 Alpha-1 receptors are usually located on target cells and have excitatory effects on them; consequently, blockage of alpha-1 receptors usually results in blocking some of the effects of norepinephrine.Drugs such as phentolamine that act on both types of receptors can produce a complex combination of both effects.In most cases when the term "alpha blocker" is used without qualification, it refers to a selective alpha-1 antagonist.