What is the mechanism of action (MOA) of phenylpropanolamine?

Phenylpropanolamine primarily acts as an alpha agonist, which increases the tone of the urethral sphincter. This action is beneficial in the treatment of urinary incontinence, particularly in dogs. By stimulating alpha-adrenergic receptors in the smooth muscle of the urethral sphincter, phenylpropanolamine enhances the contraction of these muscles, helping to maintain continence and reduce involuntary urination.

The other mechanisms listed are not the primary actions of phenylpropanolamine. While some of the options touch upon the drug's influence on the adrenergic system, this specific action at the urethral sphincter and its direct effect on increasing sphincter tone is what classifies it as a treatment for conditions characterized by a lack of sphincter control. Understanding this mechanism is crucial for effectively treating urinary issues in veterinary practice.