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Screening of neuromuscular blocking

agents

By Arijit Chakraborty

M- Pharm (Pharmacology)

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The neuromuscular blocking agents are used primarily in

conjugation with general anesthetics to provide muscle relaxation for surgery, while centrally acting muscle relaxants

are used mainly for painful muscle spasms and spastic neurological

condition.

Neuromuscular blocking agents can be two types: a) Depolarization agents. b) Nondepolarizing agents.

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Classification: A.Depolarizing agents: Succinylcholine,

Decamethonium.B. Nondepolarizing agents: 1. Long acting: d- tubocuranin 2. Intermediate acting:

Vecuronium 3. Short acting: Mivacurium

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Models:• Intact Unanaesthetised Animals: 1. Production of contractures in

birds 2. Testing of righting reflex 3. Inclined screen and inverted

grid methods 4. Rotating drum methods 5. Head drop methods 6. Injection into lymph sac of frog

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Models:

• Anesthetized Animals: 1. Nerve muscle preparations of

lower limbs 2. Phrenic nerve- diaphragm

preparations 3. Close intra- arterial injection 4. Determination of dose

causing cardiac arrest

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Models:• In vitro studies: 1. Isolated nerve- muscle preparations 2. Frog nerve- muscle preparations 3. Phrenic nerve- diaphragm

preparations 4. Lumbrical nerve- muscle

preparation in rabbit 5. Ionophoretic micro application

technique 6. Electrical activity of single muscle

fiber

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Intact Unanaesthetised Animals

Production of contractures in birds:

1. Administration of depolarization blockers in adult fowls, chicks causes a rigid extension of

the limbs and retraction of the head.

2. Curare is using for flaccid paralysis of bird, advantage is this test on avian muscle is the

ease with which the difference in the action of these two groups of drugs can be strikingly

illustrated.

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Intact Unanaesthetised Animals

Testing of righting reflex:

1. Rat and mice is use for this experiment to determine the potency of

the neuromuscular blocking agent.

2. Then drug is injected in IP route.

3. Animal is turning on its back and watching to see if the animal rolls back

over onto its sternum.

4. The rat is on its back when its head is tilted, its limbs move opposite

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Intact Unanaesthetised Animals

Inclined screen and inverted grid methods:

1. Mice or rats can also be tested for their ability to stay on an inverted grid after

administration of neuromuscular blocking agents.

2. This method is used to estimate ED50 .

3. Dose given subcutaneously to groups of 10 mice each.

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Intact Unanaesthetised Animals

Rotating drum methods:

1. Mice is use for this experiment.

2. Mice are place in the rotating drum and noted the observation time.

3. Drug should be given by subcutaneous route.

4. Place in the rotating drum and noted the falling time.

5. TO calculate the falling time and determine the Percentage of reduction.

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Intact Unanaesthetised Animals

Head drop method:

1. It is applicable for mouse, rat, guinea pig, rabbit, dog, monkey.

2. Dose given by intravenous inj. at the rate of 1 ml/ 5 seconds to groups of ten

rabbits each.3. Then the animals are kept in the box where the head is placed outside in the

box.4. Then observed for the occurrence or

absence of head drop due to muscle relaxation.

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Anaesthetized AnimalsPhrenic nerve-diaphragm

preparation:1. The effect of neuromuscular blocking agents on

isolated respiratory muscles may be studied in Phrenic nerve- diaphragm preparation from rats and

guinea pig.A parallel-sided slip of diaphragm is removed with the

Phrenic nerve.

2. Stimuli are applied alternately to the muscle directly, via electrodes at each end, and to the

Phrenic nerve.

3. Rat diaphragm preparation is relatively insensitive to Decamethonium and similar agent.

4. Diaphragm from other animals must be from very young animals or from fetuses so that they enough

for oxygen diffusion.

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In-vitro StudiesElectrical activity of single muscle

fiber:

1.Frog Sartorius muscle is used.

2. The nerve cut close to its point of entry into the muscle.

3. The muscle is mounted in a bath of ringer solution on an illuminated stage and observed

from above with a microscope.4. The bathe is divided in two groups and the muscle is drawn through a gap of partition.

5. Stimulating electrodes are placed in the 2 compartment.

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The recording equipments consists of a DC amplifier and oscilloscope

A large amount of information can be obtained in a relatively short time since the process of mounting and recording from a muscle fibers takes only a few seconds.

The average values for a large number of fibers can be determined.

Electrical activity of single muscle fiber:

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In-vitro StudiesLumbrical nerve- muscle

preparation in rabbit:

1. The rabbit Lumbrical muscle is sensitive to depolarizing blockers.

2. The lever bearing and writing tip must be chosen for minimal friction.

3. The lever must be weight loaded to stretch the muscle optimally and ensure a linear relation

between the height of the recording and the work done on the lever by the muscle.

4. The preparation is very sensitive to changes in ionic concentration.

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In-vitro Studies

6. A fall in the concn of H2 , Ca, Mg especially, Ca induces spontaneous

activity.

7. Respectively response to stimulation and irregularity of behavior, while altering

the sensitivity to all types of blocking agents.

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