Mode of action of hormones, Pathway for Water-soluble & Lipid-soluble Hormones.pptx

*Mode of action of hormones
*Hormone Receptors
*Classes of Hormones based on their solubility in water and lipids.
1) Water-soluble hormones
2) Lipid-soluble Hormones
*Pathway for Water-soluble Hormones
*Pathway for Lipid-soluble Hormones

1. Mode of Action of Hormones The mode of action of hormones on cells is not yet completely understood. Mostly hormones are involved in synthesis or activation of specific proteins, mostly enzymes. The mode of action of peptide hormones is different from that of steroid hormones.

2. Hormone Receptors Hormones activate a cellular response in the target cell by binding to a specific receptor in the target cell . Hormone receptors are of following types : Internal receptors – they can be either  nuclear or cytoplasmic. Nuclear receptors are found on the nuclear membrane while the cytoplasmic receptors are found in the cytoplasm of the cell. These receptors are for the steroid hormones. External receptors – These are the transmembrane receptors which are embedded in the lipid layer of the cell membrane. These receptors are for the protein ones .

3. Hormones fall into two general classes based on their solubility in water and lipids. 1) Water-soluble hormones: A hydrophilic hormone that is secreted by exocytosis, travel freely in the bloodstream, and bind to cell surface receptors on the plasma membrane. As these hormones are water soluble, they cannot pass through the cell membrane as it is made up of a lipid layer. Can also be called lipophobic hormones. Examples: The water-soluble hormones include polypeptide, protein and most amino acid hormones.

4. 2 ) Lipid-soluble Hormones A lipophilic hormone that passes through the plasma membrane of the cell, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells . They are poorly soluble in water. Can also be called hydrophobic hormones. Examples: The lipid soluble hormones include steroid hormones, thyroid hormone, and Vitamin D 3 .

5. Pathway for Water-soluble Hormones Water-soluble hormones bind to their extracellular receptors present on the membrane. Once they bind to the receptor, a series of reactions occur beginning with the activation of adenyl cyclase enzyme, which is located within the plasma membrane. This enzyme leads to the production of cyclic AMP or cAMP which is the secondary messenger.

6. This cAMP can now enter the cell and cause the effect it was meant to bring about . The hormone outside the cell is the primary messenger while c-AMP within the cell is the second messenger.

7. Pathway for Lipid-soluble Hormones Lipid-soluble hormones can penetrate the cell membrane due to their smaller size and lipid permeability. Within the cell, they bind to specific receptor proteins in the cytoplasm or nucleus. Upon binding by the hormone the receptor undergoes a conformational change, and the receptor together with the bound hormone causes a change in gene activity .

8. This increases transcription and mRNA production which leads to increase in protein synthesis.

9. Endocrine glands release hormones in response to one or more of the following stimuli: 1 . Hormones from other endocrine glands. e.g TSH released by pituitary gland act on thyroid gland to release thyroxin (T4). 2 . Chemical characteristics of the blood e.g amount of glucose in the blood controls the secretion of insulin and glucagons . 3 . Neural stimulation. Prolactin inhibiting factor is actually dopamine, released from hypothalamus, reach to cells in the anterior pituitary which were responsible to release prolactin and it will inhibit the release of prolactin.