What is agonist: Drug which activates Receptor.
What is antagonist: Drug which blocks the Receptor
Agonist drugs for guinea pig ileum: Acetylcholine & Histamine
Antagonist drugs for guinea pig ileum:
Atropine (Antagonise Acetylcholine)
Mepyramine (Antagonise Histamine)


Atropine at a specific concentration almost totally blocks the response to acetylcholine but has no effect on the histamine-induced contraction.

Mepyramine at a specific concentration almost totally blocks the response to exogenously applied histamine but has no effect on the acetylcholine-induced contraction. Please note that at higher concentrations mepyramine does also antagonise muscarinic cholinoceptors.

Note that if the same dose of either of the agonists is repeated, the size of the contraction varies slightly from experiment to experiment.
Procedure:

1. Arrange organ bath and rotating drum assembly as per standard protocol

2. Dissect guinea pig & Isolate guinea pig ileum as per standard protocol

3. Mount the ileum in organ bath assembly

4. Apply agonist (Acetylcholine & Histamine) and record the response.

5. Determine a 'standard dose' (one which produces approximately 75% of the maximum response) for acetylcholine and histamine. Select a low concentration to begin with and increase the dose in a geometric manner.

6. Apply antagonist Atropine (Antagonise Acetylcholine) & Mepyramine (Antagonise Histamine) and record the response.

7. Test the action of the antagonists atropine and mepyramine against exogenously applied acetylcholine and histamine. The concentrations of ACh and histamine are pre-selected as those which produce a sub-maximal (75%) contraction as determined in 'Part 1' (i.e. acetylcholine 8x10-8 M; histamine 8x10-7 M). In each experiment antagonists are added to the bathing fluid and left for 5 minutes before the agonist is added.