PHARMACOLOGY - GUINEA PIG ILEM

What is the general overview of the experiment?

In the guinea pig ileum experiment, the potency and efficacy of cholinergic agonists are assessed using a bioassay, which involves estimating the concentration of a substance by measuring its effect on a living system. Bioassays are essential because they allow the drugs to be compared based on their functional effects. The guinea pig ileum is used as the isolated tissue and is placed in an organ bath where conditions such as temperature, time and duration are kept constant. Cholinergic agonists (acetylcholine, carbachol and methacholine in our case) were applied to the tissue and resulted in smooth muscle contraction which was measured.

Describe the preparation of the tissue, equilibrium stage and finding the baseline

A segment of the guinea pig ileum is carefully excised and mounted in an organ bath. The tissue was allowed to equilibrate for a fixed period, during which it is washed at regular intervals to stabilise its baseline tone. A resting tension is applied and the initial baseline contraction is recorded. A transducer is attached to the lever and converts movements of the lever into an electrical signal which is amplified and displayed on chart recorder.

Adding the cholinergic agonists

Increasing concentrations of the cholinergic agonists are then added to the organ bath and the extent of smooth muscle contraction is measured. Cholinergic drugs bind to muscarinic receptors and stimulate contraction.

Plotting the curves

The necessary data is then collected to construct and compare the dose-response curves. For each drug, the logarithm of the concentration is plotted on the x-axis against the percentage maximal response. The resulting sigmoidal dose-response curve is fitted through the data points.

EC50 values and efficacy

The EC50 is the drugs concentration that produces 50% maximal response and is read directly from the curve. A low EC50 value indicates the drug is more potent, as smaller amounts are required to elicit the same effect.

The efficacy is measured by plotting the dose response curves and estimating what the maximal response was, with 100% being the ideal efficacy.