Correlation between log POCT/H2O and pKB estimates for a series of muscarinic and histamine H2-receptor antagonists.

1. With histamine used as agonist, pKB values were estimated for seventeen histamine H2-receptor antagonists on assays involving acid secretion by the mouse isolated stomach and contraction frequency of the guinea-pig right atrium. 2. With the exception of oxmetidine, SK&F 94,826 and SK&F 94,206 on the right atrium assay, the compounds behaved as simple competitive antagonists on both assays. Although the former three compounds produced concentration-dependent, parallel, displacement of the histamine concentration-effect curves, subsequent analysis indicated Schild plot slope parameters significantly less than unity. However, the application of a combined dose-ratio analysis indicated that their antagonistic behaviour did not differ from expectations for simple competition at dose-ratios of approximately 20, and pKB values were estimated on this basis. 3. In accordance with previously reported data, pKB values were found to be consistently lower on the stomach than atrial assays. The pKB value for tiotidine was underestimated to the same extent on the stomach assay when impromidine was used as agonist. 4. The removal of the serosal muscle from the mouse stomach, achieved by using an isolated, perfused, mucosal sheet preparation, did not significantly affect the underestimation of the pKB value for metiamide. 5. Linear regressional analysis indicated a significant, positive, correlation between lipophilicity (log POCT/H2O) of the antagonists and the degree of antagonist pKB value underestimation on the gastric secretion assay.