Studying Competitive Antagonism on Isolated Rabbit Jejunum

1.

Studying Competitive
Antagonism on Isolated Rabbit
Jejunum

2.

Principle:
Atropine is a competitive antagonist
for Ach on muscarinic receptors (M) of
the isolated rabbit jejunum.
Higher concentrations of Ach are
needed to displace atropine from M
receptors and eliciting a response.

3.

Isolated Tissue Preparations

4.

Assignment:
The figure on the next page depicts the response of isolated rabbit
jejunum to different concentrations of Ach (1-33 mM) upon
pre-treatment with a fixed concentration of atropine (1 mg/mL) prior to
each Ach concentration.
Calculate the ED50 of Ach under this condition, and comment on the
differences between Ach ED50 values in the absence/presence of
atropine.

5.

Assignment.

6.

Assignment.

7.

Assignment
Ach conc (µM)
Response (cm)
1
0
3
0
6
0
9
0
12
0.3
15
18
21
24
27
30
33

8.

Dose response curve of acetylcholine in presence of atropine on isolated rabbit
jejunum
Contraction (cm)
3,5
3
3
2,5
3
2,3
2
2
1,7
1,5
1
0,5
0,3
0
0
0
0
0
5
0,4
0,5
Ach conc µM
0
10
15
20
25
30
35

9.

Dose response curve of acetylcholine in presence of atropine on isolated rabbit
jejunum
Contraction (cm)
3,5
E max
3
3
3
2,5
2
2
1,5
1
0,5
0,5
0
0
0
0
0
5
Ach conc µM
0
10
15
20
ED50
25
30
35

10.

Comment
2
22
dose

11.

Atropine is a reversible competitive muscarinic receptor antagonist
causing parallel rightward shift of dose response curve of acetylcholine
Atropine decreases potency of acetylcholine
and not affect efficacy.
ED50 Ach in presence of atropine is greater than that of Ach alone