Pyridostigmine exerts its effects by competing with acetylcholine for its binding site on acetylcholinesterase. By interfering with acetylcholine enzymatic destruction, pyridostigmine potentiates the action of acetylcholine on both the skeletal muscle (nicotinic receptor) and the GI tract (muscarinic receptor).
Pyridostigmine also can stimulate cholinergic responses in the eyes (causing miosis) if directly applied. Different muscle groups exhibit different levels of response to anticholinesterase agents, and doses that produce stimulation of one muscle group can cause weakness, through overdose, in another.
Specific responses to cholinesterase inhibitors include: increased skeletal muscle tone (nicotinic); increased gastric motility and GI tone (muscarinic); bradycardia (muscarinic); ureteral constriction (muscarinic); stimulation of the sweat and salivary glands (muscarinic); and constriction of the bronchi (muscarinic). There is also some evidence that they have a direct action on skeletal muscle.