There are two major mechanisms, second-messenger mechanisms and direct gene activation, by which the hormone activates the target cell. Direct Gene Activation. Steroid hormones pass through plasma membrane (they're lipid soluble) and attach to receptor molecules that are inside the cell.
Fat-soluble hormones, like the sex hormone steroids estrogens and androgens, are fat soluble and water repellent. That is, they "like" lipid or fatty structures such as those surrounding cells but are generally repelled by watery areas.
Amine hormones originate from the amino acids tryptophan or tyrosine. Steroid hormones and thyroid hormone are lipid soluble. All other amino acid–derived hormones are water soluble. Hydrophobic hormones are able to diffuse through the membrane and interact with an intracellular receptor.
Water-soluble hormones include glycoproteins, catecholamines, and peptide hormones composed of polypeptides, e.g. thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone and insulin. These molecules are not lipid-soluble and therefore cannot diffuse through cell membranes.
Mechanism of Hormone Action. Each hormone has receptors that are found on the cell membrane of the target organ. Once the hormone bind to its designated receptor, a series of actions are initiated to release secondary messengers inside the cell.
Peptide hormones consist of a polypeptide chain; they include molecules such as oxytocin (short polypeptide chain) or growth hormones ( proteins ). Amino acid-derived hormones and protein hormones are water-soluble and insoluble in lipids.
Heart. When the body experiences an increase in blood volume or pressure, the cells of the heart's atrial wall stretch. In response, specialized cells in the wall of the atria produce and secrete the peptide hormone atrial natriuretic peptide (ANP).
All the steroids (androgens and estrogen, and the hormones of the adrenal cortex, the corticosteroids) are examples of lipid soluble hormones. Cortisol is a water soluble membrane; epinephrine is lipid soluble.
Although a given hormone can reach all cells of the body, only some cells have receptors for that hormone. A hormone elicits a response only from specific target cells that have the matching receptor. These signals in turn regulate neurons, muscle cells, and endocrine cells.
Types of hormones
- Steroid hormones – these are made from cholesterol.
- Eicosanoids: these are lipid hormones – hormones made from lipids, kinds of fats.
- Amino acid derived.
- Peptides, polypeptides and proteins – small peptide hormones include TRH and vasopressin.
There are at least 60 different hormones in humans, and we're only beginning to understand how what we eat affects them. If you think your thyroid hormones or other hormones could be off, it's important to see an endocrinologist. Hormone imbalances are medical conditions that often require the use of medications.)
Hormones produced by the pituitary gland
- Adrenocorticotrophic hormone (ACTH)
- Thyroid-stimulating hormone (TSH)
- Luteinising hormone (LH)
- Follicle-stimulating hormone (FSH)
- Prolactin (PRL)
- Growth hormone (GH)
- Melanocyte-stimulating hormone (MSH)
List of important hormones and their functions.
- Hormones of Thyroid. Thyroid gland basically releases two hormones Triiodothyronine (T3) and Thyroxine (T4), which helps in controlling the metabolism of our body.
- Insulin. Source:
- Estrogen.
- Progesterone.
- Prolactin.
- Testosterone.
- Serotonin.
- Cortisol.
Hormones can only affect cells that display receptors that are specific to them; cells can display receptors for many different hormones at once. The more receptors for a particular hormone that a cell displays, the more sensitive to that hormone it will be.
Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone.
Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide (peptide and proteins) hormones.
There are two major classes of hormones:
- proteins, peptides, and modified amino acids.
- steroids.
Tropic hormones are hormones that have other endocrine glands as their target. Most tropic hormones are produced and secreted by the anterior pituitary. Tropic hormones are contrasted with non-tropic hormones, which directly stimulate target cells.
1 Hypothalamic Hormones (Factors)
In addition, a variety of non-peptide neurotransmitters (e.g., acetylcholine, dopamine, GABA, glutamate, norepinephrine, serotonin) (see Figure 4-2) affect tropic hormone release through their effects at the hypothalamic level.The pituitary produces the direct action hormones: prolactin, ADH and oxytocin. The trophic hormones are TSH, ACTH, FSH and LH.
Trophic hormones from the anterior pituitary include:
- Thyroid-stimulating hormone (TSH or thyrotropin) – stimulates the thyroid gland increasing the size and number of cells.
- Adrenocorticotropic hormone (ACTH or corticotropin) – stimulates the adrenal cortex increasing the size and number of cells.
Some examples of non-tropic hormones are:
- Glucocorticoids: secreted from the adrenal glands and released directly into the blood stream where it alters blood glucose levels.
- Vasopressin (Antidiuretic hormone; ADH): secreted from the posterior pituitary and acts on the kidneys to maintain water balance in the body.
Oxytocin is produced in the hypothalamus and is secreted into the bloodstream by the posterior pituitary gland. Secretion depends on electrical activity of neurons in the hypothalamus – it is released into the blood when these cells are excited.
For example, adrenocorticotropic hormone (ACTH) is a tropic hormone which acts on the cells of adrenal cortex and stimulates synthesis and secretion of glucocorticoid hormones. Alternative name for ACTH (a tropic hormone) is corticotropin.
GnRH is a tropic peptide hormone synthesized and released from GnRH neurons within the hypothalamus. The peptide belongs to gonadotropin-releasing hormone family. It constitutes the initial step in the hypothalamic–pituitary–gonadal axis.
