Pharmacology is the study of the manner in which the function of living systems is affected by chemical agents (Rang et al., 1999), i.e. it is concerned with the uses, effects and actions of drugs. For a drug to produce a pharmacological response, it would need to 'bind' to the active site (for example, particular constituents of cells or tissues). Most drugs will be bound to protein molecules, although some drugs exert their action by binding to DNA. Once bound, the drug would affect physiological function in a specific manner.

There are four main regulatory proteins that are drug targets:

Receptors: Receptors form the sensing elements in the system of chemical communications that co-ordinates the function of all the different cells in the body (with chemical messengers such as hormones or cytokines, for example). When a drug binds to a receptor it initiates a change in cell function and can cause an effect. If the action of the receptor is activated in this way, then the drug is known as an agonist. If the receptor action is inhibited, or the binding of the drug produces no action, then it is known as an antagonist.

Enzymes: Enzymes play a major role in the action of drugs. Some drugs are analogues of enzyme substrates. They act by competitively inhibiting the

Advanced Clinical Skills for GU Nurses. Edited by Matthew Grundy-Bowers and Jonathan Davies © 2007 John Wiley & Sons Ltd enzyme, and this action can be reversible or irreversible. For example, zidovudine is a thymidine analogue and competitively inhibits the reverse transcrip-tase enzyme. Other drugs may bind to the enzyme and stop or inhibit its normal function, thereby causing an effect. Some drugs, like valaciclovir, need to undergo metabolism by an enzyme to become active: these are called prodrugs. Sometimes drug toxicity can result, when an enzyme converts a drug molecule into a harmful metabolite.

Carrier molecules: These are responsible for the transport of ions and small organic molecules, such as glucose or amino acids, across cell membranes. The carrier proteins embody a recognition site that makes them specific for a particular permeating species, and these recognition sites can also be targets for drugs whose effect is to block the transport system.

Ion channels: Ion channels are proteins that are found within cell membranes. They control the influx and efflux of ions from the cell. Drugs can act directly or indirectly on ion channels. The simplest mechanism of action is by blocking the ion channel itself, although some drugs may bind to a part of the ion channel and thus modulate its effect. In this way the effect of the ion channel can be either facilitated or impaired.

Drug specificity: For a drug to be useful, it must show specificity for its target-binding site. Specificity is reciprocal: individual classes of drugs bind only to certain targets; individual targets recognise only certain classes of drugs. However, no drug acts with complete specificity. Generally, the lower the potency of a drug, the higher the doses needed to produce an effect, and the more likely it is to bind to active sites other than the one targeted. This may give rise to unwanted side-effects.

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