Question

Which of the following effects is not observed with systemic administration of glycopyrrolate?

• A. Antisialagogue
• B. Tachycardia
• C. Ocular effects
• D. Smooth muscle relaxation

Hint:

Atropine is a tertiary amine and readily crosses the blood-brain and blood-ocular barriers, causing significant mydriasis and central nervous system signs.
Glycopyrrolate is a quaternary amine and cannot cross these barriers easily, resulting in minimal central and ocular side effects.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Glycopyrrolate is a synthetic quaternary ammonium anticholinergic (antimuscarinic) agent used in veterinary anesthesia as a preanesthetic medication.
Its primary role is to inhibit salivary and respiratory secretions (antisialagogue effect), prevent bradycardia, and reduce gastrointestinal motility.

Step 2: Detailed Explanation:
Let us analyze the pharmacological properties of glycopyrrolate to understand why certain systemic effects are observed or absent:
1. Antisialagogue Effect (A): Glycopyrrolate is a highly potent inhibitor of salivary, bronchial, and pharyngeal secretions.
It is more effective and longer-acting than atropine in this regard, meaning this effect is prominently observed.
2. Tachycardia (B): By antagonizing muscarinic receptors ($M_2$) in the sinoatrial node of the heart, glycopyrrolate increases the heart rate and prevents drug-induced bradycardia.
Although the heart rate elevation is more gradual and less severe than with atropine, mild to moderate tachycardia is still observed.
3. Smooth Muscle Relaxation (D): Antimuscarinic agents block the parasympathetic bronchoconstrictive and gastrointestinal motility pathways, leading to bronchodilation and decreased gastrointestinal smooth muscle tone.
Therefore, smooth muscle relaxation is observed.
4. Ocular Effects (C): Unlike atropine, which is a tertiary amine, glycopyrrolate is a highly polar quaternary ammonium compound.
This chemical structure prevents it from readily crossing biological membranes, such as the blood-brain barrier and the blood-ocular barrier, when administered systemically.
Consequently, systemic administration of glycopyrrolate does not produce significant ocular effects (like pupillary dilation/mydriasis or cycloplegia), unlike atropine.

Step 3: Final Answer:
Ocular effects are not observed with the systemic administration of glycopyrrolate.