Enumeration of mast cells and determination of histamine in sections of dog skin and dog lung have shown that the regions of the tissue rich in mast cells are also rich in histamine. In the skin, the outer layers of the dermis contain higher concentrations of histamine and mast cells than any other region.

Average values of the histamine content and mast cell count for dog liver, heart, muscle, and kidney, and for beef liver parenchyma and capsule, also exhibit a positive relationship between these two parameters, although the amount of histamine found per mast cell varied from 7 µµg. in dog skin to 32 µµg. in beef liver capsule.

When the size of the mast cells was taken into account, the intracellular concentration of histamine was found to be of the same order (about 0.4 molar) for all the tissues examined except skin and kidney. In both dog skins analyzed, the intracellular concentration was about 0.1 molar. In kidney both mast cell and histamine values were too low for accurate estimation.

The concentration of histamine in the mast cells investigated was much greater than the concentration in human basophils, the blood elements richest in histamine.

The mechanism of the localization of histamine in mast cells is briefly considered.

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