McKenna's Pharmacology, 2e

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P A R T 1  Introduction to nursing pharmacology

passive diffusion: movement of substances across a semipermeable membrane with the concentration gradient; this process does not require energy pharmacodynamics: the science that deals with the interactions between the chemical components of living systems and the foreign

chemicals, including drugs, that enter living organisms; the way a drug affects a body pharmacogenomics: the study of genetically determined variations in the response to drugs

pharmacokinetics: the way the body deals with a drug, including absorption, distribution, biotransformation and excretion placebo effect: documented effect of the mind on drug therapy; if a person perceives that a drug will be effective, the drug is much more likely to actually be effective receptor sites: specific areas on cell membranes that react with certain chemicals to cause an effect within the cell selective toxicity: property of a chemotherapeutic agent that affects only systems found in foreign cells without affecting healthy human cells (e.g. specific antibiotics can affect certain proteins or enzyme systems used by bacteria but not by human cells) therapeutic concentration: see effective concentration transmitter: an endogenous chemical usually released from nerve terminals to transfer nerve impulse from one neuron to the next or from a neuron to an effector cell such as muscle

T o understand what happens when a drug is admini­ stered, the health professional must understand pharmacodynamics —how the drug affects the body— and pharmacokinetics —how the body acts on the drug. These processes form the basis for the guidelines that have been established regarding drug adminis­ tration—for example, why certain agents are given intramuscularly (IM) and not intravenously (IV), why some drugs are taken with food and others are not, and the standard dose that should be used to achieve the desired effect. Knowing the basic principles of phar­ macodynamics and pharmacokinetics helps the nurse or midwife to anticipate therapeutic and adverse drug effects and to intervene in ways that ensure the most effective drug regimen for the person. Chemicals control many physiological processes in the body. Drugs are chemicals used for their thera­ peutic benefits. These chemicals are endogenous substances and are vital to the physiological functioning of the body. For example, acetylcholine, a neurotrans­ mitter is necessary to contract the muscle as well as for neuronal functioning. Drugs mimic these endogenous substances. Many drugs are therefore designed to have similar chemical groups to those of naturally occurring chemicals. So for drugs to work, interaction with bodily process is necessary. PHARMACODYNAMICS Pharmacodynamics is the science dealing with inter­ actions between the chemical components of living systems and the foreign chemicals, including drugs, which enter those systems. All living organisms function by a series of complicated, continual chemical reac­ tions. When a new chemical enters the system, multiple changes in, and interferences with, cell functioning may occur. To avoid adverse effects, drug development works to provide the most effective and least toxic chemicals for therapeutic use.

Drugs usually work in one of four ways: 1. To replace or act as substitutes for missing substances, such as transmitters. 2. To increase or stimulate certain cellular activities. 3. To depress or slow cellular activities. 4. To interfere with the functioning of foreign cells, such as invading microorganisms or neoplasms. (Such drugs are called chemotherapeutic agents .) Drug molecules do not confer any new functions on a tissue or organ in the body; they modify existing physiological, biochemical or biophysical functions. They can act in several different ways to achieve these results. They can combine with a small molecule (e.g. antacids neutralise gastric acid) or produce an altera­ tion of cell membrane activity (e.g. local anaesthetics). Many drugs act by binding to a protein target, which is called the molecular target or site of action. Four kinds of regulatory proteins are commonly involved as primary drug targets, because they mediate the actions of hormones, neurotransmitters and autocoids. These regulatory proteins are: • Enzymes • Carrier molecules • Ion channels • Receptors Drug–enzyme interactions Enzymes are proteins and biological catalysts, which speed up the rate of chemical reactions. Drugs also can cause effects by interfering with the enzyme systems that act as catalysts for various chemical reactions. Enzyme systems work in a cascade fashion, with one enzyme acti­ vating another and then that enzyme activating another, until a cellular reaction eventually occurs. If a single step in one of the many enzyme systems is blocked, normal cell function is disrupted. Acetazolamide ( Diamox ) is a diuretic that blocks the enzyme carbonic anhydrase, which subsequently causes alterations in the hydrogen ion and water exchange system in the kidney, as well as in the eye.