The action of drugs on the human body (or any other organism's body) is called

pharmacodynamics Pharmacodynamics (PD) is the study of the biochemical and physiologic effects of drugs (especially pharmaceutical drugs). The effects can include those manifested within animals (including humans), microorganisms, or combinations of organisms (fo ...

, and the body's response to drugs is called

pharmacokinetics Pharmacokinetics (from Ancient Greek ''pharmakon'' "drug" and ''kinetikos'' "moving, putting in motion"; see chemical kinetics), sometimes abbreviated as PK, is a branch of pharmacology dedicated to determining the fate of substances administered ...

. The drugs that enter an individual tend to stimulate certain

receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...

ion channels Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of io ...

, act on

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

transport proteins A transport protein (variously referred to as a transmembrane pump, transporter, escort protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that serves the function of moving other materials within ...

. As a result, they cause the human body to react in a specific way. Based on drug action on receptors, there are 2 different types of drugs: *

Agonist An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the ago ...

s – they stimulate and activate the receptors *

Antagonist An antagonist is a character in a story who is presented as the chief foe of the protagonist. Etymology The English word antagonist comes from the Greek ἀνταγωνιστής – ''antagonistēs'', "opponent, competitor, villain, enemy, riv ...

s – they disable the agonists from stimulating the receptors Once the receptors are activated, they either trigger a particular response directly on the body, or they trigger the release of

hormones A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...

and/or other endogenous drugs in the body to stimulate a particular response.

Short note on receptors

Drugs interact with receptors by bonding at specific binding sites. Most receptors are made up of proteins, and the drugs can therefore interact with the amino acids to change the conformation of the receptor proteins. These interactions are very basic, just like that of other chemical bondings:

Ionic bonds

Mainly occur through attractions between opposite charges; for example, between protonated amino (on

salbutamol Salbutamol, also known as albuterol and sold under the brand name Ventolin among others, is a medication that opens up the medium and large airways in the lungs. It is a short-acting β2 adrenergic receptor agonist which works by causing rel ...

) or quaternary ammonium (e.g.

acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...

), and the dissociated

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

group. Similarly, the dissociated carboxylic acid group on the drug can bind with

amino In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...

groups on the receptor. This type of bond is very strong, and varies with the inverse of the distance between the atoms so that it can act over large distances. Cation-π interactions can also be classified as ionic bonding. This type of interaction occurs when a cation, e.g. acetylcholine, interacts with the negative

π bonds In chemistry, pi bonds (π bonds) are covalent bond, covalent chemical chemical bond, bonds, in each of which two lobes of an atomic orbital, orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap oc ...

on an aromatic group of the receptor. Ion-dipole and

dipole-dipole An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction or repulsion which act between atoms and other types of neighbouring particles, e.g. a ...

bonds have similar interactions, but are more complicated and are weaker than ionic bonds.

Hydrogen bonds

There is a small but significant attraction between

hydrogen atoms A hydrogen atom is an atom of the chemical element hydrogen. The Electric charge, electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic ...

and polar functional groups (e.g. the

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

OHgroup). These so-called hydrogen bonds only act over short distances, and are dependent on the correct alignment between functional groups. Receptors are located on all cells in the body. The same receptor can be located on different organs, and even on different types of tissues. There are also different subtypes of receptor which elicit different effects in response to the same agonist. For example, there are two types of

histamine receptor The histamine receptors are a class of G protein–coupled receptors which bind histamine as their primary endogenous ligand. There are four known histamine receptors: * H1 receptor * H2 receptor * H3 receptor * H4 receptor Comparison There ...

: H1 and H2. Activation of the H1 subtype receptor causes contraction of smooth muscle, whereas activation of the H2 receptor stimulates gastric secretion. It is this phenomenon that gives rise to drug specificity. Of course, drugs do not only act on receptors: they also act on

, and cell transporter proteins. These hydrogen bonds are so powerful than the

ionic bonds Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. ...

How shape of drug molecules affect drug action

When talking about the shape of molecules,

biochemists Biochemists are scientists who are trained in biochemistry. They study chemical processes and chemical transformations in living organisms. Biochemists study DNA, proteins and Cell (biology), cell parts. The word "biochemist" is a portmanteau of ...

are mainly concerned with the three-dimensional conformation of drug molecules. There are many

isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. ...

of a particular drug, and each one will have its own effects. Differences in isomer affect not only what the drug activates, but also changes the

potency Potency may refer to: * Potency (pharmacology), a measure of the activity of a drug in a biological system * Virility * Cell potency, a measure of the differentiation potential of stem cells * In homeopathic dilutions, potency is a measure of how ...

of each drug.

Potency

Potency is a measure of how much a drug is required in order to produce a particular effect. Therefore, only a small dosage of a high potency drug is required to induce a large response. The other terms used to measure the ability of a drug to trigger a response are: *Intrinsic Activity which defines: **Agonists as having Intrinsic Activity = 1 **Antagonists as having Intrinsic Activity = 0 **and, Partial Agonist as having Intrinsic Activity between 0 and 1 *Intrinsic Efficacy also measures the different activated state of receptors, and the ability for a drug to cause maximum response without having to bind to all the receptors.

The specificity of drugs

Drug companies invest significant effort in designing drugs that interact specifically with particular receptors , since non-specific drugs can cause more side effects. An example is the

endogenous Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, es ...

drug acetylcholine (ACh). ACh is used by the

parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...

to activate muscarinic receptors and by the neuromuscular system to activate nicotinic receptors. However, the compounds muscarine and nicotine can each preferentially interact one of the two receptor types, allowing them to activate only one of the two systems where ACh itself would activate both.

Affinity

The specificity of drugs cannot be talked about without mentioning the affinity of the drugs. The affinity is a measure of how tightly a drug binds to the receptor. If the drug does not bind well, then the action of the drug will be shorter and the chance of binding will also be less. This can be measured numerically by using the dissociation constant K_D. The value of K_D is the same as the concentration of drug when 50% of receptors are occupied. The equation can be expressed as K_D =

\frac

But the value of K_D is also affected by the conformation, bonding and size of the drug and the receptor. The higher the K_D the lower the affinity of the drug.

References

{{reflist

External links

Drug Action
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Merck Manual ''The Merck Manual of Diagnosis and Therapy'', referred to as ''The Merck Manual'', is the world's best-selling medical textbook, and the oldest continuously published English language medical textbook. First published in 1899, the current print e ...

Home Health Handbook
Pharmacology. Physiologic Action Of Drugs
Pharmacodynamics Pharmacokinetics