Hormones Definition
Hormones Definition, Hormones are small molecules or proteins that are produced in one tissue, released into the bloodstream, and carried to other tissues, where they act through specific receptors to bring about changes in cellular activities. We also include in this discussion of short-lived signals such as NO, which acts locally, on neighboring cells.
Hormones serve to coordinate the metabolic activities of several tissues or organs. Virtually every process in a complex organism is regulated by one or more hormones: maintenance of blood pressure, blood volume, and electrolyte balance: embryogenesis, sexual reproduction, development: eating behavior, digestion, and fuel allocation.
Hormones act through Specific High-Affinity Cellular Receptors:
Hormones act as a high specific receptor in hormone-sensitive target cells, to which the hormone receptors, which the hormones bind with high affinity. Each cell type has its combination of hormone receptors, which define the range of its hormone responsiveness.
Moreover, two cell types with the same kind of receptors may have different intracellular targets of hormone action and thus may respond differently to the same hormone. The specificity of hormone action results from structural complementarity between the hormone and its receptors, The high affinity of the interaction allows cells to respond to very low concentrations or hormonal regulation, we need to know the relative specificity and affinity of the drug and the natural hormone.
- Water soluble Hormones
- Water insoluble Hormones
Water soluble Hormones (Hormones Definition)
Water soluble peptides and amine hormones such as insulin and epinephrine, act extracellularly by binding to cell surface receptors that span the plasma membrane. When the hormone binds to the extracellular domain, the receptors undergo a conformational change analogues to that produced in an allosteric enzyme by binding to an effector molecule. The conformational change triggers the effect on the hormone.
Water-insoluble Hormones
Water-insoluble hormones, including steroid, retinoid, and thyroid hormones, readily pass through the plasma membrane of their target cells to reach their receptor proteins in the nucleus. The hormone receptors complex itself carries the message it interacts with DNA to alert the expression of specific genes, changing the enzyme complement of the cell and thereby changing cellular metabolism.
Hormones Are Chemically Diverse
Hormones are also classified by the way they get from their point of release to their target tissue.
1- Endocrine Tissue:
Endocrine tissue( from the GreRendondon, “within” akinesinein, to “release”) hormones are released into the blood od are carried to target cells throughout the Examples are insulin and glucagon.
2- Paracrine Tissue:
Hormones are released into the extracellular space and diffuse to neighboring target cells ( the eicosanoid hormones are this type ).
3- Autocrine Tissue:
Hormones that affect the same cell that release, them bind to receptors on the cell surface. Mammals are hardly unique in processing hormonal signaling systems. Insects and nematode worms have highly developed systems for hormonal regulation with fundamental mechanisms similar to those of mammals. Plants too, use hormone signals to coordinate the activities of their tissues. To illustrate the structural diversity and range of action of mammalian hormones, we consider representative examples of each major class.
Classes of Hormones:
- Peptide Hormone
- Catecholamine Hormone
- Eicosanoid Hormone
- Steroid Hormone
- Vitamin D Hormone
- Retinoid Hormone
- Thyroid Hormone
- Nitric oxide Hormone