In endocrinology, permissiveness is a biochemical phenomenon in which the presence of one hormone is required in order for another hormone to exert its full effects on a target cell. Hormones can interact in permissive, synergistic, or antagonistic ways. The chemical classes of hormones include amines, polypeptides, glycoproteins and steroids. Permissive hormones act as precursors to active hormones and may be classified as either prohormones or prehormones.
Thyroid hormone increases the number of receptors available for epinephrine at the latter's target cell, thereby increasing epinephrine's effect on that cell. Without the thyroid hormone, epinephrine would have only a weak effect. The lipophilic hormones (steroids and thyroid hormones) bind to nuclear receptor proteins, which function as ligand-dependant transcription factors. Some steroid hormones bind to cytoplasmic receptors, which then move into the nucleus. Other steroids and thyroxine bind to receptors already in the nucleus. Each receptor binds to both the hormone and to a region of DNA called a hormone response element.
The effects of a hormone in the body depend on its concentration. Permissive actions of glucocorticoids like cortisol generally occur at low concentrations. Abnormally high amounts of a hormone can result in atypical effects. Glucocorticoids function by attaching to cytoplasmic receptors to either enhance or suppress changes in the transcription of DNA and thus the synthesis of proteins. Glucocorticoids also inhibit the secretion of cytokines via post-translational modification effects.
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