Question

Which of the following effects are least observed with systemic administration of glycopyrrolate?

• A. Antisialogogue
• B. Tachycardia
• C. Ocular effects
• D. Muscle relaxation

Hint:

Anticholinergic drugs block muscarinic receptors, not nicotinic receptors.
Therefore, they never cause skeletal muscle relaxation, which is the domain of neuromuscular blockers like atracurium or centrally acting muscle relaxants.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Glycopyrrolate is a synthetic quaternary ammonium anticholinergic agent used as a pre-anesthetic medication in veterinary medicine to block muscarinic receptors.

Step 2: Detailed Explanation:
Let us analyze the physiological effects of glycopyrrolate:
- Antisialogogue effect: Glycopyrrolate is a highly potent inhibitor of salivary and respiratory secretions, lasting longer and being more effective than atropine. (Commonly observed)
- Tachycardia: By blocking M2 muscarinic receptors on the sinoatrial node, glycopyrrolate increases the heart rate, although it causes less severe and smoother tachycardia compared to atropine. (Commonly observed)
- Ocular effects: Glycopyrrolate can cause mydriasis (pupillary dilation) and cycloplegia (loss of accommodation) by blocking pupillary constrictor muscles. However, because it is a polar quaternary amine, it does not cross the blood-brain barrier easily, and its ocular effects are less pronounced than those of atropine, but still occur peripherally. (Observed)
- Muscle relaxation: Skeletal muscle relaxation is mediated by nicotinic acetylcholine receptors ($N_m$) at the neuromuscular junction. Anticholinergics like glycopyrrolate act selectively on muscarinic receptors ($M_1$-$M_5$) of the autonomic nervous system and have no effect on nicotinic receptors. Therefore, skeletal muscle relaxation is not observed.

Step 3: Final Answer:
The effect least observed with the systemic administration of glycopyrrolate is muscle relaxation.