Hormones- Chemical messengers are those who alter the metabolism of many cells. The release of the hormones- Chemical messengers are regulated by negative feedback. It is delivered from an organ to the target organ via blood.
Structure of Hormones
Three chemical families that form the structure of hormones.
1. Amino acid derivatives
E.g., epinephrine, thyroid hormones
2.Peptides and proteins
E.g., insulin, ADH
Steroids (e.g., testosterone), Eicosanoids (e.g., prostaglandins)
A protein in the cell membrane, or within the cytoplasm or nucleus, to which a hormone specifically binds to trigger its actions on a target cell. The binding of hormones is specific to the target cell receptors. After the release of hormones from the endocrine cells, the hormone is distributed throughout the body and is bound to its target receptor forming hormone-receptor complex.
Classes of Hormone Receptor
2. Intracellular Receptors
It is located in the cell membrane. It is targetted by amino acid derivatives, peptides, Eicosanoids. It is operated through a second messenger such as cyclic-AMP.
Mechanism of Hormonal action in Extracellular Receptors
- The membrane receptor binding hormones (the water-soluble hormones) bind to the membrane which leads to the activation of G protein
- The G protein now activates adenylyl cyclase which catalyses the conversion of ATP to cAMP (secondary messenger)
- cAMP then activates protein kinases that phosphorylate protein in the cytoplasm
This now leads to the alteration of the cell activity which is a response of the target cell.
It is located in the cytoplasm or nucleus. It is targeted by steroid and thyroid hormones. It is operated through changes in gene expression. Hormones like thyroid hormones also act on mitochondria.
Mechanism of Hormonal action in intracellular Receptors
- The steroid hormones (lipid-soluble hormones) diffuse through the plasma membrane binding to the cytoplasm forming receptor- hormone complex.
- The complex then enters the nucleus and triggers gene transcription.
- The mRNA is then transcribed and proteins are translated which leads to cell activity.
The thyroid hormones bind to mitochondria leading to an increase in the production of ATP which then alters the cell activity leading to a response.
Hormone Secretion and Distribution
- Hormones directly enter into the capillaries. It is distributed widely in the circulation. Most of the hormones travel free in solution while steroid and thyroid hormones bind to blood proteins for their transport.
- Hormones are inactivated by binding to cell receptors or removal by the liver, kidney cells or through breakdown by extracellular enzymes
Control of Endocrine Secretion
Humoral (fluid) stimuli- E.g., blood level of Ca2+ directly controls parathyroid hormone and calcitonin release
Hormonal stimuli- E.g., thyroid-stimulating hormone triggers thyroid hormone release
Neural stimuli – E.g., epinephrine release from the adrenal gland
Hypothalamic Control over Endocrine Organs
There are 3 mechanisms of action by Hypothalamus,
- Hypothalamus produces hormones as an endocrine organ which is released by posterior pituitary gland ie. ADH (Antidiuretic hormone) and oxytocin
- Hypothalamus secretes regulatory hormones to control pituitary gland endocrine cells which leads to the control of other endocrine organs.
- Autonomic centres exert direct neural control of adrenal medulla which leads to the secretion of epinephrine and norepinephrine