Types of receptors

• Ligand gated ionic channel receptors

  • they are membrane receptors located on the gate of ionic channels.

  • Binding of the ligand to the receptor will lead to conformational change in the receptor and change in cell membrane permeability to ions.

  • Milliseconds between binding of the ligand and cellular response.

  • e.g.

  • Acetylcholine (A.Ch) + Nicotinic receptors → increase Na influx (depolarization).
  • GABA + GABA receptors → Cl influx hyperpolarization
• G. protein-coupled receptors and second messenger

  • They are cell surface receptors which facilitate binding of guanosine triphosphate (GTP) to specific proteins located on the cytoplasmic surface of plasma membrane known as Gprotein which regulates the activity of membrane enzyme and ion channels.

  • The G-protein is a membrane protein comprising three subunits (α, β, γ)

  • G-protein either:

  • regulate intracellular 2nd messenger (α-subunits with GTPase activity) or
  • control opening of ion channel (β, γ subunits)
• here are different G subunits (Gs - Gi - Gq)

  • Few seconds - minutes between binding of the ligand and cellular response:

  1. Adenyl cyclase (A.C) is regulated via Gas (increase c-AMP) and Gai (decrease c-AMP)
  2. Phospholipase C activated via Gq responsible for generation of IP3 (inositol triphosphate) and DAG (diacylglycerol) to regulate free calcium concentration.
• G subunits examples

  • Adrenaline + b receptor→ Gas-protein → increase A.C. → increase CAMP concentration
  • Adrenaline + a2 receptor → Gai-protein → decrease A.C. . → decrease CAMP concentration
  • A.Ch + M1 &3 receptors → Gaq-protein → Phospholipase C enzyme →inositol triphosphate (IP3) & Di-acyl glycerol (DAG) → Ca2 + Calmodulin → Response.
  • Acetylcholine + M2-receptors → GBy → increase K Efflux → Hyperpolarization
• Enzyme (Tyrosine kinase)-Iinked receptors


  • They are polypeptide receptors consisting of an extracellular ligand-binding domain (to which insulin or growth hormone bind).
• Enzyme (Tyrosine kinase)-Iinked receptors

  It is connected to cytoplasmic enzymatic domain containing tyrosine kinase enzyme. which when activated will phosphorylate and activates:

  1. signal transducer → non genomic actions→ seconds – minutes e.g: hypoglycemia of insulin.
  2. activators of transcription that will separate from the receptor to cross the nuclear membrane and modulate gene transcription. → genomic actions → hours e.g. anabolic effects of Insulin and growth hormone.
• Gene-active intracellular receptors (DNA linked receptors)


  • They are cytosolic or nuclear receptors which modulate the transcription of genes in the nucleus leading to protein synthesis.

  • e.g. Lipid soluble ligands (steroid hormones, thyroxin, vit. D) + Intracellular receptors → DNA transcription → mRNA → change in protein synthesis.

  • Delayed long-lasting effect ( hours between binding of the ligand and cellular response).
• The receptor has two main functions

  ligand binding and generating effector response by interacting with closely associated cellular proteins called signaling systems.