Apriority nursing assessment for a client who is to receive an alpha- or beta- adrenergic blocking agent would be to:

assess heart rate

Explanation:

Adrenergic receptors are sensitive to certain neurotransmitters like epinephrine and norepinephrine which is responsible for fight and flight response in the body. There are two types of adrenergic receptors alpha and beta receptors.  

Alpha- or beta-adrenergic blocking agents are the drugs that are used to treat patients with high blood pressure. These drugs block the alpha and beta adrenergic receptors which results in decreasing the constriction of blood vessels and slowing heart rate.

So before giving alpha or beta adrenergic blocking agent the assessment of heart rate is the priority.