16
P A R T 1
Introduction to nursing pharmacology
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.
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
