine system.

The pituitary gland
The pituitary gland is located within the brain. It is divided into two major parts that are considered to be two separate glands. The anterior pituitary gland, or front portion of the pituitary gland, produces and releases six major hormones. Many of these hormones in turn stimulate other endocrine glands to produce their hormones.
The anterior pituitary hormones include:

1. 1) thyroid-stimulating hormone (TSH), which increases the release of the major thyroid hormone;
   
2. 2) adrenocorticotropic hormone (ACTH), which regulates the action of the outer adrenal gland tissue;
   
3. 3) growth hormone (GH), also called somatotrophic hormone (STH), which stimulates cell metabolism to "build" and controls growth and size increases;
   
4. 4) prolactin (PRL), which has many effects, including udder development and milk production; and
   
5. 5) follicle-stimulating hormone (FSH) and 6) luteinizing hormone (LH), which stimulate cells in the ovaries in mares and the testicles in stallions.
   

Control of the anterior pituitary gland and associated hormones is controlled by the hypothalamus, which is also located within the brain. The hypothalamus is also the most important part of the brain for controlling subconscious internal body functions of the horse, including thirst, hunger, body temperature, heart rate, and blood pressure.
The posterior pituitary gland, or back portion of the pituitary gland, produces and releases two important hormones. Antidiuretic hormone (ADH) stimulates water retention in the kidneys. Oxytocin stimulates smooth-muscle contractions in the uterus and therefore foaling. In addition, oxytocin is responsible for milk letdown when foals are nursing.

The thyroid gland
The thyroid gland is located in the throat latch area just behind the jaw. The thyroid gland produces and releases several hormones. The most important one is thyroxine (T4). Iodine is critical to the production of thyroxine. Just about every cell in a horse's body is a target for thyroxine. Thyroxine helps regulate the metabolic action of cells. It steps up the metabolic rate of target cells. Thus, more calories are burned, the heart pumps more rapidly and with greater force, alertness is increased, intestinal tract motility can increase, and appetite may also be stimulated. The base metabolic rate of a horse may be increased in prolonged cold weather of several weeks duration as a result of an increase in thyroxine production and release. Another hormone, T3, is very similar to thyroxine, but it has one less iodine molecule attached to it. T3 is produced in smaller amounts than thyroxine, has similar effects as thyroxine, but the effects occur more rapidly than those of thyroxine.
Calcitonin is another hormone produced and released by the thyroid gland. It functions to control calcium metabolism and bone development. It is a calcium-depressing hormone and acts to lower blood calcium levels. Calcitonin acts by increasing the deposition of calcium into bones.

The parathyroid glands
The parathyroid glands are located near the back of the thyroid gland. Parathormone is the only hormone produced by parathyroid glands. It controls the calcium levels in the blood. This is done by stimulating increased calcium absorption in the intestinal tract, decreased kidney loss of calcium by increasing its reabsorption, and by stimulating its release from bone. Parathormone is the primary calcium-regulating hormone.

The thymus and adrenal glands
The thymus gland is located within the chest and has important hormonal and nonhormonal functions. The thymus generates lymphocytes that are critical to a horse's immunity.
The adrenal glands are located near the kidneys. They are made up of two distinct regions. The outer layer, called the adrenal cortex, releases very important steroid hormones, including cortisol and aldosterone. Aldosterone is one of the hormones that regulate body salt (mineralocorticoid hormones). Cortisol is the most important of the glucose-regulating (and also fat and protein) hormones (glucocorticoid hormones). They also cushion a horse's body from shock, trauma, toxins, and stress.
The inner region of the adrenal glands is called the adrenal medulla. The two hormones produced in the adrenal medulla are not steroids. These two hormones were previously called adrenalin and noradrenalin. They are now called epinephrine and norepinephrine. These hormones are "emergency stimulators" and effect the "flight or fight" reaction in a horse.

The gonads and pancreas
The testes in stallions and ovaries in mares produce steroid hormones. In mares, these are the estrogens, and in stallions, testosterone. These sex hormones regulate sexual characteristics and the reproduction process.
The pancreas is located near the first part of the small intestine in the abdomen. It has both hormonal and nonhormonal functions. The pancreas releases two important hormones: 1) insulin; and 2) glucagon. Insulin regulates blood sugar levels by decreasing blood glucose levels and also has an important impact on the metabolism of carbohydrates, proteins, and fats. Glucagon raises the blood sugar levels.

Conclusion
The endocrine system, through messenger compounds called hormones, is one of the primary means for cell and tissue communication within a horse's body. The increasing knowledge of the effects of the various hormones' actions and the regulatory feedback mechanisms that keep everything in check permit the augmentation of certain hormone levels and the manipulation of the endocrine communication system. Care and caution are needed in order to prevent deleterious side effects.

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Brad J. Gordon, DVM, specializes in surgery, lameness, and therapeutics in the Midwest and Caribbean.