Bioassay of d-tubocurarine is an essential practical topic for B. Pharm students, focusing on pharmacodynamics, experimental design and standardization of neuromuscular-blocking agents. This introduction covers competitive nicotinic antagonism, commonly used isolated tissue preparations (rat phrenic nerve‑diaphragm, frog rectus abdominis, chick biventer), dose–response and inhibition curves, ED50/ID50 determination, Schild analysis and interpretation of reversibility by acetylcholinesterase inhibitors. Key laboratory variables—temperature, stimulation frequency, cumulative dosing and data plotting—are emphasized for reliable results. Also note d‑tubocurarine’s quaternary ammonium chemistry, poor CNS penetration, histamine release and cardiovascular effects relevant to bioassay safety and interpretation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary pharmacological mechanism of d‑tubocurarine at the neuromuscular junction?
- Irreversible inhibition of acetylcholinesterase
- Non‑competitive blockade of voltage‑gated sodium channels
- Competitive antagonism of nicotinic acetylcholine receptors
- Activation of muscarinic acetylcholine receptors
Correct Answer: Competitive antagonism of nicotinic acetylcholine receptors
Q2. Which isolated tissue preparation is commonly used for bioassay of d‑tubocurarine in undergraduate pharmacology labs?
- Guinea‑pig ileum
- Rat phrenic nerve‑diaphragm
- Isolated rabbit heart
- Mouse vas deferens
Correct Answer: Rat phrenic nerve‑diaphragm
Q3. In a neuromuscular bioassay, which primary endpoint is usually measured to quantify d‑tubocurarine effect?
- Change in resting membrane potential
- Twitch height (peak amplitude) of electrically evoked contractions
- Rate of spontaneous contractions
- Length of refractory period
Correct Answer: Twitch height (peak amplitude) of electrically evoked contractions
Q4. Chemically, d‑tubocurarine is best described as which of the following?
- A tertiary amine with high lipid solubility
- A quaternary ammonium compound
- An ester local anesthetic
- A benzodiazepine derivative
Correct Answer: A quaternary ammonium compound
Q5. Which drug class is used to reverse d‑tubocurarine‑induced neuromuscular blockade in bioassay antagonism studies?
- Acetylcholinesterase inhibitors (e.g., neostigmine)
- Sodium channel blockers (e.g., lidocaine)
- M2 muscarinic agonists
- Beta‑adrenergic agonists
Correct Answer: Acetylcholinesterase inhibitors (e.g., neostigmine)
Q6. In a concentration–response experiment, how does a competitive antagonist like d‑tubocurarine typically alter the agonist curve?
- Leftward shift with increased Emax
- Rightward parallel shift without change in Emax
- Decrease in slope and decrease in Emax
- Complete abolition of the curve at all concentrations
Correct Answer: Rightward parallel shift without change in Emax
Q7. Which analytical method is standard for quantifying antagonist potency and obtaining pA2 values in d‑tubocurarine bioassays?
- Scatchard plot analysis
- Schild plot analysis
- Lineweaver–Burk plot
- Henderson–Hasselbalch plot
Correct Answer: Schild plot analysis
Q8. How does lowering the organ bath temperature typically affect d‑tubocurarine bioassay results?
- Increases twitch amplitude and speeds onset of blockade
- No effect on drug kinetics or twitch responses
- Decreases twitch amplitude and slows onset and recovery kinetics
- Converts competitive antagonism into irreversible antagonism
Correct Answer: Decreases twitch amplitude and slows onset and recovery kinetics
Q9. A known adverse effect of d‑tubocurarine relevant in experiments is:
- Excessive CNS stimulation
- Histamine release causing hypotension and bronchospasm
- Profound cholinergic bradycardia via muscarinic agonism
- Severe hepatic necrosis
Correct Answer: Histamine release causing hypotension and bronchospasm
Q10. Which receptor subtype is primarily targeted by d‑tubocurarine at the motor end plate?
- Muscarinic M3 receptor
- Nicotinic neuronal (Nn) receptor only
- Nicotinic muscle (Nm) receptor at the neuromuscular junction
- GABA‑A receptor
Correct Answer: Nicotinic muscle (Nm) receptor at the neuromuscular junction
Q11. What describes the cumulative dosing method used in organ bath bioassays?
- Each tissue receives a single fixed dose, then is discarded
- Successive increasing concentrations of the drug are added to the same bath without washing between doses
- Different tissues each get a single concentration for comparison
- Drug concentrations are decreased stepwise during the experiment
Correct Answer: Successive increasing concentrations of the drug are added to the same bath without washing between doses
Q12. ED50 in a bioassay context means:
- The dose that produces maximal effect
- The dose that inhibits 50% of enzyme activity
- The dose that produces 50% of the maximal biological effect
- The dose lethal to 50% of tissues
Correct Answer: The dose that produces 50% of the maximal biological effect
Q13. The pA2 value of an antagonist is defined as:
- The negative log of antagonist dose that prevents any agonist effect
- The negative log of antagonist concentration that requires a two‑fold increase in agonist concentration to restore the original response
- The negative log of the agonist EC50 in presence of antagonist
- The ratio of antagonist concentration to agonist concentration at EC50
Correct Answer: The negative log of antagonist concentration that requires a two‑fold increase in agonist concentration to restore the original response
Q14. Which control agonist is typically used to generate a reference concentration–response curve when assaying d‑tubocurarine?
