PHARMACOLOGY

 TARGETS OF DRUG ACTION ::

A drug may act at different sites It may act at the point of application e.g. corticosteroids Some times drugs act during its transport e.g. urea as diuretic or probenecid as blocker of uric acid reabsorption in kidney A drug may act by reflex action e.g. turpentine oil or by reaching a definite concentration in particular organ e.g. digitalis 

    In most instances a drug acts by binding to a protein in a mammalian cell These proteins can be divided into four : 

(a) Receptors (b) ion channels (c) enzymes (d) carrier molecules 

DRUG RECEPTOR INTERACTIONS ::

The term receptor was first coined by Langly in 1878 to define the sites to which the drug combines and produces a response Thus a receptor can be any clearly defined target molecule with which a drug molecule combines to produce the effect in this way sodium channels can be called as receptors for local anaesthetics Similar is the case with Na+ K+ ATPase for digitalis however the conventional receptors (e.g.nicotinic muscarinic etc ) can be differentiated from Na+ K+ ATPase etc by the fact that these receptors do nothing unless a complex is formed with a drug (exogenous substance ) or endogenous substance like neurotransmitters or hormones The functions of Na+ channels or Na+ K+ ATPase do not require any complex formation 

            Receptors were defined as hypothetical specific cellular components with which drug combines chemically to evolve the biological response A large number of drugs produce the effects by interacting with the receptors The receptors are now no longer hypothetical entities With the advancement of radiolabelling techniques it has now been possible to characterize the receptors as the macromolecular structure They can now be counted as well in a particular tissue The receptor is now defined as protein molecule which is capable of binding selectively a drug hormone or neurotransmitter thereby eliciting a physiological response