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  1. Affinity
    the tendency of a ligand to bind to its receptor.
  2. Agonist
    A ligand that binds to a receptor and alters the receptor state resulting in a biological response.
  3. Antagonist
    A drug that reduces the action of another drug, generally an agonist. Many antagonists act at thesame receptor macromolecule as the agonist.
  4. Chemical antagonism
    the antagonist combines in solution directly with the chemical being antagonised(e.g., chelating agents, used to treat lead poisoning, bind to heavy metals and form a less toxic chelate). Theantagonist action does not directly involve a receptor.
  5. Desensitisation
    spontaneous decline in the response to a continuous application of agonist, or to repeatedapplications or doses. A direct consequence of receptor activation.
  6. Dose ratio
    The factor by which the agonist concentration must be increased to restore a given response(e.g., EC50) in the presence of an antagonist.
  7. Drug
    A chemical that affects physiological function in a specific way.
  8. Drug potency-
    An expression of the activity of a drug, in terms of the concentration or amount needed toproduce a defined effect such as EC50Drug potency depends on both receptor (affinity, efficacy) and tissue (receptor numbers, drug accessibility)parameters.
  9. EC50
    the concentration that produces 50% of the maximal response (sometimes referred to as [A]50). Oftenused as a measure of agonist potency.
  10. Efficacy
    the tendency for an agonist to activate the receptor
  11. Inverse Agonist
    A ligand that by binding to receptors reduces the fraction of them in an active state. This can occur if some of the receptors are in the active form (R*), in the absence of a conventional agonist.
  12. Ligand
    A substance that is bound to a protein.From the Latin word ‘ligare’ meaning ‘to bind’.
  13. pA2
    The negative log of the molar concentration of an antagonist that makes it necessary to double the concentration of the agonist needed to elicit the original response obtained in the absence of antagonist. pA2 is a measure of potency for reversible competitive antagonists.
  14. Partial agonists
    An agonist that in a given tissue, under specified conditions, cannot elicit as large an effect (even when applied at high concentration with 100% receptor occupancy) as can another agonist acting through the same receptors in the same tissue.
  15. Pharmacokinetic antagonism
    The ‘antagonist’ reduces the concentration of the active drug at its site of action (e.g., phenobarbital increases hepatic metabolism of the anticoagulant drug warfarin). The antagonist action does not directly involve a receptor.
  16. Physiological antagonism
    Two agonists that produce opposing physiological actions and cancel each other out. Each drug acts through its own receptors (e.g., adrenaline relaxes bronchial smooth muscle reducing the bronchoconstriction of histamine). The antagonist action does not directly involve a receptor.
  17. Potency
    An expression of the activity of a drug, in terms of the concentration or amount needed to produce a definedeffect, such as EC50, pA2. Drug potency depends on both receptor (affinity, efficacy) and tissue(receptor numbers, drug accessibility) parameters.
  18. Receptor
    a term used in pharmacology to denote a class of cellular macromolecules that are concerned specifically and directly with chemical signalling.
  19. Receptor down regulation
    A gradual decrease in the number of receptors expressed on the cell surface caused from a prolonged exposure to an agonist.
  20. Tachyphylaxis
    a decline in the response to repeated applications or doses of agonist
  21. Tolerance
    A general term used to describe the gradual decrease in the response to a drug, usually taking days or weeks to develop
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