Screening and bioassay of drugs · Are drugs which mimic parasympathetic nerve actions. They are...
Transcript of Screening and bioassay of drugs · Are drugs which mimic parasympathetic nerve actions. They are...
Screening and bioassay
of drugs
by
Dr. Magdy M. Awny
Lecture 5
1
Are drugs which mimic parasympathetic nerve actions.
They are classified into:
1-Directly acting: activate M also N receptors 2-Indirect acting (Anticholinesterase):
A-Choline ester:
Acetylcholine & Carbacol.
B-Cholinomimetic alkaloids:
pilocarpine.
A-Reversible:
neostigmine, physostigmine (Eserine)
B-irreversible:
organophosphorus compounds
Screening & Bioassay of
Parasympathomimetics
3
Screening of Parasympathomimetics
1- Primary general test:
Injection of parasympathomimetic to mice causes &hypotension
Red skin & tears,
Urination,
Miosis,
Bradycardia,
Lacrimation,
Salivation
2- Cardiovascular test: (hypotensive action)
-Small dose Ach → brief ↓ in BP (due to M action on heart, BV)
that is blocked by atropine
-If Ach given in large dose after atropine (a fully atropinized animal
i.e. M receptors are blocked) →↑BP not fall in BP,
This Ach reversal due to the nicotinic action of Ach, include:
1- Stimulation of Adrenal medulla (release of E, NE →↑BP)
2-Stimulation of sympathetic ganglia (release of NE from the nerve end →↑BP)
5
3- Miotic action:
parasympathomimetics cause miosis through stimulation of circular muscle.
4- Sialagogue action:
A dog is anesthetized, Wharton's duct is cannulated and the cannula is connected
to a drop counter.
Muscarinic agonists cause a copious flow of salivary secretion, which is potency
and dose dependant.
This effect is blocked by prior administration of atropine.
I- Methods based on Nicotinic action II- Methods based on Muscarinic action
1-The dorsal muscle of leech (hirudo medicinalis)
SK .muscle preparation
Principle:
Ach cause contraction of the muscle, which is
proportional to the dose.
Procedures:
- muscle is removed, suspended in ringer
solution at room temp.
-Dose response curve is carried out and the
relative potency is calculated using matching
technique
- Physostigmine (Eserine) is added to increase
the sensitivity of the muscle
2-Frog rectus abdominis (SK.m preparation)
Principle: Ach cause contraction of the muscle,
Which is blocked by curare (NMB)
Procedures: as above
1-Isolated rabbit intestine
Principle:
Ach stimulate M receptor of rabbit intestine;
→ contraction which is proportional to dose
Procedures:
piece of intestine is suspended in tyrode
solution at 37 C , matching technique is done
2-Isolated guinea pig ileum:
As intestine but more sensitive
3-Anesthsized cat BP:
- Cat is anesthetized with chloralose
- I.V. Inj of Ach →transient brief fall in BP
(due to hydrolysis of Ach by cholinesterase)
As a result of vasodilatation & bradycardia
- apply the matching technique
Bioassay of Parasympathomimetics
1-1ry
general test [neuropharmacological tests]
Anticholinesterase differ from other parasympathomimetics in
They have M, N actions due to accumulation of endogenous Ach.
They produce gradual effect
They have longer duration of action
More stimulant action of skeletal muscle so produce more muscle fasciculation
[twitch of a group of muscle fibers] & fibrillation [twitch of individual muscle fibers]
8
2-cardiovascular test:
When anticholinesterase injected to anesthetized cat they produce
Prolonged fall in BP
Potentiate the effect of Ach & Vagal stimulation in tetrad system
Potentiate the hypertensive effect of Ach when injected to atropinized Cat
3-cat nictitating membrane:
Anticholinesterase potentiate response of the membrane to preganglionic ES
as they protect Ach in autonomic ganglia from destruction by cholinesterase
Screening & Bioassay of
Parasympatholytics
10
1-1 ry general test [neuropharmacological tests]
Muscarinic antagonists (Atropine-like agents) →
Mydriasis, cycloplegia, ↑IOP
Bronchodilatation, tachycardia,
↓ Secretion (Lacrimation, salvation& sweating)
↓ Gut motility
Antispasmodic action
Screening of Parasympatholytics
2- Cat blood pressure:
Atropine like drugs will
Block the effect of both injected ACh and vagal stimulation on blood pressure and
heart rate in the tetrad system.
Reverse the hypotensive action of ACh when it is given in large dose after atropine ??
Bioassay of Parasympatholytics1-Mydriatic action [mouse pupil method]
Principle:
Atropine causes passive mydriasis with loss of light reflex & cycloplegia
i.e. ↑ pupil diameter which is proportional to dose
Procedures:
-The pupil diameter is measured before administration of the drug then
measured At 10, 20, 30 min after drug administration
-The mean increase in the pupil diameter for S & T is determined and
relative potency is calculated
2-Isolated organ preparation 50% 50%
• Principle: Ach atrop Ach atrop
Atropine blocks the Ach-induced contraction of rabbit intestine or G. p. ileum
which is proportional to dose
• Assay is indirect [i.e. detect dose of atropine that cause 50% reduction in response
to Ach] & done by using matching technique
A- Chorda tympani induced salivation B- Pilocarpine induced salivation
Rabbit is anesthetized, Wharton’s duct is
cannulated & chorda tympani nerve is
exposed, up on ES →↑salivary secretion,
the volume of secretion collected in 1 hr is
recorded
Atropine is injected & the nerve is ES again
↓volume of salivary secretion is the
response to the given dose of atropine
The dose of atropine that causes 50% ↓ in
salivation is determined & compared with a
standard
S.C pilocarpine use to induce salivation
Atropine given 70 min before pilocarpine
, Dose of atropine causing 50% reduction
in volume of saliva is calculated
(indirect assay)
3-Anti-sialogogue action:
Principle: Atropine ↓ volume of the salivary secretion induced by pilocarpine
or ES of chorda tympani nerve supplying salivary gland.