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Classification of Hormones

 

1. By chemical composition, hormones may be classified into 3 groups:

A. derivatives of amino acids. For example, adrenaline, a hormone produced by adrenal medulla, is a derivative of tyrosine. Iodine-containing thyroid hormones are also derivatives of tyrosine;

B. hormones of protein origin. Here 3 subgroups can be distinguished:

a) oligopeptides – low-molecular-weight proteins, e.g., antidiuretic hormone (vasopressin), oxytocin, gastrin, etc.;

b) proteins proper (insulin, glucagon, etc.);

c) glucoproteids, e.g., follicle-stimulating hormones, leuteinizing hormones;

C. Steroid hormones (hormones of adrenal cortex, sex hormones.

 

2. By mechanism of action hormones are divided into 2 groups:

a) hormones of membrane action (insulin, glucagon). Receptors sensitive to these hormones, are embedded into biological membranes of target cells. Thus, a hormone recognizes a receptor on the membrane of a target cell and interacts with it. After that a hormone-receptor complex activates second messengers that act to couple processes occurring on the membrane and those occurring in the cytoplasm. Second messengers may be the substances that participate in synaptic transmission:

- adenylate cyclase – cyclic adenosine monophosphate (cAMP)

- guanilate cyclase – cyclic guanosine monophosphate (cGMP)

- phospholipase C – inositol-3-phosphate;

- Ca++ - calmodulin;

b) hormones of intracellular (cytosolic) action. This group includes steroid hormones and reproductive hormones. Receptors sensitive to these hormones are located inside cells. Therefore, to be able to act on a cell a hormone must first penetrate a cell and interact with the receptor. Hormones of cytosolic action realize their action via hereditary apparatus of the cell located in the nucleus.

 

3. by physiological action hormones are classified into:

a) effector hormones acting on target cells in tissues and organs;

b) tropic or regulatory hormones. Hormones of this group are synthesized in the adenohypophysis (anterior pituitary). Their primary function is to regulate the activity of pituitary-dependent endocrine glands. This group includes adrenocorticotropic hormones, thyrotropic hormones and gonadotropic hormones;

c) neurohormones (regulators of regulators). This group includes releasing hormones (liberins and statins) produced in the hypothalamic region. The main function of this group of hormones is to regulate production of tropic hormones by cells of the anterior pituitary.

 

Hormones are continuously synthesized by the endocrine glands. The intensity of synthesis is determined not only by controlling signals, but also by the amount of produced hormones. In other words, the activity of the endocrine glands is controlled both by input and by deviation. The principle of inhibition by the end product known in biochemistry, works to suppress hormone synthesis if this hormone accumulates in the producer cells. Release of hormones by endocrine glands leads to increase in their concentration in the internal environment. Activity of the endocrine glands also depends on concentration of hormones in the internal environment of the body. If concentration is high, activity of producer cells decreases. This phenomenon requires careful use of hormones for practical purposes. For example, recently some anabolics of steroid nature have been widely used in sports due to their ability to build up muscles. But, because the structure of anabolics of this group is very much similar to the structure of male reproductive (sex) hormones, their use may lead to hypofunction of reproductive glands in males with their further hypotrophy and atrophy. And at last, the amount of hormones synthesized and released into the internal environment of the body, in some cases depends on the amount of substrates in the internal environment which is controlled by this hormone. For example, activity of pancreatic β-cells for synthesis and release of insulin into the internal environment depends on concentration of glucose, and an increase in the latter enhances synthesis and secretion of insulin by β-cells.



 


Date: 2015-01-02; view: 1138


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