Pharmacodynamics

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  Pharmacodynamics is a branch of pharmacology concerned with the study of
biochemical and physiological effects of drugs  and the mechanisms by which they
produce such effects.
  Drug actions result from the dynamic interactions between drug molecules and
cellular components resulting in perturbation of the normal physiology. Any cellular
macromolecule may act as a drug  receptor.  This receptor may be a cell surface
receptor or an intracellular receptor  (ion channel,  enzyme, protein, microsome or
nuclear material).
  Certain drug receptors normally act as physiological receptors for endogenous
components, e.g. adrenergic receptors for adrenaline and noradrenaline. Drugs whose
responses mimic the response elicited by the endogenous component are termed
agonists. An agonist therefore is a drug having both affinity and intrinsic activity at
the receptor. For example, bethanechol is a cholinergic receptor agonist as its action
on the cholinergic receptor resembles that of the endogenous component
acetylcholine.
  Drugs that have affinity but lack intrinsic activity at the receptor site are termed
antagonists as they block the action of the endogenous component without having an
action themselves. These may be  competitive  or  noncompetitive.  Competitive
antagonists act by interfering with binding of the endogenous ligand to the receptor in
a reversible manner. For example, propranolol is a competitive blocker of β
adrenergic receptors. Alternatively, a noncompetitive antagonist acts by interacting
with the nonligand binding site of the receptor, e.g. through covalent modification, so
that the normal binding of the endogenous ligand to the receptor is irreversibly
inhibited. For example, the monoamine oxidase (MAO) inhibitor tranylcypromine
forms covalent adducts with MAO enzyme so that the enzyme is irreversibly
inhibited.
  Partial agonists act by binding to the receptor thus inhibiting  the endogenous
ligand from binding  to the receptor but they have some intrinsic activity.  For
example, nalorphine is a partial agonist for opiate receptor.
  Physiological antagonism  occurs when the drug acts independently at
different receptors but yielding opposite actions. For example, adrenaline acts as a
bronchodilator, by acting on adrenergic receptors in the bronchial smooth muscles,
that antagonizes the bronchoconstrictor action of acetylcholine that acts on
muscarinic receptors.
  Neutralizing  or  chemical antagonism  occurs when  a drug binds to another
drug directly thus inactivating each other. For example, digoxin binging antibodies
are used as antidotes for digoxin toxicity by forming inactive complexes with the
drug.  

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