Station 6 — Acetylcholine Dose–Response & Ceiling Effect
🎯 Task
You are provided with an isolated rabbit ileum preparation and a tracing obtained after administration of increasing doses of acetylcholine. Describe the correct experimental approach, interpret the recorded responses and construct or interpret the corresponding dose–response graph.
⚡ Model Station Answer
Technique: I would record contractions of the isolated rabbit ileum after progressively increasing doses of acetylcholine under constant experimental conditions. Response: Increasing doses produce progressively greater contractions until a maximum response is reached. Ceiling effect: A further increase in dose produces no appreciable increase in contraction; this plateau represents the ceiling or maximal effect. Graph: Dose or log dose is plotted on the X-axis and response, usually percentage of maximum response, on the Y-axis. The log dose–response curve is typically sigmoid.
🔎 Stepwise Procedure / Approach
- Mount the isolated rabbit ileum in an organ bath containing Tyrode solution and maintain appropriate aeration and physiological temperature.
- Connect the tissue to the recording system and allow it to equilibrate until a stable baseline is obtained.
- Administer an initial low dose of acetylcholine and record the height of the contraction.
- Wash the tissue appropriately and allow it to return to baseline before the next response when using a non-cumulative recording method.
- Give progressively increasing doses of acetylcholine while keeping the experimental conditions and recording cycle constant.
- Observe the graded increase in contraction and record the response produced by each dose.
- Continue until increasing the dose produces no further meaningful increase in contraction. Confirm the plateau with a subsequent higher dose.
- Measure the recorded responses and take the maximum response as 100% when percentage response is required.
- Plot dose or log dose on the X-axis against response or percentage response on the Y-axis and identify the maximal plateau.
🖼️ Visual Learning
Rabbit Ileum Experiment, Tracing & Dose–Response Curve
🎥 Practical Video
Watch the practical demonstration of acetylcholine acting on isolated rabbit ileum. Focus on the experimental setup, addition of acetylcholine and the resulting ileal contraction relevant to interpreting the dose–response experiment.
📝 Important Viva Questions
Q1. What effect does acetylcholine produce on isolated rabbit ileum?
It produces contraction of intestinal smooth muscle, predominantly through activation of muscarinic receptors.
The magnitude of response increases progressively as the dose or concentration of the agonist is increased, until the maximum response is reached.
It is the maximum response of the tissue beyond which increasing the agonist dose produces no further meaningful increase in effect.
Successive higher doses produce contractions of essentially the same maximum height rather than progressively larger responses.
Dose or log dose is plotted on the X-axis, while the measured response or percentage of maximum response is plotted on the Y-axis.
It is typically sigmoid or S-shaped, with a lower portion, a relatively steep middle portion and an upper plateau.
Washing removes the previous drug from the organ bath and allows the tissue to recover toward baseline before the next dose is tested.