The biochemicals that initiate such change are also known as Second Messengers.
Antagonists also bind to receptors but they do not have the ability of agonists to initiate changes in cellular functions. Instead, their prime characteristics are that they tend to occupy receptor surfaces thereby blocking agonist bindings on the same surface. This diminishes or even totally inhibits agonist functions that initiate changes in cellular functions. That is why they are termed as antagonists or blockers. There are two kinds of antagonists: competitive and irreversible. Competitive antagonists bind reversibly with receptors and high dosage of agonists may displace them and reinstate agonist interaction. On the other hand, irreversible antagonists bind irreversibly with receptors and do not allow displacement with increasing dosage of agonists. (Piascik, M.T., 2005)
In the first half of the first set of experiments acetylcholine (Ach) was used in isolation in 8 varyingly increasing concentrations to separate segments of the guinea pig ileum tissue. The response times to the dosages were noted and recorded. Ach is a known agonist and binds with receptors in smooth muscle cells to cause contraction.