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The classical competitive antagonism of phentolamine on smooth muscle preparations, investigated by two procedures

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Summary

1 In isolated smooth muscle tissues taken from rats, rabbits and guinea-pigs, all at 37.5 °C, the equilibrium dissociation constant (Kβ) of the competitive, reversible α-adrenoceptor antagonist phentolamine varied between 4 and 28 nm.

2 The concentration of the antagonist required to inhibit contractions to direct- or indirect-acting α-adrenenoceptor agonists by 50% (IC50) also varied between 5 and 30 nm.

3 From one tissue to another, the IC50/Kβ ratio of the blocker varied from 1 to 2.5, the values being close to those predicted by classical receptor theory based on the law of mass action.

4 At 27.5 °C, using phenylephrine as the spasmogen in rat aorta, the IC50/Kβ ratio for phentolamine was 3.1.

5 A significantly higher IC50 compared with Kβ for phentolamine indicates that the procedures for estimating affinity constants for a competitive antagonist are not equivalent.
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Keywords: IC50/Kβ ratios; drug-antagonism phentolamine; drug–receptor interactions; tyramine; various agonists; vascular muscles; α-adrenoceptor

Document Type: Research Article

Publication date: January 1, 2007

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