PHARMACOLOGY

 Direct regulation of ionic permeability ::

This is the fastest type of receptor -mediated response that occurs when a neurotransmitter acts on the post -synaptic membrane of a nerve or muscle cell The stimulation of receptor transiently increases the permeability of particular ions such as Na+ K+ or Ca++ Accordingly various specific ion channels such as Na+K+Ca++Cl- etc have been identified and well defined The onset and the peak of action by this mechanism occurs within a fraction of millisecond and usually decays within a few milliseconds 

Regulation of the effect via an intracellular second messenger and /or G-Proteins ::

In many instances stimulation of a receptor by a drug hormone or transmitter causes an activation of G-protein which inturn activates some membrane enzyme (adenylate cyclase or phospholipase -C) These enzymes generate either cyclic adenosine monophosphate (c AMP) from AMP or hydrolyze another phospholipid component of the membrane (phosphatidyl-inositol 4,5-triphosphate (IP3) leading to the formation of diacylglycerol and inositol -1,4,5 triphosphate cAMP and diacylglycerol activate their respective specific protein kinases the enzymes that catalyze the transfer of phosphate group from ATP to particular protein molecule These phosphorylated proteins finally produce cellular responses such as contraction of a muscle secretion from gland etc The phosphate groups are continuously removed from these proteins through phosphateses 

    The inositol -1,4,5 triphosphate triggers the release of intracellular calcium The elevated cytoplasmic Ca++ promotes the binding of Ca++ to calcium -binding protein calmodulin which finally produces cellular response