General Description
Investigations and demonstrations of the pharmacological properties
of drugs which affect the function of skeletal muscle and the motor
nervous system can be performed on several animal preparations.
Rat phrenic nerve - diaphragm, frog rectus abdominis and chick biventor
cervicis are commonly used in vitro and various preparations of
the gastrocnemius, soleus or anterior tibialis muscles (stimulated
via the sciatic nerve or one of its branches) are use in vivo. Recordings
of resting tension and of twitch tension (in response to single
shocks or to a brief tetanus) can be made from a spinal, or, where
the law allows, from an anaesthetised animal (usually a dog, cat
or rat). This simulation is of the anterior tibialis muscle-sciatic
nerve preparation and may be used to fulfil some of the teaching
objectives normally achieved by use of several of the above neuromuscular
preparations in the laboratory.
The program consists of a brief introduction which sets out in
diagrammatic form the major mechanisms through which drugs produce
effects on the neuromuscular junction. This section is intended
only as an aide-memoire for students. It is not intended as a complete
instruction on the pharmacology of the neuromuscular junction.
The major part of the program consists of a simulation of the anterior
tibialis - sciatic nerve preparation. Various drugs can be administered
mostly intravenously but some by close intra-arterial injection.
The dose of drug used and the time sequence of administration is
entirely in the hands of the user. The responses produced by the
drugs themselves or their effect on the twitch response to 0.05Hz
or 0.5Hz electrical stimulation of the sciatic nerve are displayed
on the screen. Stimulation can be switched off or applied as a brief
tetanus (30Hz for 5 sec). It is possible to review the record produced
earlier in the experiment and to move backwards and forward in the
record of the experiment. Parts of the record may be printed. Local
printers work well but network printers can give problems in that
output can go astray or take hours to be printed. Use with a network
printer is not recommended.
A great variety of experiments can be performed either as set by
the tutor or as devised by students to answer questions set by the
tutor. These can be designed to be relatively straight-forward or
to be highly complex and demanding as appropriate to the particular
course. The program is intended to be used with specific tasks set
by the tutor and to this end a series of schedules has been included
(see later) which students can be asked to perform. Tutors can of
course design their own exercises or questions to suit particular
groups of students.
Drugs and procedures available.
Saline, in an appropriate volume, can be injected to act as a control
- an essential part of any well designed experiment. A small effect
on twitch tension may be produced especially if inappropriately
large volumes of saline are used. Electrical stimulation can be
applied to the sciatic nerve using supramaximal rectilinear shocks
of 0.05msec duration at a frequency of 0.05 or 0.5Hz. Only at the
latter frequency can be demonstrated the ability of triethylcholine
to produce blockade by depletion of acetylcholine stores. Alternatively,
the stimulus can be switched off completely (to investigate the
effects of close intra-arterial injections of acetylcholine or potassium
for example). Tetanic stimulation (30Hz supramaximal stimulation
for 5 sec) can be applied to distinguish between a competitive (non-depolarising)
block (poorly-sustained tetanus results) and a depolarising blockade
(a smaller but well maintained tetanus). Note that tetanic stimulation
is affected by competitive (non- depolarising) agents at lower doses
than are required to affect the twitch response to single shocks.
A variety of drugs is available for injection and the dose required
for each to produce an overt but submaximal effect is shown in Table
1. These doses will be affected by the frequency of stimulation,
the presence of other drugs and the design of the experiment. Note
that large doses of muscarinic cholinoceptor stimulants will produce
cardiac arrest unless the animal has been premedicated with atropine.
After an effective dose of drug has been given the effects wear
off with an appropriate half-life. Since each administration of
drug is accompanied by and indication of the time from the start
of the experiment comparisons of duration of action can be made.
A display of simulation time (about 20 times faster than real time
at 0.05Hz stimulation) is part of the screen display. Time can be
`advanced' by any chosen number of minutes so drugs are cleared
from the preparation (at a rate depending on their half-life) without
wasting hours of real time. The record will be marked appropriately
whenever this facility is selected. Preparations will last indefinitely;
anybody who has done a cat sciatic nerve - anterior tibialis preparation
knows the preparation frequently outlasts the lab worker. The unknown,
which is provided by the program for students to characterise, may
be a depolarising or a non-depolarising (competitive) neuromuscular
blocker. The half-life and potency are fixed at random by the computer.
At the end of the program run the nature of the unknown is revealed
together with the ratio of the binding affinity of the unknown to
that of a standard agent. Note that in vivo both relative potency
and half-life may depend on the particular design of the experiments
used to determine these parameters and it is interesting to ask
students to explain why the potency ratios they measure in the simulation
are different from those indicated by ligand binding measurements.
Once this information has been given a new unknown with different
characteristics will be chosen for the next program run.
|