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Renin Angiotensin Mechanism.

Classification of Hormones and their Mechanisms of action.

 

Introduction.

  • The Endocrine System consists of endocrine glands separated from each other by a wide distance.

  • Endocrine glands consist of secretory cells present between a network of capillaries.

  • Secretion of the endocrine gland is called “Hormone”, the hormones secreted diffuse in blood directly and transfer to the target organs.

  • Endocrine glands are also called “Ductless Glands”.

  • It plays a vital role in maintaining homeostasis along with the Nervous system.

  • The branch of science that deals with the study of Endocrine glands is called Endocrinology.

Important Endocrine Glands:

  • Pituitary Gland.

  • Thyroid Gland.

  • Parathyroid Glands.

  • Adrenal Glands.

  • Pineal Gland.

Definition and Classification of Hormones:

  • The chemicals secreted by Endocrine Glands are called Hormones.

  • Hormones are classified on the basis of the chemical structures as follows,

  • A) Amine Hormones:

    • The hormones that are derived from single amino acids with modified groups in their structures.

    • E.g.

      •  Noradrenaline.

      • Melatonin

      • Thyroxin

  • B) Peptide Hormones:

    • The hormones derived from multiple amino acids and contain shorter peptide chains than proteins are called peptide hormones.

    • E.g.

      • ADH (AntiDiuretic Hormone)

      • Oxytocin.

      • Calcitonin

  • C) Protein Hormones: 

    • The hormones derived from multiple amino acids and contain longer peptide chains than peptide hormones are called protein hormones.

    • E.g. 

      • Insuline.

      • Growth Hormone.

      • Glucagon.

  • D) Steroid Hormones:

    • Hormones containing steroidal nuclei in their structure are called steroidal hormones.

    • E.g. 

      • Testosterone

      • Progesterone.

      • Estrogen.

Mechanisms of Hormone Action.

  • The message sent by the endocrine gland through hormones is received in the cell by hormone receptors.

  • The receptor then processes the message by initiating various cellular mechanisms to produce target cells' response.

  • The processing is done by one of the either pathways,

    • Pathway involving intracellular hormone receptors.

    • Pathway involving extracellular hormone receptors.

  • Pathway involving intracellular hormone receptors:

    • The lipid soluble hormones like steroidal hormones easily pass through the cell membrane.

    • They then attach to the hormone receptor present in the cytoplasm or nucleus of the cell.

    • The hormone receptor complex then attaches to a chromatin and generates a mRNA (messenger RNA) which comes out into cytoplasm.

    • The information written in mRNA is used to synthesize the proteins which in turn produce the target cell’s response.

  • Pathway involving extracellular hormone receptors:

    • Hydrophilic hormones such as Angiotensin II  having lesser lipid solubility cannot pass easily inside a cell.

    • Such hormones attach with a receptor located on a cellular membrane to form a “Hormone - receptor complex”.

    • The formed hormone receptor complex then initiates a cascade of events by forming a second molecule (Second Messenger), resulting in the target cell’s action.

    • The Hormone receptor complex works from either of the following pathways,

      • cAMP

      • Phospholipase C.

    • Cyclic Adenosine Mono Amino Phosphate (cAMP) Pathway:

      • The hormone receptor complex activates a cellular component called “G Protein”.

      • The activated G Protein then activates an enzyme “Adenylate cyclase” which causes conversion of ATP (Adenosine Triphosphate) to cAMP.

      •  cAMP activates a type of enzyme called “Protein kinase” which causes a cascade of phosphorylation events of many proteins activating different enzymes resulting in target cell’s action.

      • The cAMP action is checked by an enzyme “Phosphodiesterase”.

      • E.g. Calcitonin, Glucagon.

    • Phospholipase C pathway:

      • The hormone receptor complex activates a cellular component called “G Protein”.

      • The activated G Protein then activates an enzyme “Phospholipase C” which causes cleavage of a phospholipid from the cellular membrane to produce,

        • diacylglycerol (DAG) 

        • inositol triphosphate (IP3)

      • Diacylglycerol activates a protein kinase like cAMP while inositol triphosphate causes mobilisation of calcium ions from the endoplasmic reticulum, which activates many proteins or enzymes.

      • E.g. Angiotensin II

Commonly Asked Questions.

  1. Define and classify Hormones.

  2. Discuss in detail various mechanisms of action of hormones.
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