Some effects of histamine in the normal and Haemophilus pertussis vaccinated rat.

The effects of histamine in the normal and H. pertussis vaccinated rat have been investigated. In the normal animal small doses of histamine (from 1 to 30 mg./kg., i.v.) produced significant, reversible alterations in the blood pressure, electrocardiogram and electroencephalogram. Doses of 100 mg./kg. or more caused bronchoconstriction and doses of 200 mg./kg. or more an effect on the muscular contractions after electrical stimulation of the nerve. Acute histamine intoxication resulted on rapid injection of 500 mg./kg. intravenously; death was probably due to cardiovascular collapse. In the vaccinated rat, a distinction could be made between the immediate and delayed effects of histamine. With a sublethal dose of histamine (5 mg./kg.) alterations in the blood pressure, electrocardiogram and electroencephalogram were noted. The neuromuscular system, which in the normal animal was the least reactive to histamine, showed the most marked increase in sensitivity in the vaccinated rat. The delayed effects of histamine observed after an interval of 5 to 30 min. were characteristic and irreversible, leading to death of the animal. The mechanism of death in the vaccinated rat after histamine appears to differ from that of the normal animal and has been discussed in the light of present knowledge.