Hormonal Regulation

regulation of the vital activities in animals and man through the entrance of hormones into the blood; one of the systems of functional self-regulation, closely connected with the nervous and humoral systems of regulation and coordination of functions.

Hormones, secreted into the blood by endocrine glands, are carried throughout the body and influence the states and activities of various organs and tissues. Hormones may be divided into two groups, according to their type of action. Some hormones act upon specific “target” organs: thyrotropic hormone acts chiefly upon the thyroid gland, adrenocorticotropic hormone (ACTH) upon the adrenal cortex, and estrogens upon the uterus. Other hormones—corticosteroids, growth (somatotropic) hormone, and certain others —exert a generalized effect upon all body tissues. Insulin, for example, affects hydrate metabolism by activating the hexokinase reaction, and it can also stimulate the biosynthesis of protein. Testosterone and other androgens intensify processes of assimilation (anabolic effect); their introduction is accompanied by the retention of nitrogen in the body. Glucocorticoids produce various changes in metabolism, stimulate the formation of glycogen in the liver, inhibit the utilization of glucose in the periphery, and intensify the breakdown of proteins (especially in connective and lymphoid tissues). Estrogens stimulate the synthesis of phospholipids and protein in the uterus and cause hydration of the tissues of that organ. Growth hormone intensifies protein synthesis and influences fat, phosphorus, and calcium metabolism.

The effect of hormones upon metabolism is apparently associated with a change in the rate of enzyme reactions; in the majority of cases this is accomplished by activating the enzymes. The effect of hormones on protein biosynthesis is associated with the stimulation of messenger ribonucleic acid (messenger RNA) formation. (Messenger RNA determines the structure of the protein to be synthesized.) Hormonal regulation of metabolism ensures the normal functioning of organs and tissues. Growth and sexual maturation of the body are regulated by the growth and sex hormones. Mobilization of the body’s forces in case of need is also accomplished by hormonal regulation. For example, in the event of danger and the muscular tension produced by it, the entry of adrenalin into the blood is increased, raising the blood sugar level and increasing the blood supply to the heart and brain. Under intensely harmful influences, the manufacture of adrenocorticotropic and other hormones is increased.

The results of many experiments make possible the supposition that hormones are capable of activating genes. Thus, introduction of the molting hormone ecdysone to insects causes the formation of special swellings on the giant chromosomes. Analysis of these swellings has shown that intensive RNA formation goes on in them. Since the changes relating to the chromosomes and RNA synthesis precede the onset of pupation, it is believed that the first result of the action of ecdysone is the activation of the genes, followed by stimulation of RNA biosynthesis and formation of the appropriate enzymes. The last two steps ensure the process of metamorphosis.

The diversity of hormonal activity requires, in order to ensure normal body activity, the precise correspondence of hormone output to the body’s needs. This precise and delicate correspondence is ensured by the mutual influence of nervous, humoral, and hormonal factors. In some cases communication of the nervous system with the endocrine gland is direct. This has been proved for the adrenal medulla: stimulation of the splanchnic nerve leads to the increased secretion of adrenalin. In other cases the stimulus is transmitted along nerve fibers first to the hypothalamus, where substances are then formed (releasing factors) that enter the pituitary and produce the additional secretion of pituitary (so-called tropic) hormones, which in turn stimulate the formation of the appropriate hormone in the peripheral gland. Although releasing factors have not been obtained in pure form, their formation in the hypothalamus has been proved for adrenocorticotropic, luteinizing, follicle-stimulating, somatotropic, and several other hormones.

Hormone secretion is also regulated according to the principle of the feedback mechanism (plus-minus interaction). If for any reason the amount of a given hormone in the body is increased, this leads to inhibition of releasing factor secretion by the hypothalamus, which leads to a decrease in secretion of the corresponding tropic hormone by the pituitary, and finally to a decrease in hormone secretion by the peripheral gland. If, on the other hand, the concentration of a given hormone in the blood decreases (for example, through its accelerated decomposition in the tissues), this leads to increased secretion of the releasing factors, increased secretion of the tropic hormones by the pituitary, and increased biosynthesis of the hormone in the peripheral glands.

The self-regulation mechanism has definite significance in hormonal regulation. Thus, it has been shown that increased concentration of glucose in the blood leads to increased secretion of insulin and, consequently, to decreased concentration of glucose. Sodium salt deficiency stimulates secretion of aldosterone, a hormone of the adrenal cortex, whose effect is due to acceleration of the processes of reabsorption of sodium salts in the renal tubules, and thereby to their retention in the body. Thus, the system that regulates hormone production ensures the hormonal regulation of metabolism and other body functions.