Which anticholinergic reduces equine colonic peristalsis to facilitate rectal examination?

• A. Glycopyrrolate
• B. Pralidoxime
• C. Detomidine
• D. Propantheline

Answer: (D)

Blockade of muscarinic receptors in the gut reduces parasympathetic stimulation of smooth muscle, lowering colonic tone and peristalsis. In horses, slowing or dampening colonic movements helps relax the bowel and allows a smoother, safer rectal examination because the colon is less distended and more easily palpated.

Propantheline is a muscarinic antagonist with strong antispasmodic effects on the gastrointestinal tract and acts predominantly peripherally, producing the desired reduction in colonic motility with minimal central effects. This makes it a practical choice when the goal is to facilitate rectal examination by quieting the colon.

Glycopyrrolate is also an anticholinergic with peripheral action and can reduce GI motility, but propantheline is the classic agent specifically noted for reducing equine colonic peristalsis to aid rectal exams. Pralidoxime is an acetylcholinesterase reactivator, not an anticholinergic. Detomidine is an alpha-2 agonist sedative that can alter GI motility via central mechanisms, not by muscarinic blockade.