Question textYou are testing the hypothesis that your unknown is an agonist at the muscarinic receptor. The results of your experiment are shown below. What is the MOST LIKELY purpose of histamine in this experiment? To demonstrate whether: Multiple choice 1 Question 2atropine is acting selectively at the chosen concentration.atropine is effective at the chosen concentration.the unknown agonist is acting selectively.the tissue’s reactivity has changed during the equilibration period with atropine. Based on the results shown above, which one of the following conclusions can you MOST CONFIDENTLY make? Multiple choice 2 Question 2The unknown agonist might be acting non-selectively.The unknown agonist is inducing this response via the muscarinic receptor alone.Atropine might be acting non-selectively at the chosen concentration.Histamine might be acting non-selectively.

Question

BlackTom AI · Accepted Answer

Histamine is used as a control agonist at a NON‑muscarinic receptor to assess whether the tissue’s responsiveness changed during the atropine equilibration.

1) Relevant concepts
- Atropine is a muscarinic receptor antagonist; it should block responses mediated by muscarinic receptors but not responses mediated by other receptors (e.g., histamine receptors).  
- Including a non‑muscarinic agonist (histamine) tests two possibilities: (A) atropine is selectively blocking muscarinic responses, or (B) something about the tissue (viability, contractile machinery) changed during atropine incubation so all responses are reduced.

2) Step‑by‑step reasoning using the plotted dose–response curves
- Compare histamine control (open purple) with histamine + atropine (filled purple). The histamine response is substantially reduced in the presence of atropine.  
- If atropine were purely selective and the tissue remained healthy, histamine responses should be unchanged by atropine. Therefore the observed reduction shows either atropine is affecting non‑muscarinic pathways (non‑selective action) or the tissue lost reactivity during the atropine equilibration.  
- The purpose of including histamine is therefore to distinguish those two explanations — i.e., to demonstrate whether the tissue’s reactivity changed during the equilibration period with atropine.

3) Why the correct answers fit the data
- Purpose: the histamine control is intended to reveal whether tissue reactivity changed during atropine incubation. If histamine were unchanged, one would conclude the tissue remained reactive and any block of the unknown is receptor selective. Because histamine changed, that diagnostic purpose is fulfilled.  
- Conclusion: given that atropine reduced histamine responses, the most confident interpretation is that atropine might be acting non‑selectively at the chosen concentration (it’s inhibiting a non‑muscarinic agonist response).

4) Why other choices are incorrect
- “Atropine is acting selectively” is contradicted by blockade of histamine responses.  
- “Unknown agonist is acting selectively” cannot be confidently stated because responses persist in atropine and histamine data indicate possible atropine non‑selectivity.  
- “Tissue reactivity changed” is what histamine tests for (first answer), but the direct experimental conclusion favored by the data is atropine’s non‑selective effect.

Conclusion: purpose = the tissue’s reactivity has changed during the equilibration period with atropine; most confident conclusion = Atropine might be acting non‑selectively at the chosen concentration.

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