Three-week-old male rats were fed for two weeks diets supplying inadequate, adequate, or excess amounts of histidine. After the 2-week feeding of the experimental diets, the rats were killed. Brain, gastrocnemius muscle, kidney and stomach were removed and analyzed for histamine and free-histidine as well as for the degradative enzyme, HMT, and the histamine-synthesizing enzyme HDC. The following results were obtained: As the levels of dietary histidine increased, (1) tissue concentrations of free-histidine and of histamine increased in all the tissues analyzed. (2) The increase of histamine was greatest in brain and stomach (5- and 4-fold, respectively), but less in kidney and muscle (2-fold). (3) HDC activity was not detected in muscle, but doubled from the lowest to the highest histidine intake in brain and increased almost 6-fold between the lowest and the highest histidine levels in stomach. (4) Kidney HDC decreased from the lowest to the two higher levels of dietary histidine. (5) HMT activity increased nominally in brain and not significantly in kidney; none was detected in either muscle or stomach. (6) Brain and kidney, tissues with considerable HMT activity, had almost no histamine. The increases in tissue histamine concentrations observed in the tissues analyzed generally reflected the changes and magnitudes of enzyme activities for HMT and HDC. The results in the rat differ in important ways from those previously observed in chickens as follows: (1) Histamine concentrations as a function in dietary histidine decreased in the chick. (2) Both HDC and HMT activities were present in chick muscle tissue. (3) HDC activity in chick stomach decreased sharply as a function of dietary histidine.

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The effect of age and of varying the dietary histidine level, with special emphasis on histamine metabolism, was studied in male, white Leghorn chicks. The birds were fed a 19% amino acid diet with histidine supplied from 0 to 0.8% of dict. In Experiment 1, 7-day-old chicks were fed the experimental diets for 7 days. In Experiment 2, chicks were fed the experimental dlets for different time intervals to equalize their body weight at time of killing. In Experiment 3, blrds were killed at 5, 9, 12 and 15 days of age when their body weights reached 69, 96, 136 and 184 g, respectively. Concentrations of anserine, carnoslne, free-hlstldlne and histamine, and actlvitles of histldine deearboxylase (HDC) and histamine methyl transferase (HMT) were assayed in whole brain, pectoralis major muscle and in proventrlculus of 3 or 4 chicks per treatment group.The following results were obtained: As the level of dietary hlstidlne increased, (1) tissue concentrations of freehistidine and of carnosine increased; (2) the activity of HMT increased; (3) the concentratlon of anserine remained constant; (4) the activity of HDC increased in brain and muscle, but decreased sharply in the proventrieulus; (5) the concentration of histamine decreased. (6) Free-histidine concentration, HDC and HMT activities increased with an increase in age and body weight; however, histamine concentrations decreased with age and body weight. The results point to the possibility that the relatively greater increase in HMT activity compared to HDC may be responsible for the decrease in histamine concentration with increase in dietary histidlne or increased age. The very high HDC activity in the proventrlculus of birds fed low histidine diets may have contributed to the higher histamine concentrations in brain and muscle of blrds fed low versus higher histldine-containing diets.

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