3. Competitive antagonism a. Equilibrium competitive b. Nonequilibrium competitive

4. Noncompetitive antagonism

Chemical Antagonism

Chemical antagonism involves a direct chemical interaction between the agonist and antagonist in such a way as to render the agonist pharmacologically inactive. A good example is the use of chelating agents to assist in the biological inactivation and removal from the body of toxic metals. Chelation involves a particular type of two-pronged attachment of the antagonist to a metal (the agonist). One chemical chelator, dimercaprol, is used in the treatment of toxicity from mercury, arsenic, and gold. After complexing with the dimercaprol, mercury is biologically inactive and the complex is excreted in the urine.

Functional Antagonism

Functional antagonism is a term used to represent the interaction of two agonists that act independently of each other but happen to cause opposite effects. Thus, indirectly, each tends to cancel out or reduce the effect of the other. A classic example is acetylcholine and epi-nephrine. These agonists have opposite effects on several body functions. Acetylcholine slows the heart, and epinephrine accelerates it. Acetylcholine stimulates intestinal movement, and epinephrine inhibits it. Acetylcholine constricts the pupil, and epinephrine dilates it; and so on.

Competitive Antagonism

Competitive antagonism is the most frequently encountered type of drug antagonism in clinical practice. The antagonist combines with the same site on the receptor as does the agonist, but unlike the agonist, does not induce a response; that is, the antagonist has little or no efficacy. The antagonist competes with the agonist for its binding site on the receptor. Competitive antagonists can fall into either of two subtypes, depending on the type of bond formed between the antagonist and the receptor. If the bond is a loose one, the antagonism is called equilibrium competitive or reversibly competitive. If the bond is covalent, however, the combination of the antagonist with the receptor is not readily reversible, and the antagonism is termed nonequilibrium competitive or irreversibly competitive.

If the antagonism is of the equilibrium type, the antagonism increases as the concentration of the antagonist increases. Conversely, the antagonism can be overcome (surmounted) if the concentration of the agonist in the biophase (the region of the receptors) is in-

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