Methylation of histamine in the gastric mucosa.

Methylation of histamine in the gastric mucosa of various species has been studied in vitro and in vivo. When gastric mucosal homogenates of rat, guinea pig, cat, dog, and pig were incubated with 14C-histamine in the presence of S-adenosyl methionine Ntau-methylhistamine was formed as the only methylated histamine derivative. Excessive concentration of Ntau-methylhistamine inhibited the formation of Ntau-methylhistamine. Pretreatment of guinea pigs with prednisolone, aminoguanidine, or pentagastrin neither influenced in vitro the formation of Ntau-methylhistamine significantly nor shifted the methylation reaction towards the side chain. Pretreatment of guinea pigs with the histamine H2-receptor blocker cimetidine significantly enhanced the formation of Ntau-methylhistamine in vitro. In pooled samples of guinea pig gastric mucosa histamine and Ntau-methylhistamine were found. Under none of these circumstances was Nalpha-methyl-or Nalpha,Nalpha-dimethylhistamine found. It is concluded that in rat (to a small extent), guinea pig, cat, dog, and pig gastric mucosa the formation of Ntau-methylhistamine is the only methylating pathway of histamine. In cases where other methylated histamine derivates (Nalpha-methyl- and/or Nalpha,Nalpha-dimethylhistamine) are found they must origin in sources other than the gastric mucosa.