Types of Signaling in the Body / Endocrine System

Endocrine signaling plays a critical role in regulating various physiological processes, such as metabolism, growth, reproduction, and stress responses. Understanding these processes is essential for diagnosing and treating endocrine disorders.

In this blog we’ll cover types of signaling in the body, hormone synthesis, variations in peptide synthesis.

Types of Signaling in the Body / Endocrine System

Endocrine Signaling

It is a system in which :- 

• Gland secretes the hormone which releases into and circulates through the bloodstream.
• These hormones interact with the target cell having specific receptors to get a specific biological response 

There are various types of endocrine signaling that are explained below: -

1. Paracrine signaling: It is a system in which :-
   • The hormone / ligand acts on the targeted cell without ever passing through the systemic circulation 
   • For e.g., 
     • ECL cell in the stomach releases histamine which acts on the parietal cell to cause secretion of HCL
2. Autocrine Signaling: It is a type of system in which :-
   • The hormone can act on the cell that itself secretes the hormone.
   • Important Information 
     • There is one additional type of signaling known as the Neurocrine signaling 
     • It is a system in which the neurons release the ligands which exhibits an endocrine behaviour.

Types of hormones / hormone families 

• There are three types of hormones :-  
  • Peptide or protein hormones 
  • Steroid hormones 
  • Derivatives of a single amino acid 

Peptide / Protein Hormones

• These are further classified into groups or families that can be studied through the following table. 

Growth hormones family	Glycoprotein family	Insulin family	Secretin family	Some other small families
It includes: GH Prolactin  hPL	It includes: FSH LH TSH hCG	It includes: Insulin IGFs Relaxin	It includes: Secretin  Glucagon  VIP GIP	It includes: Calcitonin family including :- Calcitonin  Calcitonin gene released peptide (CGRP) Amylin Adrenomedullin  Parathyroid hormone (PTH) family including :- PTH  PTH - RP  Somatostatin family including :-  Somatostatin  Cortistatin

Important Information 

• The Ligand which has both paracrine, endocrine & neurocrine signaling is somatostatin.

Steroid Hormones 

• All of these hormones are synthesized from cholesterol 
• Synthesis of cholesterol requires enzymatic steps which can only be performed by the specialized tissues
• For e.g. : 
  • Glucocorticoids and mineralocorticoids secreted by the adrenal cortex gland.
  • Testosterone secreted by testis.
  • Estrogen and progesterone secreted by ovaries.
• One common characteristic of all steroid hormones is that they have a CPP ring (cyclo pento-perhydrophenanthrene ring).

Derivatives Of A Single Amino Acid

• These are the hormones that are derived from Tyrosine (A single Amino Acid).
• These hormones include :- 
  • Thyroid hormone 
  • Catecholamines 

Physiology Related Articles:

Endocrine Glands: Anterior pituitary and Posterior pituitary - NEET PG Physiology	The Thyroid Gland - NEET PG Physiology	Body Fluid Compartments and its Measurement
Important Topics in Physiology for NEET-PG by Dr. Vivek Nalgirkar	How to Prepare Physiology for PG Entrance Exams | PrepLadder

HORMONE SYNTHESIS: GENERAL PRINCIPLES

General principles for various hormone synthesis can be studied through following points:

Peptide Hormones 

• These hormones are always synthesized from a precursor hormone called “Prehormone”.
• This prehormone is cleaved to form prohormone which is again cleaved to form the final hormone.
• Peptides are water soluble due to which they cannot cross the cell membrane easily.
• The receptors for these hormones are present in the cell membrane only.
• These hormones move freely once they are released i.e. without binding with the plasma protein, Except, IGFs.
• As these hormones are not bound to plasma protein, their half-life is short in circulation. 

Steroid hormone 

• These hormones are not stored into the gland or synthesizing cell i.e. they are immediately released into the circulation.
• It happens because steroid hormones are lipid soluble due to which they can cross cell membranes easily.
• Steroid hormones have an intracellular receptor i.e. it is present in the cytoplasm or nucleus of the cell.
• In Particular, estrogen has intra intra-nuclear receptor whereas Glucocorticoids have an intra-cytoplasmic receptor.
• These hormones are bound to plasma protein into circulation i.e. they cannot move freely.
• Since these hormones are bound, they have a longer half-life.

Important Information 

• Vit. D has the longest half-life i.e. almost 15 days 

Latent Period 

• It is the time interval between stimulus and actual reaction. 
• Different hormones have different latent periods.
• E.g. Oxytocin has the shortest latent period → i.e. milk ejection occurs in a few seconds.
• Peptide hormone - TRH has one of the shortest latent periods.
• The Thyroid hormone has the longest latent period that is 48-72 hours.

VARIATIONS IN PEPTIDE SYNTHESIS

• One precursor can form multiple copies of the same hormone. For e.g. TRH.
• One precursor can form multiple peptide hormones, for e.g. POMC is synthesized by the basophil cell in the pituitary gland that forms ACTH & -LPH on getting cleaved.
  • Further cleavage of ACTH will form MSH and CLIP. 
  • Further cleavage of LPH will form endorphin.
• One precursor can form only a single copy of one hormone, for e.g. Insulin - When preproinsulin is cleaved into proinsulin which is further cleaved into final insulin and C-Peptide.

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