Aim: Bioassay of Acetylcholine Using Rat Ileum or Colon by Four-Point Bioassay
References
1. Ghosh, M.N. (2008). Fundamentals of Experimental Pharmacology (6th ed.). Hilton & Company.
2. Rang, H.P., Dale, M.M., Ritter, J.M., & Flower, R.J. (2015). Rang & Dale’s Pharmacology (8th ed.). Churchill Livingstone.
3. Schild, H.O. (1947). The bioassay of acetylcholine on the rat ileum. British Journal of Pharmacology, 2(3), 189-195.
Introduction
Acetylcholine is a neurotransmitter that causes smooth muscle contraction. The rat ileum or colon is commonly used in bioassays to measure the potency of acetylcholine. The four-point bioassay method involves comparing the responses of the unknown sample to four standard concentrations of acetylcholine to determine its potency.
Objective
To determine the concentration of acetylcholine in an unknown sample using the rat ileum/colon by the four-point bioassay method.
Materials and Equipment
Male rats (to isolate the ileum/colon)
Dissection tools (scissors, forceps, etc.)
Physiological saline (Tyrode’s solution or Krebs solution)
Acetylcholine standard solutions (known concentrations)
Unknown acetylcholine solution
Tissue bath setup
Aeration system (Oxygen supply)
Isometric transducer or force transducer
Recording device (kymograph or digital recorder)
Micropipettes and tips
Data analysis software (optional)
Procedure
1. Preparation of Rat Ileum/Colon:
- Euthanize the rat using an appropriate method (e.g., CO2 inhalation or anesthetic overdose).
- Quickly dissect the abdomen to isolate the ileum or colon.
- Cut a suitable length of the ileum or colon and mount it in a tissue bath filled with Tyrode’s solution or Krebs solution, maintained at 37°C and continuously aerated with oxygen.
2. Equilibration:
- Allow the ileum/colon strip to equilibrate in the tissue bath for about 30 minutes, with constant aeration.
- Apply a resting tension of 1 gram to the tissue.
3. Baseline Recording: Record the baseline muscle tension to ensure stability and viability of the preparation.
4. Standard Acetylcholine Response:
- Prepare four standard acetylcholine solutions of known concentrations (e.g., 0.5, 1.0, 2.0, 4.0 µg/mL).
- Add the lowest concentration of acetylcholine to the tissue bath and allow the ileum/colon to contract.
- Record the maximum contraction.
- Wash the ileum/colon with fresh Tyrode’s solution or Krebs solution and allow it to return to baseline tension.
- Repeat the process for each standard concentration and plot the dose-response curve.
5. Testing Unknown Acetylcholine Solution:
- Add the unknown acetylcholine solution to the tissue bath and allow the ileum/colon to contract.
- Record the maximum contraction.
6. Four-Point Bioassay Method:
- Compare the contraction response of the unknown sample to the four standard acetylcholine dose-response points.
- Use the interpolation method to determine the concentration of acetylcholine in the unknown sample by fitting it within the range of the four points.
Data Analysis
1. Plotting Standard Dose-Response Curve: Plot the maximum contraction (response) on the y-axis against the concentration of acetylcholine on the x-axis for the four standard solutions.
2. Determining Unknown Concentration:
- Locate the response produced by the unknown acetylcholine solution on the y-axis.
- Interpolate the response between the closest two standard points to determine the concentration of acetylcholine in the unknown sample.
Sample Data Table
| Acetylcholine Concentration (µg/mL) | Maximum Contraction (g) |
| 0.5 | 0.4 |
| 1.0 | 0.8 |
| 2.0 | 1.5 |
| 4.0 | 2.8 |
| Unknown | 1.2 |
Sample Plot
To visualize the data, plot the dose-response curve using the sample data provided. The unknown concentration can be identified by interpolating the response between the four points.
Here’s the plot for the four-point bioassay of acetylcholine using the rat ileum/colon. The standard acetylcholine solutions’ dose-response curve is shown in blue, with the responses at 0.5, 1.0, 2.0, and 4.0 µg/mL. The red dashed line represents the maximum contraction (1.2 g) for the unknown acetylcholine solution. The interpolated concentration of acetylcholine in the unknown sample is approximately 1.75 µg/mL, as indicated by the red point on the graph.