Experimental Preparation and Setup

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Figure 1.1 – Response of the isolated guinea-pig ileum to the addition of Carbachol and comparing the response of the control curve and the time curve at various periods.

With the addition of Carbachol (CCh) to the guinea-pig ileum, both the control curve and the time curve showed an increased in twitch responses (g) as tabulated in Table 1.1. Figure 1.1 shows that as there is an increase in the concentration of CCh added, the twitch response of the guinea-pig ileum also increases. However, the maximum response of the two curves does not further increase past approximately 2.7g and 3.3g for the control curve and the time curve respectively. From Figure 1.1, it is determined that the time curve has an EC50 of 1.5x10-7M and the control curve has an EC50 of 2.5x10-7M. This indicates that time curve has a higher potency than the control curve.

Discussion

A response in the guinea-pig ileum is able to be produced due to Carbachol binding to the muscarinic receptor of the muscles. Carbachol is classified as an agonist of the muscarinic receptors because once it is bound to the muscles, it produces a response. The maximum response reached with the addition of Carbachol is 3.33g indicating the point where all muscarinic receptors are occupied. Any further addition of greater Carbachol concentrations will not induce a twitch response greater than 3.33g.

From the class data, it is shown that there is great variation in the maximum responses produced by different samples of guinea-pig ileum tissues. This variation could be attributed to a variety of factors such as rough handling of the tissue sample. During the preparation of the guinea-pig ileum sample, each sample was handled by different groups and some of which may have been damaged during set up. These different groups would also use different techniques in handling the sample. Furthermore, each tissue sample would contain a wide range of receptor densities in which the number of receptors present within each tissue sample obtained would be different. In response to the addition of a drug (Carbachol), the maximum response produced would be proportional to the number of receptors present in that tissue. As a result, tissue samples with a lower muscarinic receptor density would have less receptors for Carbachol to bind to. Hence, this may also account for the lower maximum

responses seen in some tissue samples.

From Figure 1.1, it is seen that the addition of Carbachol has increased the potency of the time curve compared to the control curve. From this, it is able to be concluded that with increasing time, the effectiveness of Carbachol to produce a response on the guinea-pig ileum increases. This increases in potency could be a result of many factors such as less experimental errors as knowledge was gained from setting up the first curve. Hence, less mistakes would have occurred the second time round for the time curve. Also, the tissue may have increased its sensitivity to the addition of drugs and as a result, adding Carbachol to the tissue the second time would elicit a greater response.

When investigating the effects of an antagonist in conjunction with Carbachol, the shift in potency with time can be involved. When plotted on a CR curve, the responses produced by the guinea-pig ileum to Carbachol and to the antagonist of interest would look very similar to the CR curve produced above in Figure 1.1. That is, a right shift parallel to the agonist curve would be seen due to decreased potency as an effect of the antagonist. Analysing the effects of an antagonist would therefore be difficult, as there would be no way of knowing whether the shift in potency was due to the antagonist effect, or due to the variability of time taken during the experiment.