They are secreted in small amounts by one type of tissue (called endocrine glands) and carried directly by the blood to act in another tissue (called targettissue) elsewhere in the body
They can also act on adjacent cells (paracrine action) and on the cells in which they were synthesized (autocrine action) without entering the systemic circulation
They can alter plasma membrane permeability, stimulate protein synthesis, activate or deactivate enzyme systems, induce secretory activity, or stimulate mitosis
1. Lipophilic hormone traverses the plasma membrane and binds to cytoplasmic or nuclear receptor
2. The hormone-receptor complex binds to specific region of DNA called hormone-receptor element (HRE) and activates or inactivates specific genes, resulting in changes in the amounts of specific proteins, which influence the metabolic processes
Mechanism of action of G-protein coupled receptors
1. In the absence of hormone, the heterotrimeric G-protein complex (α,β,γ) is in an inactive GDP-bound form
2. On binding of hormone to the receptor, there are conformational changes within the receptor, resulting in exchange of GDP with GTP on the α subunit, after which α and βγ dissociate
3. The α subunit binds to and activates the effector (E), which can be adenylyl cyclase, Ca2+, phospholipase Cβ, or cGMP phosphodiesterase
Mechanism of action using calcium and phosphoinositides as second messengers
1. Binding of hormone to GPCR activates Phospholipase C enzyme which catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol trisphosphate (IP3) and 1,2-diacylglycerol (DAG)
2. IP3 increases release of Ca2+ from intracellular storage sites in the endoplasmic reticulum
3. The resulting elevations of cytosolic Ca2+ activate Ca2+-calmodulin–dependent kinases and many other Ca2+-calmodulin–dependent enzymes
Mechanism of action using cGMP as second messenger
1. Cyclic GMP is formed from GTP by the action of the enzyme guanylate cyclase
2. Membrane bound guanylate cyclase is activated by atrial natriuretic factor (ANF), leading to effects like natriuresis, diuresis, inhibition of aldosterone secretion, and vasodilatation
3. Soluble guanylate cyclase is activated by compounds like nitroprusside, nitroglycerin, nitric oxide and sodium nitrate, leading to smooth muscle relaxation and vasodilation
4. cGMP activates cGMP-dependent protein kinase (PKG) which phosphorylates smooth muscle proteins like myosin light chain
1. Insulin, EGF, and IGF-1 have receptors with intrinsic protein kinase activities in their cytoplasmic domains
2. Binding of the hormone induces autophosphorylation on tyrosine residues of the receptor, which stimulates the activity of many intracellular enzymes like protein kinases and lipid kinases
3. Growth hormone and prolactin promote activation of cytoplasmic protein tyrosine kinases which phosphorylate one or more cytoplasmic proteins
Insulin increases the activity of some enzymes by dephosphorylation (e.g. glycogen synthase) and decreases the activity of some enzymes by dephosphorylation (e.g. phosphorylase)
They are secreted in small amounts by endocrine glands and carried directly by the blood to act on target tissues
They can also act on adjacent cells (paracrine action) and on the cells in which they were synthesized (autocrine action) without entering the systemic circulation