Mechanism of Action of Hyoscine Butylbromide
Hyoscine butylbromide functions primarily as an antimuscarinic (anticholinergic) agent that blocks muscarinic receptors on gastrointestinal smooth muscle cells, preventing acetylcholine from binding and thereby producing smooth muscle relaxation and antispasmodic effects. 1, 2
Primary Mechanism: Muscarinic Receptor Antagonism
Hyoscine butylbromide has high affinity for muscarinic receptors (particularly M3 receptors) located on smooth muscle cells of the gastrointestinal tract, where it acts as a competitive antagonist blocking acetylcholine binding 2, 3
The drug demonstrates stronger inhibition through M3 receptor blockade compared to M2 receptor blockade, with M3 being the predominant receptor mediating its antispasmodic effects 3
This muscarinic blockade inhibits cholinergic contractions from both enteric motor neurons and potentially non-neuronal sources of acetylcholine in the colon 3
The antimuscarinic action produces smooth muscle relaxation (spasmolytic effect) by preventing acetylcholine-mediated muscle contraction 2, 4
Secondary Mechanism: Nicotinic Receptor Effects
At higher concentrations (10 μmol/L), hyoscine butylbromide also exhibits nicotinic receptor antagonism, producing a ganglion-blocking effect 2, 4
This nicotinic blockade plays only a moderate modulatory role compared to the dominant antimuscarinic action 4
The nicotinic antagonism can moderately decrease nerve activity, secretion, and motility at these higher concentrations 4
Site of Action and Pharmacokinetic Considerations
Hyoscine butylbromide is a quaternary ammonium compound with very poor systemic absorption (bioavailability <1%) and does not cross the blood-brain barrier 2
Despite minimal systemic availability, the drug maintains high tissue affinity for muscarinic receptors at the intestinal site of action, where it exerts its local spasmolytic effect 2
This localized action explains why it produces gastrointestinal effects without significant central nervous system anticholinergic side effects 2
Additional Pharmacological Effects
Hyoscine butylbromide inhibits epithelial secretion through muscarinic receptor blockade, representing an antisecretory action that may be clinically relevant for secretory disorders 4
The drug reduces bethanechol-induced action potential discharge in enteric neurons, contributing to its overall antispasmodic profile 4