Cardiac and gastric effects of histamine H_2 receptor antagonists: no evidence for a correlation between lipophilicity and receptor affinity

摘要

1 A series of histamine H_2 receptor antagonists with different lipophilicity were tested in cardiac and gastric assays in order to reveal possible differences in receptor affinity. Lipophilicity of the compounds was expressed as CLOG P (theoretically-determined logarithm of octanol:water partition coefficient) and log κ' (logarithm of capacity factor, experimentally-determined by reverse-phase high performance liquid chromatography). 2 Aminopotentidine (APT) and iodoaminopotentidine (I-APT), which are both lipophilic compounds, behaved as insurmountable antagonists of histamine responses in rat isolated gastric fundus (pK_B = 6.20 ± 0.16 and 6.89 ± 0.19, respectively) and guinea-pig isolated papillary muscle (pK_B = 6.34 ± 0.37 and 6.81 ±0.26, respectively). They were approximately as effective as ranitidine (RAN) in reducing histamine-induced acid secretion in the anaesthetized rat, ID_(50) values being 0.018 ± 0.02, 0.020 ± 0.03 and 0.036 ± 0.01 μmol kg~(-1) i.v. for APT, I-APT and RAN, respectively. Both APT and I-APT had a significantly longer duration of action than RAN. 3 The hydrophilic compound, SK&F 92857, was inactive up to 10 μM in modifying histamine-induced acid secretion in the isolated rat stomach. In the papillary muscle, low concentrations (0.1-1 μM) of this compound produced a competitive antagonism of the histamine responses (pA_2 value = 7.38± 0.11), while a higher concentration (10 μM) significantly reduced the maximal response to histamine. 4 RAN competitively antagonized histamine effects with a comparable affinity in cardiac and gastric preparations (pA_2 values were 6.42 ± 0.09 and 6.78±0.38 in heart and stomach, respectively). 5 Results obtained in this study clearly showed that the discrepancies between gastric and cardiac effects observed for some H_2 antagonists are not explained solely by differences in lipophilicity of compounds. Moreover, the significant correlation found between CLOG P and log κ' parameter, which takes into account, besides their lipophilicity, the ionization of the molecules, suggests that ionization has a similar influence for all the molecules on the partition between the lipophilic and aqueous phase.