drug receptors

 drug receptors

receptor is target macromolecule present on the cell surface or intracellularly where the drug binds to.

After attachment to a receptor site, a drug may either initiate a response or prevent a response from occurring.

An agonist is a drug which produces a stimulation type response. The agonist is a very close mimic and "fits" with the receptor site and is thus able to initiate a response.

An antagonist drug interacts with the receptor site and blocks or depresses the normal response for that receptor because it only partially fits the receptor site and can not produce an effect.

Antagonists can be competitive or noncompetitive.

• A competitive antagonist competes with the agonist for receptor sites. Because this type of antagonist binds reversibly to the receptor site, administering larger doses of an agonist can overcome the antagonist’s effects.
  
  A noncompetitive antagonist
• binds to receptor sites and blocks the effects of the agonist. Administering larger doses of the agonist can’t reverse the antagonist’s action.
  
  

  -If a drug acts on a variety of receptors, it’s said to be non-selective and can cause multiple and widespread effects. In addition, some receptors are classified further by their specific effects. For example, beta receptors typically produce increased heart rate and bronchial relaxation as well as other systemic effects.

  Beta receptors, however, can be further divided into beta₁ receptors (which act primarily on the heart) and beta₂ receptors (which act primarily on smooth muscles and gland cells).

  -  Desensitization of receptors
  Repeated or continuous administration of an agonist (or an antagonist) may lead to changes in the responsiveness of  the receptor. To prevent potential damage to the cell (for example, high concentrations of calcium, initiating cell
  death), several mechanisms have evolved to protect a cell from excessive stimulation. When repeated
  administration of a drug results in a diminished effect, the phenomenon is called tachyphylaxis. 

  The receptor becomes desensitized to the action of the drug . In this phenomenon, the receptors are still present on the cell surface but are unresponsive to the ligand.

  - Dose Response Relationships 

  -The magnitude of the drug effect depends on the drug concentration at the receptor site.
  

   -Drug potency refers to the relative amount of a drug required to produce a desired response. Drug potency is also used to compare two drugs. If drug X produces the same response as drug Y but at a lower dose, then drug X is more potent than drug Y.

   -efficacy

   This is the ability of a drug to illicit a physiologic response when it interacts
  with a receptor.
  

  - a dose-response curve is used to graphically represent the relationship between the dose of a drug and the response it produces.

  

  
  

  Maximum effect

  On the dose-response curve, a low dose usually corresponds to a low response. At a low dose, a dosage increase produces only a slight increase in response. With further dosage increases, the drug response rises markedly. After a certain point, however, an increase in dose yields little or no increase in response. At this point, the drug is said to have reached maximum effectiveness.

  - therapeutic index

  it is The relationship between a drug’s desired therapeutic effects and its adverse effects . It’s also referred to its margin of safety.

  The therapeutic index usually measures the difference between:

  • an effective dose for 50% of the patients treated
  • the minimal dose at which adverse reactions occur.