Hormones

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism.

Endocrine hormone molecules are secreted (released) directly into the blood stream. Exocrine hormones (or ectohormones) are secreted directly into a duct, and, from the duct, they flow either into the bloodstream or from cell to cell by diffusion in a process known as paracrine signalling.

Video (AP1 EPITHELIA EXOCRINE & ENDOCRINE GLANDS)

Chemically, hormones are of four types: 1. Hormonal amine. 2 Chemically, hormones are of four types: 1. Hormonal amine. 2. Peptide, protein, or glycoprotein. 3. Steroid. 4. Eicosanoid.

1. Hormonal Amines

2. Steroid Hormones

Four kinds of steroid hormones differ in structure and action; they are the androgens (C19), the estrogens (C18), the progestins (C21), and the corticosteroids (C21). All are synthesized from cholesterol .

3. Peptide, Protein, and Glycoprotein Hormones

Mechanism of Hormone Action

Hormone receptors can be classified into three types on the basis of their locations in the cell and the types of hormone they bind: Nuclear receptors, which bind triiodothyronine (T3) after it enters the cell.

Cytosolic receptors, which bind steroid hormones as they diffuse into the cell. Cell surface receptors, which detect water-soluble hormones that do not enter the cell (peptides, proteins, glycoproteins, catecholamines).

Nuclear receptors of thyroid hormones

Video (Thyroxine _ Mechanism of Action of Thyroid Hormones)

Steroid Hormone Receptors

Video (Steroid Hormones _ Mechanism of Action)

Cell Surface Receptors intracellular second messenger is utilized to implement the hormonal action and involves: G-protein-coupled adenylate cyclase cAMP system. G-protein-coupled Phosphatidylinositol Ca2+ pathway.

G-Protein-Coupled Adenylate Cyclase-cAMP System

G-Protein-Coupled Phosphatidylinositol-Ca2+ Pathway

Video (How Hormones Use G-protein Signaling Pathways_ A Video Review of the Basics)

Hypothalamus

Hypophysiotropic Peptides released by hypothalamus

Thyrotropin-releasing hormone (TRH) Thyrotropin-releasing hormone is a tripeptide amide. TRH principally stimulates the synthesis and release of thyroid-stimulating hormone (TSH, thyrotropin) in the anterior pituitary, and also stimulates the release of prolactin (PRL). Both effects are mediated by membrane receptors coupled to the GQα-phospholipase C-β-calcium-protein kinase C second-messenger system.

2.Gonadotropin-releasing hormone (GnRH) or luteinizing hormone-releasing hormone (LHRH) Gonadotropin-releasing hormone (GnRH) is a decapeptide which, like TRH, has a pyroglutamic acid residue in its N terminus. GnRH stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), the two gonadotropic hormones produced by the pituitary.

3. Corticotropin-releasing hormone (CRH) Corticotropin-releasing hormone is a 41-amino-acid polypeptide. CRH stimulates the release of ACTH and β-endorphin by the anterior pituitary corticotrophs.

4. Growth hormone-releasing hormone (GHRH) Growth hormone-releasing hormone (GHRH) is a 44-amino-acid polypeptide. GHRH stimulates the synthesis and release of GH in anterior pituitary somatotrophs by a cAMP-mediated mechanism.

5.Somatostatin Somatostatin (growth hormone release-inhibiting hormone) is a tetradecapeptide [14 amino acids] with an intrachain disulfide bridge. Somatostatin inhibits the synthesis and release of GH from the somatotropes by neutralizing the effect of GHRH, an effect mediated by Giα inhibition of adenylyl cyclase. Within the hypothalamus, somatostatin inhibits the release of GHRH by the same mechanism and therefore exerts an inhibitory effect at two levels of GH control.

6. Prolactin-inhibiting hormone (PIH) It is known as dopamine 6.Prolactin-inhibiting hormone (PIH) It is known as dopamine. It functions as a neurotransmitter in the CNS and as a precursor of norepinephrine and epinephrine in the adrenal medulla. Dopamine is a potent inhibitor of PRL release by the lactotropes (and mammosomatotropes) of the anterior pituitary, and this effect is mediated by D2 receptors that are coupled to Giα inhibition of adenylate cyclase.

Neurohypophyseal Peptides ADH (Vasopressin)

Video (GTAC - Chemical Signalling_ Antidiuretic hormone (ADH)

Oxytocin

The principal action of oxytocin is ejection of milk from the lactating mammary gland ("milk let-down"), and it also participates in parturition. The mechanism of action of oxytocin does not involve cAMP but may involve regulation of increased intracellular Ca2+.