Hormones
Introduction to Hormones
Almost all organisms produce organic compounds that serve as intracellular signals. Cells secrete hormones in small amounts and transport them through the blood to target cells, where they regulate metabolism. Hormones manage processes such as growth and differentiation at the cellular, tissue, and organ levels, metabolic pathways, digestive processes, and the maintenance of ion concentration (homeostasis). Consequently, they control not only the metabolism of various substances but also cell and tissue growth, heart rate, blood pressure, kidney function, digestive tract peristalsis, secretion of digestive enzymes, lactation, and reproductive system activity.
Types of Hormones
Considering the distance hormones travel from the synthesis site to the target cell, we classify them as endocrine, paracrine, and autocrine.
Endocrine Hormones
Endocrine hormones are synthesized in one tissue or gland (e.g., pituitary gland, hypothalamus, thyroid gland, pancreas, adrenal glands, sex glands). They move long distances through the blood and interact with specific receptors on target cells. Many other cells throughout the body also synthesize hormones. The diffuse neuroendocrine system influences the body, consisting of neuroendocrine cells, enterochromaffin cells, central nervous system neurons, platelets, basophilic leukocytes, and others. These cells typically synthesize peptide hormones and amines, leading to autocrine and paracrine effects. This system plays a crucial role in regulating metabolism, maintaining homeostasis, and transmitting nerve impulses.
Paracrine and Autocrine Hormones
Cells secrete paracrine hormones, which travel relatively short distances and interact with specific receptors on neighboring cells (e.g., gastrointestinal hormones). Cells that possess specific receptors for autocrine hormones secrete these hormones. These hormones interact with the cells that secrete them and with neighboring cells if they have specific receptors (e.g., prostaglandins). Tumor cells frequently exhibit autocrine effects, promoting the proliferation of tumor cells. Endocrine hormones, which travel long distances to target cells, demonstrate more stability than paracrine and autocrine hormones, which act near the secretion site.
Biological Effects of Hormones
Many other cells throughout the body (e.g., liver, gallbladder, kidneys, skin, heart, digestive system, respiratory organs) synthesize hormones. Hormones exhibit specific biological effects:
- Endocrine glands and their secreted hormones form a unified system regulated by direct and feedback mechanisms.
- Hormones showcase high biological activity and function at low concentrations (10–6–10–9 mol/l).
- Hormones regulate metabolism by altering the quantity of enzymes, their activity, or membrane permeability to ions and small molecules in target cells.
- Hormones act biologically through receptors, which are specific proteins located inside the cell or on the cell membrane.
- Hormones display absolute specificity, meaning one hormone cannot replace another.
Hormone Structures and Functioning
Hormones come in various structures:
- Steroid hormones (aldosterone, cortisol, estradiol)
- Derivatives of amino acids (adrenaline, thyroxine, auxins)
- Peptides (oxytocin, vasopressin)
- Proteins (insulin, glucagon)
- Fatty acid derivatives (prostaglandins)
All hormones operate by binding to receptors on the cell target membrane (insulin, adrenaline) or inside the cell (steroid hormones). Often, 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, altering cell metabolism or membrane permeability to ions or molecules.
Additional Roles and Transport
Some hormones can function as neurotransmitters, such as acetylcholine and certain peptide hormones. Hormones travel either freely or bound to specific or nonspecific proteins. In the blood, hormones travel non-covalently bound to plasma proteins. Albumins transport somatotropin, globulins transport steroid hormones, and some steroid hormones have specific transport proteins, for example, transcortin transports cortisol.
Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė