Hormones

HORMONES (from Greek hormao—to excite), synonym for incretins, a term proposed by Starling to denote specific chemical products of internal secretion glands that are secreted directly into the bloodstream or lymph, rather than through excretory ducts into a cavity or to the outside. Hormones possess a regulating effect on various functions and establish a connection between organs through the blood. Under this definition fall the specific products of the thyroid and parathyroid glands, the adrenal medulla, the anterior lobe of the pituitary gland, the pineal gland, the thymus, the duodenum, and the large intestine. Products of the sex glands and the pancreas, which has an excretory duct, also fall here; in addition to the secretion of the so-called external secretion to the outside, they also release specific substances directly into the blood. In recent times, the liver has been included among the internal secretion glands (Lowi and others); hormones of the vagus and sympathetic nerves (Gley, Lowi and others), and the sinus heart hormone of Bayliss have been discussed. Bayliss and Starling applied the term hormone to secretin, which activates pancreatic secretion. It was later found that hormones possess not only an excitatory but also an inhibitory influence; for this reason, Sharpey-Schafer proposed to distinguish hormones and halons (from Greek chalao—to weaken), which are united under the general name "autacoids" (from Greek autos—self and akos—active substance). Roux and Abderhalden proposed the term "incretins," which they oppose to excretins, which are secreted to the outside and do not possess a regulating effect. The concept of hormones undergoes constant changes, but none of the existing definitions can be considered entirely successful. Only when the chemical nature of hormones is deciphered will a more satisfactory definition be reached. Guggenheim attempts to unite a number of hormones based on common chemical properties, generalizing them under the name "biogenic amines" because their main active constituent is amino groups; however, substances of non-hormonal nature also belong to this group of biogenic amines. Hormones were interpreted very broadly by Schiff and Brown-Sequard. They assumed that every cell of the organism, besides its direct functions, possesses a fermentative action (through the blood) on individual organs. Under this definition fall products of metabolism, for example, carbonic acid formed in muscles during their activity and influencing the respiratory organs through the blood. Fat extracts, when injected into rabbits (females), have an excitatory effect on the growth and function of the mammary glands. But such a broad interpretation of the concept of hormones risks depriving it of all definiteness and practical applicability. The above more limited definition is also not free from certain contradictions. The first contradiction is the presence of excretory ducts in some internal secretion glands (pancreas, sex glands)—this is only apparent, since these ducts belong not to the secretory but to the excretory function of the given organ. The characteristic epithelial-glandular structure of internal secretion glands is also not obligatory for them. Thus, the adrenal glands and the posterior lobe of the pituitary gland can be rightfully referred to as hormonal organs, although they are not of glandular structure. Mesenchymal derivatives also produce hormones. The spleen is not a glandular but a lymphatic organ, yet Gley and others refer it to hormonal organs. Finally, some glands do not secrete their secretions directly into the blood or lymph; for instance, the anterior lobe of the pituitary gland, at least partially, secretes its secretion into the cerebrospinal fluid, and the secretion of the adrenal glands, in the opinion of some authors, enters directly the peripheral nerve trunks. If, in solving the question of whether a given organ is hormonal, contradictions cannot be avoided based on a morphological principle, then a considerable number of contradictions also arise when applying a chemical criterion. The most indisputable criterion is the detection of hormones in the venous blood draining the organ, but this has been proven only for the adrenal glands; it has not been possible to detect hormones in the venous blood draining the thyroid gland; nevertheless, no one doubts the hormonal nature of the thyroid gland. Fewer contradictions are found when applying a physiological and, to some extent, clinical criterion based on the specific action of hormones, which is determined by the method of extirpation of the gland, the study of the phenomena of their atrophy, and the influence of subsequent substitution therapy. For a general characterization of hormones, one has to cite mainly negative signs. Hormones are not substances of protein nature, nor do they belong to the class of enzymes. They are not antigens; true, they have something in common with the latter, since they act in minimal quantities, but they differ in that they do not cause the formation of antibodies and anaphylaxis, since otherwise the action of hormones would be immediately destroyed. Hormones do not cause phenomena of habituation. Hormones do not possess species specificity. Thus, thyroid insufficiency in humans can be compensated by preparations of the same gland from animals; this indicates, among other things, the relative simplicity of the chemical structure of hormones; on the other hand, since hormones must pass through the walls of blood vessels, they must be characterized by a relatively low molecular weight. The hormonal effect does not differ in duration; according to Starling, this eliminates the possibility of overexcitation of the tissue on which the hormone acts. Finally, it is important to note that there is a definite dependence between the amount of hormone and the physiological effect. Gley proposed a classification based on the general physiological effect of the products of internal secretion glands. He distinguishes: first, secretions playing the role of nutrients (e.g., glucose, fat, fibrinogen); second, harmonons, substances regulating chemical processes in tissue construction (these include thyroid, pituitary, sex gland, thymic and pancreatic secretions, and hepatic antithrombin); third, hormones proper, influencing the functions of tissues in an excitatory manner (these include secretin and adrenaline); and fourth, parhormones—products of metabolism playing the role of stimulants (e.g., carbonic anhydride stimulating respiration, and urea). This classification cannot be considered successful, since it is impossible to draw a line between individual types of hormones and parhormones (Bogomolets). For example, carbonic anhydride possesses all the signs of a hormone. It has a specific influence on the respiratory centers, i.e., on the most important functions of life; an excess of carbonic acid excites respiration, a deficiency causes paralysis of the respiratory center. The harmonon of the thyroid gland is at the same time a dissimilatory hormone.

M. Jewish.