Which of the following is an example of a physiological antagonist?

• A. Insulin use against glucocorticoid hormones
• B. Protamine sulfate for heparin
• C. Naloxone for opioid receptor
• D. Atropine for muscarinic acetylcholine receptor

Answer: (A)

Physiological antagonism occurs when two agents produce opposite effects in the same physiologic system by acting on different receptors or pathways, counteracting each other without one blocking the other’s receptor. Here, insulin lowers blood glucose by promoting uptake and storage, while glucocorticoids raise blood glucose by stimulating gluconeogenesis and reducing glucose utilization. Using insulin to oppose the hyperglycemic action of glucocorticoids is a classic example of physiological antagonism because the two act through separate mechanisms to produce opposite effects on glucose homeostasis.

The other options involve direct receptor blockade or chemical neutralization rather than opposing actions via separate pathways. Protamine sulfate neutralizes heparin by binding it directly (chemical antagonism). Naloxone blocks opioid receptors (pharmacologic receptor antagonism). Atropine blocks muscarinic receptors (pharmacologic receptor antagonism).