Estimation of pKB values for histamine H2-receptor antagonists using an in vitro acid secretion assay

1 Histamine H2-receptor antagonism by burimamide, metiamide and cimetidine was analysed under apparent equilibrium conditions in the lumen-perfused isolated stomach preparation of the mouse. 2 The behaviour of these compounds was not incompatible with simple competitive antagonism but the estimated pKB values (-log KB) were all significantly lower, by about 1 log unit, than reference values reported for guinea-pig atrium or rat uterus. 3 There seems no need to propose that the parietal cell receptors are somehow different from the others; metiamide continually appears in the gastric juice so that a steady-state but not equilibrium can presumably be reached with respect to the bath concentration and this could keep the antagonist concentration artificially low in the region of the receptors.