Veterinary Physiology-III

RECEPTOR PROPERTIES

• The properties of the receptors are
• Each cell may have receptors for more than one hormone and therefore is a target for several hormones
• Specificity spill over – hormones with similar structure may bind to one another’s receptors usually with lower affinity and ability to activate the receptors.
• Activation of receptor by a ligand is an all or none phenomena
• Message carried by the ligand is in its three dimensional structure which is important to bind to the receptors
• Once a hormone binds to its receptor the message is delivered and the hormone plays no further role
  Hormone receptor combination initiates a cascade of reactions
• All receptors are proteins
  Specificity: The binding site of the receptor is highly specific for each signaling molecule(hormone, neurotransmitter); molecules that bind specifically to receptor are called ligands.
  However, some molecules having structural similarity to ligands can also bind with receptors and produce a physiological response and such molecules are called as agonists. Some molecules having structural similarity with hormone may bind with receptors without producing a response and inhibit the hormone effect are called as antagonists.
  High affinity: Usually the concentration of the hormone in the blood is very low(in pmol/l to μmol/l). Because the receptors have high affinity to hormones, binding occurs rapidly
  Binding capacity: Number of receptors available on the target cells at any given time is the limiting factor that determines number of hormone molecules bound to target organs; i.e. the binding capacity of target cells is decided by the receptor numbers.
  The concentration of receptors is quite variable
  When a target cell is exposed to high level of hormone for a period of time,the number of active receptors will be reduced (by receptor degradation) known as down regulation.
  During deficiency of a hormone, there is an increase in the number of active receptors known as up regulation.
  When a receptor becomes bound to a hormone, the receptor undergoes a conformational change (known as receptor activation), which allows it to interact productively with other components of the cells, leading ultimately to an alteration in the physiologic state of the cell.