Hormones

Almost all organisms produce organic compounds that serve as intracellular signals. Hormones are substances that are secreted in small amounts by cells, transported by blood to target cells, where they regulate metabolism. Hormones regulate processes such as cell, tissue, and organ growth and differentiation, metabolic pathways, digestive processes, maintenance of ion concentration (homeostasis). Thus, they regulate not only the metabolism of various substances but also cell and tissue growth, heart rate, blood pressure, kidney function, peristalsis of the digestive tract, secretion of digestive enzymes, lactation, and activity of the reproductive system. Considering the distance they need to travel from the synthesis site to the target cell, hormones can be classified as endocrine, paracrine, and autocrine. Endocrine hormones are substances synthesized in one tissue or gland (e.g., pituitary gland, hypothalamus, thyroid gland, pancreas, adrenal glands, sex glands), transported over a long distance through the blood, and interact with specific receptors on target cells. In addition to endocrine glands, hormones are also synthesized by many other cells throughout the body. Therefore, the body is influenced by the diffuse neuroendocrine system, which consists of neuroendocrine cells, enterochromaffin cells, neurons of the central nervous system, platelets, basophilic leukocytes, and others. These cells usually synthesize peptide hormones and amines, which have autocrine and paracrine effects. This system is important in regulating metabolism, maintaining homeostasis, and transmitting nerve impulses. Paracrine hormones are secreted by cells, travel relatively short distances, and interact with neighboring cells’ specific receptors (e.g., gastrointestinal hormones). Autocrine hormones are secreted by cells that have specific receptors for them. These hormones interact with the cells that secrete them as well as with neighboring cells if they have specific receptors (e.g., prostaglandins). Autocrine effects are more common in tumor cells, promoting the proliferation of tumor cells. Endocrine hormones, which travel long distances to target cells, are more stable than paracrine and autocrine hormones, which act near the site of secretion. Thus, in addition to endocrine glands, hormones are synthesized by many other cells throughout the body (e.g., liver, gallbladder, kidneys, skin, heart, digestive system, respiratory organs). Hormones have specific biological effects: 1) endocrine glands and their secreted hormones form a unified system regulated by direct and feedback mechanisms; 2) hormones exhibit high biological activity: active at low concentrations (10^–6–10^–9 mol/l); 3) hormones regulate metabolism by altering the quantity of enzymes, their activity, or membrane permeability to ions and small molecules in target cells; 4) they act biologically through receptors – specific proteins located inside the cell or on the cell membrane; 5) hormones exhibit absolute specificity, meaning one hormone cannot be replaced by another. Hormones have various structures: they can be steroid hormones (aldosterone, cortisol, estradiol), derivatives of amino acids (adrenaline, thyroxine, auxins), peptides (oxytocin, vasopressin), proteins (insulin, glucagon), fatty acid derivatives (prostaglandins). All hormones act by binding to receptors on the cell target membrane (insulin, adrenaline) or inside the cell (steroid hormones). In many cases, hormones act as primary messengers, activating secondary messengers (often cyclic AMP) inside the cell. cAMP can activate protein kinase, which in turn activates specific enzymes, thereby altering cell metabolism or the permeability of the cell membrane to ions or molecules. Some hormones can act as neurotransmitters, for example, acetylcholine and certain peptide hormones. Hormones are transported either freely or bound to specific or nonspecific proteins. In the blood, hormones are transported non-covalently bound to plasma proteins. Albumins transport somatotropin, globulins transport steroid hormones, some steroid hormones have specific transport proteins, for example, cortisol is transported by transcortin.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė