Transcript Effect & Sites of Action of Different Agonist Drugs on The Isolated

Effect & Sites of Action of Different
Agonist Drugs on The Isolated Rabbit
Intestine
Lab # 1
Purpose of Pharmacological Experiments
Qualitative
To determine the activity & mechanism of drugs action e.g.
analgesic, effect on blood pressure
Quantitative
To assay (measure) the activity of drugs e.g. toxic level
Types of pharmacological experiments
In vivo
In vitro
-
Study the action of drugs on
the whole body (intact animal)
-
Study the action of drugs on
pieces of tissue
-
There is interference by
compensatory mechanisms of
body
Easier to study effect of drugs
-
There is no interference by
compensatory mechanisms of
body
Easier to study MOA & potency
of drugs
-
-
In vitro experiments
- In vitro experiments involve the use of isolated organ or muscle
- Conditions of the experiment should be kept similar to body
Experiment conditions:
1- Electrolytes
2- Nutrients
3- Physiological pH
4- Constant temperature
5- Aeration
Electrolytes
Physiological solution:
Ex. Tyrode’s & Kreb’s
Ingredients:
1- Nacl → adjust the tonicity
2- Ca+2, K+, Mg+2 → for normal contraction
3- Sodium bicarbonate → to render the pH similar to that of blood
4- NaH2PO4 → to compensate for any change in the pH
5- Glucose → nutrient
Aeration
Examples
-
Pure oxygen → for heart tissues
Air → for intestine
Carbogen (95% O2, 5% CO2) → used for uterus
Temperature
Temperature should be kept constant during the experiment
Physiological temp. (37°C) is needed for mammalian tissues e.g.
rabbit intestine
The temp. should be reduced in some experiments to decrease
spontaneous contraction e.g. isolated guinea-pig ileum
Amphibian tissues can survive in room temp. e.g. isolated frog
rectus abdominis
Intestine
Intestine is composed of duodenum, jejunum & ileum
It is a smooth muscle & has myogenic contraction
The intestine is involuntary structure which undergoes the control of
nervous system
Autonomic nervous system innervations of the
intestine
Sympathetic
CNS AG
e
PGF
PostGF
Key:
AG= autonomic ganglion
E= effector organ
PGF= preganglionic fiber
POSTGF= postganglionic fiber
Parasympathetic
CNS
AG e
PGF
PostGF
Autonomic nervous system innervations of the
intestine
Sympathetic
Transmitter:
Receptors
(intestine):
Action
(intestine):
NA, Adrenaline
Parasympathetic
Ach
adrenergic
a1, b2
cholinergic
a-central-Nicotinic (Nn)
b-periphral-muscarinic(M3)
inhibitory (↓cont.)
excitatory (↑cont.)
Drugs Acting on Intestine
1- STIMULANTS
They are called Spasmogenic
2- DEPRESSANTS
They are called Spasmolytic
Stimulants
1- Autonomic Cholinergic stimulant
A- Central nicotinic neuronal (Nn) R agonist (ganglionic stimulant)
- Act on Nn R present in autonomic ganglia
- Ex. Small dose of nicotine ( dil. Nicotine)
- MOA of dil.nicotine:
Dil. Nicotine stimulates Gs protein in cell membrane
→ activates Na channels → rapid ↑ intracellular Na
→ depolarization and AP → contraction
Stimulants
B- Muscarinic receptors agonist (cholinomimetic)
- Act on muscarinic M3 receptors
- Ex.: 1- Ach & its derivatives e.g. methachloine, bethanecol, carbacol
2- Naturally occurring alkaloids e.g. Pilocarpine
- MOA of Ach:
Ach ++ M3R → ++ Phospholipase C
→ Splitting of phosphatidylinositol biphosphate (PIP2) into:
1- DAG (diacylglycerol) → ↑opening of Calcium channels
2- IP3 (inositol triphosphate) → mobilization of cal. from its stores
1, 2 lead to ↑ intracellular calcium → contraction
Stimulants
2- Direct acting stimulant
e.g. Bacl2
- Acts directly on muscle fibers
Depressants
1- Autonomic adrenergic stimulants (sympathomimetic)
- e.g. adrenaline & noradrenaline
- They bind to a1, b2 receptors causing relaxation
2- Direct depressants
e.g. Mgcl2 & papaverine
Experiment
N
0.1
0.1
Dil.nic. Ach
0.1
Bacl2
0.1
adr.
0.1
Mgcl2
Definitions
Dose cycle: time between different drug additions
Contact time: time from addition of drug until its removal by
washing
↑ amplitude
↓ amplitude
↑ tone
↓ tone
↑ tone & amplitude
↓ tone & amplitude