Histamine synthesis by intact mast cells from canine fundic mucosa and liver

The synthesis and degradation of histamine by dog fundic mucosa was studied by using cells dispersed by enzymatic digestion and separated sequentially by velocity sedimentation in an elutriator rotor, and by density gradient. Histidine decarboxylase activity was found in appreciable amounts in fractions highly enriched in mast cells when these cells were studied intact, whereas only trace activity was detected in homogenates of these mucosal mast cells or of whole mucosa. Unlike the rat gastric mucosal histamine cell, dihydroxyphenylalanine decarboxylase activity was not present in the canine fundic mast cell. Serotonin, which is found in the rat peritoneal mast cell, was not detectable in the canine mast cell. The histamine-degrading enzyme, histamine methyltransferase, was also present in gastric mucosal cells, but not diamine oxidase. This methyltransferase activity was primarily associated with parietal cells and was not found in the mast cell-enriched functions. For comparison, fractions containing 60%-80% mast cells were enriched by elutriation from enzyme-dispersed cells of canine liver. As with the gastric mast cells, histidine decarboxylase activity was found in intact cell, but it was lost upon cell disruption.