Role of mast cells in health: daily rhythmic variations in their number, exocytotic activity, histamine and serotonin content in the rat thyroid gland.

A chronobiologic transverse study on rat thyroid has been carried out to investigate whether mast cells and their content in biogenic amines normally undergo daily variations and whether these are related to circadian activity of the gland. The mean number of mast cells per microscopic field presents daily variations ranging from 10.9 +/- 3 to 14.6 +/- 3.8 in males and from 8.4 +/- 1.9 to 14.8 +/- 3 in females: these variations show a circadian trend in both sexes, with a 12 hrs period and two peaks at about 11:10/23:10. The mean percentage of degranulated mast cells per microscopic fields shows daily variations ranging from 51 +/- 11 to 60.4 +/- 14.2 in males and from 49.8 +/- 12.5 to 58.3 +/- 13.6 in females; these variations present a circadian rhythm with a 24 hrs period and a mean peak at 02:00. The histamine content of the gland varies in 24 hrs from 20.93 +/- 1.19 micrograms/g w w to 38.08 +/- 1.7 micrograms/g w w, without any sex-related difference: these variations show a rhythmic trend with a 12 hrs period and two peaks at 09:10/21:10. Serotonin content of thyroid presents circadian variations from 15.98 +/- 0.83 to 23.23 +/- 0.61 micrograms/g w w, with a 12 hrs period and two peaks at 04:20/16:20. Whereas the variations of mast cell exocytosis and of serotonin content seem to be chronobiologically linked to circadian variations of gland activity, evaluated on the basis of free and total tetraiodothyronine serum levels, the variations of mast cell number appear to be related to those of thyroid and blood histamine. The present data support the hypothesis that mast cell activity should not be considered as only linked to inflammation or allergic responses.