Circadian patterns in histamine concentrations and mast cell number in the rat thyroid gland.

We carried out a cross-sectional chronobiological investigation on blood histamine, thyroid histamine and thyroid mast cell number in Wistar rats. Daily, blood histamine varied from 0.38 +/- 0.01 (12.00 h) to 0.60 +/- 0.01 mg/g wet weight (20.00 h) and thyroid histamine from 21.2 +/- 1.19 (04.00 h) to 38.3 +/- 1.54 mg/g wet weight (08.00 h). The number of mast cells per microscopic field ranged from 10.8 +/- 0.6 (16.00 h) to 14.6 +/- 0.6 (12.00 h) in males and from 8.3 +/- 0.2 (04.00 h) to 14 +/- 0.4 (12.00 h) in females. Chronobiologic analysis indicates that the levels of all three variables follow a circadian pattern with a period of 12 h. Peak levels were noted at 07.36 h and 19.36 h for the blood histamine concentration, at 09.00 and 21.00 h for the tissue histamine concentration, and at 11.00 and 23.00 h for the mast cell number. The consistent, consecutive relationship of these data supports the hypothesis that thyroid mast cell number is conditioned by the blood histamine level and thyroid histamine concentration.