Antagonist competitive

H2-antagonists competitively interact with the H2-receptor. They are very specific for the H2-sub-type of the histamine receptor. 

Pharmacological - actions at the same site. This is the typical agonist-antagonist competition for the same receptor. Atropine blocks the binding of acetylcholine. Naloxone blocks the binding of morphine. 

Methotrexate, for example, a folic acid antagonist, competitively inhibits dihydrofolate reductase, preventing the synthesis of tetrahydrofolic acid (the coenzyme that is important in synthesis of amino and nucleic acids). This drug also provides a cogent illustration of the need to exploit every possible... 

Nonsteroidal estrogen receptor antagonist competitively binds to estrogen receptors on estrogen-dependent breast cells... 

Propantheline, a muscarinic cholinergic receptor antagonist, competitively blocks acetylcholine s actions at cholinergic neuroeffector sites, decreasing GI motility and inhibiting gastric acid secretion. 

Fig. 3.4 Schild analysis of agonist-antagonist competition. (A) Concentration-effect relationship for an agonist in the absence (curve 0) and the presence of three increasing concentrations of antagonist (curves 1, 2 and 3). (B) Schild plot generated from curve A (see text for explanations).

Antagonist An antagonist is a receptor ligand preventing the action of an agonist, in a direct (competitive) or indirect (allosteric) manner. 

For control of gastric acid secretion, the H2 antagonists have encountered competition from the potent -ATPase inhibitors such as... 

In contrast, interactions of competitive antagonists and receptors are relatively straightforward. It can be shown by comparing the equal responses provided by an agonist, [ ], alone or in the presence of a competitive antagonist, so that... 

The dissociation content for the competitive antagonist, iCg, can be deterrnined without knowing the relationship between receptor occupancy and response. Equation 17 is often written in logarithmic form ... 

Because iCg values for competitive antagonists represent tme dissociation constants, these make possible quantitative interpretations of SARs. Significant use also has been made of iCg values in the quantitative comparison of receptors to determine whether receptors that respond to the same agonists are identical or whether responses produced by different agonists are initiated at the same receptors (44,46). Thus, beta-adrenoceptors in human and guinea pig preparations can be direcdy compared and selective and antagonists quantitated (Table 3). 

Peripheral Antagonists. The relatively long duration of action of the thyroid hormones makes it desirable to have compounds capable of blocking them competitively at their site of action. This is desirable in the treatment of thyroid storm where the reduction of circulating hormone levels brought about by the inhibition of their synthesis is too slow. 

Potassium-Sparing Diuretics. Potassium-sparing diuretics act on the aldosterone-sensitive portion of cortical collecting tubules, and partially in the distal convoluted tubules of the nephron. The commonly used potassium-sparing diuretics are triamterene, amiloride, and spironolactone (Table 3). Spironolactone is a competitive aldosterone receptor antagonist, whereas triamterene and amiloride are not (44,45). 

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