Is [3H]-tiotidine a specific ligand for the H2-receptor?

The H2-antagonist tiotidine inhibited the H2-receptor-mediated, histamine-induced increase in cyclic AMP in dispersed mucosal cells from guinea pig stomach (Ki, 4 X 10(-8) M). The radiolabeled [3H]-tiotidine bound specifically and reversibly to the same cells with a half-maximal binding occurring at 5 X 10(-7) M tiotidine. The dissociation of bound [3H]-tiotidine from gastric cells and the Scatchard analysis of the binding binding data suggest the existence of additional binding sites for tiotidine. Eight other antagonists which inhibited the H2-receptor-mediated increase in cyclic AMP also inhibited [3H]-tiotidine binding. However, the potencies of these agents for binding did not agree with their effects on cyclic AMP. The selective H2-agonists impromidine and dimaprit which increased cyclic AMP caused only partial inhibition of [3H]-tiotidine binding. These results demonstrate that [3H]-tiotidine has limited binding to the H2-receptors and as such [3H]-tiotidine is not a suitable ligand for labelling the H2-receptor on gastric mucosal cells.