For the purpose of clarifying the pathophysiology of allergic reactions the role of serotonin was investigated in anaphylactic shock in experimentally induced rabbits as well as dogs, and in patients with bronchial asthma. Furthermore, the mast cells in the skin as well as the lung tissue of rabbits under the shock were observed. Results thus obtained were as follows. 1) The average serotonin levels of normal rabbits without sensitization were 0.70±0.16 μg/ml in the plasma, 3.04±0.24 μg/ml in the whole blood and 3.49±0.58 μg/g in the lung tissue. 2) After sensitization of the rabbits with egg white, they were 0.75±0.13 μg/ml in the plasma, 3.80±0.37 μg/ml in the whole blood and 3.75±0.66 μg/g in the lung tissue. Comparing these serotonin levels with those of normal rabbits, serotonin levels of only the whole blood were noted to be significantly, increased after the sensitization. 3) Determining the serotonin levels in plasma, whole blood, as well as lung tissue after administration of the antigen to the sensitized rabbits in an attempt to induce an anaphylactic shock, they were noted to be as follows. The plasma serotonin level increased three times as much as the level before the provocation from thirteen to thirty seconds after injection of the antigen and almost returned to normal level after one minute, however thereafter, it decreased to lower level than those before provocation from three to five minutes after the antigen was given. On the other hand, the whole blood serotonin demonstrated remarkable decrease from thirteen to thirty seconds after injection of the antigen showing no signs of recovery until after five minutes. 4) The serotonin level of the lung tissue from six to seven minutes after injection of the antigen was 11.08±0.91 μg/g in average which level was significantly higher than those of normal as well as sensitized rabbits. From sixty to ninty minutes after injection of the antigen, the serotonin level of the lung tissue was noted to return to normal level. 5) In rabbits under anaphylactic shock the mast cells of the skin as well as the lung tissue revealed various histopathological changes such as swelling, degranulation and destruction. These changes were more striking than those of the non-sensitized group given the antigen. 6) When an anaphylactic shock was induced in rabbits, their leucocyte and platelet counts decreased remarkably. These changes ran almost parallel with the rise and fall of the whole blood serotonin. 7) The average plasma serotonin levels were 0.083±0.0022 μg/ml in normal dogs without sensitization 0.128±0.018 μg/ml in the dogs sensitized with horse serum and 0.235±0.032 μg/ml in the latter thirty seconds after injection of the antigen to induce an anphylactic shock. There was no significant difference in the plasma serotonin level between the non-treated dogs and the sensitized dogs, whereas the plasma serotonin level of dogs during anaphylactic shock showed a remarkable increase which value was noted to be significantly higher than the same of the non-treated as well as the sensitized dogs before provocation of the anaphylactic shock. The blood pressure of the sensitized dogs began to decrease at thirty minutes after the provocation, plasma serotonin level was noted to already return to the pre-provocation level three minutes after provocation injection of the antigen. The clinical symptomes of anaphylactic shock appeared a little later after the plasma serotonin level reached its peak. 8) The average plasma serotonin level of healthy person was 0.80±0.019 μg/ml in average. The average plasma serotonin level in the patients with bronchial asthma was 0.083±0.035 μg/ml in their symptom-free periods and 0.148±0.083 μg/ml during their paroxysm periods, which level was noted to reveal a significant increase in comparison with the plasma levels of both healthy persons and asthmatics without paroxysm. 9) Thirty minutes after the intravenous injection of dexameth
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