Toluidine blue released histamine from isolated rat mast cells suspended in a buffered physiological solution at 37℃, in concentrations ranging from 10 to 250μg/ml. The highest histamine release was seen at concentrations from 50 to 100μg/ml. In concentrations between 25 and 50μg/ml, mast cells responded with a typical degranulation quite similar to that seen under the action of 0.5μg/ml of compound 48/80, while at 100μg/ml the cell surface showed
indentation with a number of small protrusions in which the granules were occasionally contained. However, a smooth cell surface was seen in higher concentrations over 100 μg/ml of toluidine blue. This indicates that toluidine blue releases histamine without accompanying degranulation. When the time course of histamine release was determined by abrupt cooling to stop the reaction at various time sequence, it was shown that histamine release at 37℃ is a very rapid process actually accomplishing within 5 sec. All morphological changes of mast cell completed at longest within 15 sec after the addition of toluidine blue, while the dye could not be seen inside the cytoplasm before 1 min after the addition. These observations suggest that toluidine blue reacts with receptive sites on the cell surface of mast cell and this may trigger the further processes leading to the histamine release as well as to morphological changes. The histamine release by toluidine blue was inhibited by respiratory inhibitors or uncouplers of oxidative phosphorylation and these inhibitions were overcome by the presence of lucose. Toluidine blue released histamine from the mast cells of rat mesentery but the effective concentrations were higher than that in the isolated mast cells. In the dog, a large amount of toluidine blue injected i. v. caused a severe hypotension, but there was no marked increase in the plasma histamine differing from the action of compound 48/80. With respect
to the mechanisms of histamine release the mode of action of toluidine blue may be in common to that of other basic histamine releasers, although distinct difference existed in some aspects of the action between toluidine blue and compound 48/80.