Cell populations of Tetrahymena pyriformis GL were kept in nutrient-free (Losina) milieu and treated with different (10-6-10-21M) concentrations of serotonin, histamine or insulin for 30min. Following that the hormone (serotonin and adrenocorticotropin (ACTH) content of the cells were measured by immunocytochemical flow cytometric method. Serotonin reduced histamine when applied in 10-12and 10-15M concentrations, while elevated ACTH levels when applied in 10-6, 10-9and 10-21M concentrations. Histamine reduced serotonin concentration at 10-9-10-21M concentrations and increased ACTH in 10-6M. Insulin elevated both hormones' content in each concentration except at 10-12M. The results demonstrate that (1) in nutrient-free conditions the hormonal effects differ from that of nutrient-rich (tryptone+yeast) condition; (2) there is an optimal hormone concentration, which causes the strongest effect and this is different for each hormones; (3) the hormone receptors of Tetrahymena are very sensitive; as they react to zeptomolar concentrations. Such small concentration is even more effective than higher ones. Since hormones must become highly diluted in the natural environment of Tetrahymena, it seems that such low concentrations are the actual physiological concentrations. © 2011 Elsevier Inc.
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