- Norepinephrine
- Acetylcholine
- Histamine
- Serotonin
Correct Answer: Acetylcholine
Q15. The antagonism produced by d‑tubocurarine is classified as which of the following?
- Irreversible non‑competitive antagonism
- Allosteric potentiation
- Reversible competitive antagonism
- Enzyme inhibition of acetylcholinesterase
Correct Answer: Reversible competitive antagonism
Q16. In the presence of a competitive antagonist like d‑tubocurarine, increasing the concentration of acetylcholine will:
- Fail to restore the response — antagonism is insurmountable
- Partially restore the response due to surmountable antagonism
- Convert the antagonist into an agonist
- Only increase side effects without affecting twitch
Correct Answer: Partially restore the response due to surmountable antagonism
Q17. In inhibition bioassays, ID50 refers to:
- The dose that inhibits 50% of twitch response
- The dose producing maximum stimulation
- The dose that kills 50% of muscle fibers
- The inhibitory constant for enzyme metabolism
Correct Answer: The dose that inhibits 50% of twitch response
Q18. Which electrical stimulation frequency is typically used to evoke single twitches in neuromuscular bioassays?
- 100 Hz
- 50 Hz
- 0.1 Hz
- 10 Hz
Correct Answer: 0.1 Hz
Q19. Which preparation allows assessment of both direct muscle responses and prejunctional modulation useful for d‑tubocurarine studies?
- Guinea‑pig ileum
- Isolated rat heart
- Chick biventer cervicis muscle
- Mouse isolated trachea
Correct Answer: Chick biventer cervicis muscle
Q20. Besides the neuromuscular junction, d‑tubocurarine also blocks which of the following, contributing to cardiovascular effects?
- Beta‑adrenergic receptors in myocardium
- Autonomic ganglionic nicotinic receptors
- Muscarinic receptors in the gut
- Voltage‑gated calcium channels in smooth muscle
Correct Answer: Autonomic ganglionic nicotinic receptors
Q21. What is an appropriate laboratory safety precaution when handling d‑tubocurarine in the pharmacology practical?
- Use bare hands since it is non‑toxic through skin
- Wear appropriate PPE (gloves, goggles) and avoid skin contact or inhalation
- Only refrigeration without PPE is sufficient
- Neutralize by mixing with strong acids on bench
Correct Answer: Wear appropriate PPE (gloves, goggles) and avoid skin contact or inhalation
Q22. When plotting a concentration–response curve for agonist in log concentration format, the x‑axis typically represents which of the following?
- Log of drug concentration
- Time in minutes
- pH of organ bath
- Temperature of preparation
Correct Answer: Log of drug concentration
Q23. A Schild plot slope equal to 1 indicates which characteristic of antagonism?
- Non‑competitive irreversible antagonism
- Competitive reversible antagonism with single receptor class
- Allosteric modulation with multiple binding sites
- Agonist efficacy enhancement
Correct Answer: Competitive reversible antagonism with single receptor class
Q24. To confirm tissue viability in an organ bath before starting the bioassay, which agent is commonly used to evoke a direct muscle contraction?
- Acetylcholinesterase inhibitor (e.g., physostigmine)
- Potassium chloride (KCl)
- D‑tubocurarine
- Atropine
Correct Answer: Potassium chloride (KCl)
Q25. Compared to depolarizing blockers like succinylcholine, d‑tubocurarine typically shows which onset and duration profile?
- Very rapid onset and ultra‑short duration
- Slow onset and prolonged duration of action
- Immediate onset and brief duration
- No onset because it is inactive in vitro
Correct Answer: Slow onset and prolonged duration of action
Q26. What is the principal route of elimination for d‑tubocurarine in the body relevant to pharmacokinetic considerations?
- Primarily renal excretion
- Exhalation via lungs
- Complete biliary excretion only
- Metabolism to active CNS metabolites
Correct Answer: Primarily renal excretion
Q27. Which parameter derived from a concentration–response curve best indicates the potency of an agonist?
- Maximum effect (Emax)
- EC50 (concentration producing 50% of Emax)
- Hill coefficient only
- Baseline spontaneous activity
Correct Answer: EC50 (concentration producing 50% of Emax)
Q28. In a classic competitive antagonism scenario with d‑tubocurarine, how is Emax affected?
- Emax is decreased to zero
- Emax is unchanged but requires higher agonist concentrations to achieve
- Emax is increased above control values
- Emax alternates unpredictably
Correct Answer: Emax is unchanged but requires higher agonist concentrations to achieve
Q29. Why is it important to include a standard reference drug and replicate runs when calibrating a d‑tubocurarine bioassay?
- To increase subjective interpretation
- To provide a reproducible reference dose–response curve and check experimental variability
- To eliminate the need for proper stimulation parameters
- To ensure irreversible tissue damage
Correct Answer: To provide a reproducible reference dose–response curve and check experimental variability
Q30. Regarding antagonist potency, a higher pA2 value indicates:
- Lower antagonist potency
- Greater antagonist potency
- No relationship to potency
- Only increased toxicity, not potency
Correct Answer: Greater antagonist potency

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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