suspect the presence of a hormone in 1890, aithough Sir "Ernie" (Starting) for the first time in 1905 actually established the presence öf seaetin.

RECEPTORS Chemical Signals like hormone molecules reach the tar-get cells where they become bound by receptors Receptors äre proten macromolecules At least, vast majority of cells of our body (if not all cells of out body) have receptors There are many types of receptors, each ceil posseses several types of receptors Most (but not all)drugs aiso act via receptors that is. the drug moiecule is bound by the specific receptor and subsequently the drug produces its effectsit may sound surpnsing becouse most drugs are toreign materials in our body Sometimes bactenai invasion occurs in aur body. these bxtena have many receptors (for example. receptors for anbbiotic drugs) which we do not have Antibotics therefore can combne with the bxtena but not with human (host's) clls Thus anbbiotics can kill the bactena but not the host cells In short receptors (henceforth, abbreviated äs R) can combne with endogenous Chemical Signals äs well äs withdrugs A ligand ,is a substance which can combine with the receptor A ligand .thus may be a hormone or a drug molecule, R s are specific That is it a partcular type of R. destned combine with a specfac ligand , say A. it not combine with any other type of ligand (henceforth abbrrvieated, L) Agonist Thus each type of R is specific tof the corresponding type of L For example, R for adrenalin wil combine with adrtealin and following the combination, the posr receptor events -will occur and effect . say gtycogenolysis in hepatocyte occufs Adrenalin is thus said to be the agonist that receptor Antagonists, aiso called blockers are motecuies which have a close (chemical) rsembiance with the gonist motecule Therpfore, the antagonista, molecule can and do occupy the receptor sits but rhere is some structural drfference also so that post receptor events d not occur ano because of the fact that the rtceptor sitare are now occupied,the agonist moleculest cannot occupy the R sites for example, the acbion of adrenaln (the agonist) is bocked by oropranolot, therefore, propranoloi is a (ß) blocker adrenttn antagomst Srte (i)Receptors which bind the peptided hormonnes are presend on the cell membrane, that is ,they partiy protude in the ECF ard partly remain