Protein-derived hormones bind to cell membrane receptors that transmit the hormonal signal into the cell while cholesterol-derived hormones bind to nuclear receptors that interact either directly with the regulatory portions of genes (promoter) or via other transcription factors to alter gene expression. However, there is one exception to this. One class of peptide derived hormone, Thyroxine (T4) and Thyronine (T3), whose structure is based on two tyrosine amino acids fused together exert its effects through binding to nuclear receptors.
Hormonal functioning can be viewed in three different styles. They are . – one hormone – many functions. one hormone – specific function and one function – many hormones. The ability of one hormone to exert multiple effects in multiple organs is due to (1) the extensive distribution of hormones throughout the body via the circulatory system and (2) the presence of different receptors that exhibit differential affinity for the hormone and variable signal transduction properties. Hormone action can be limited to certain tissues because of: (1) the limited distribution of its receptors. For example, ACTH secreted by the anterior pituitary, although it circulates freely in the body, only acts on the adrenal glands because only the adrenal cortex has receptors to ACTH. (2) Circulation of the hormone in restricted blood supply. For example, CRH is secreted by the hypothalamus into the pituitary venous plexus and acts on the pituitary. Very little CRH can be found circulating in the rest of the body. Hormones act in a concerted fashion to maintain the normal function of the organism.
Hormone secretion is generally under a feedback control mechanism. The integral feedback control mechanism is the characteristic of the endocrine system. Hormones and their end products feedback to inhibit or stimulate further secretion of stimulating or inhibitory hormones.