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Chapter 8: General Pharmacology

An adult farm worker requires treatment for exposure to an insecticide containing parathion (cholinesterase inhibitor). To reverse the potentially dangerous side effects of parathion, a tertiary amine muscarinic antagonist such as atropine is recommended over a quaternary amine muscarinic antagonist such as glycopyrrolate. This choice of atropine is based on which pharmacokinetic or pharmacodynamic parameter?

(A) Atropine has a larger volume of distribution than glycopyrrolate.

(B) Atropine is a competitive muscarinic antagonist while glycopyrrolate is a noncompetitive muscarinic antagonist.

(C) Atropine has a shorter terminal half-life than glycopyrrolate.

(D) Atropine has a slower rate of clearance than glycopyrrolate.

(E) Atropine has decreased oral bioavailability compared to glycopyrrolate.

The answer is A. Atropine is a tertiary amine alkaloid while glycopyrrolate is a quaternary amine. Both are competitive antagonists of acetylcholine at muscarinic receptors. However, the structure of glycopyrrolate means that is it positively charged and unable to cross the blood-brain barrier as easily as atropine. This means atropine is more widely distributed than glycopyrrolate and can reverse both the peripheral and CNS effects of parathion.

Atropine has a longer half-life, an increased rate of clearance, and greater oral bioavailability than glycopyrrolate.

Ref: Katzung BG, Masters SB, Trevor AJ. Basic and Clinical Pharmacology. 12th ed. (LANGE Basic series). New York, NY: McGraw Hill; 2012.

An oral dose of morphine is subject to metabolism in the liver. After hepatic extraction, approximately 33% of the morphine enters the systemic circulation unchanged. Which of the following terms best describes this fraction of drug that reaches the systemic circulation?

(A) volume of distribution

(B) first-pass elimination

(C) extraction ratio

(D) clearance

(E) bioavailability

The answer is E. Bioavailability is the portion of drug that reaches the systemic circulation unchanged.

Volume of distribution (Vd) is the ratio of drug in the body to concentration in plasma or blood.

First-pass elimination is the metabolism of drug before it reaches the systemic circulation. Extraction ratio is the clearance of drug from the liver divided by the hepatic blood flow. Clearance is the rate of drug elimination divided by the drug concentration.

Ref: Katzung BG, Masters SB, Trevor AJ. Basic and Clinical Pharmacology. 12th ed. (LANGE Basic series). New York, NY: McGraw Hill; 2012.

A patient is taking ...

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