A large number of p-blockers are on the market in the United States. Of these, propranolol, a nonselective p-antagonist, was the first to be introduced and is the prototypical drug with which the others are compared. Metoprolol was the first p1-selective drug and timolol the first p-blocker approved for ophthalmic use.
As a class, p-blocking agents have greater structural similarity to their corresponding agonists than do the ablockers. This structural similarity also accounts for the greater specificity of action exhibited by the p-receptor blocking drugs than by the a-adrenoceptor blocking drugs.
The similarity in structure to p-agonists is most certainly responsible for the finding that some p-blockers activate p-receptors; that is, they have some intrinsic sympathomimetic activity. The intrinsic activity of these compounds is generally modest in comparison with an agonist, such as isoproterenol, and they are generally referred to as partial agonists (see Chapter 2).
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