Antagonist X (competitive) has an affinity for RB but not Ra. Thus, it specifically antagonizes agonist B. It does not antagonize agonist A. Antagonist Y acts on a receptor associated with the cellular translocation of calcium and inhibits the increase in intracellular free calcium. It will therefore antagonize the effects of both A and B, since they both ultimately depend on calcium movement to cause contraction.
The effect of a noncompetitive antagonist on the dose-response curve for an agonist would be the same as the effect of a non-equilibrium-competitive antagonist (Fig. 2.6).The practical difference between a noncompet-itive antagonist and a nonequilibrium-competitive antagonist is specificity. The noncompetitive antagonist antagonizes agonists acting through more than one receptor system; the nonequilibrium-competitive antagonist antagonizes only agonists acting through one receptor system. The antihypertensive drug diazoxide is one of the few examples of therapeutically useful noncompetitive antagonists (see Chapter 20).
^ Study Questions
1. Receptors are macromolecules that
(A) Are designed to attract drugs
(B) Are resistant to antagonists
(C) Exist as targets for physiological neurotrans-mitters and hormones
(D) Are only on the outer surface of cells
(E) Are only inside of cells
2. All of the following are capable of initiating a signal transduction process EXCEPT
(A) Combination of an agonist with its receptor
(B) Combination of an antagonist with its receptor
(C) Combination of a neurotransmitter with its receptor
(D) Combination of a hormone with its receptor
3. Which of the following chemical bonds would create an irreversible combination of an antagonist with its receptor?
(A) Ionic bond
(B) Hydrogen bond
(C) Van der Waals bond
(D) Covalent bond
4. Potency is determined by
(A) Affinity alone
(B) Efficacy alone
(C) Affinity and efficacy
(D) Affinity and intrinsic activity
(E) Efficacy and intrinsic activity
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