Each stage of this amplification that occurs allows one hormone molecule to stimulate millions or billions of responses in the target cell in just a few minutes. When a hormone is no longer present at its receptor sites, phosphodiesterase breaks down cAMP and restores cell functions to their non-hormone stimulated state. The tropic hormones of the anterior pituitary, some of the hypothalamic releasing hormones, PTH, adrenaline, glucagon, gastrin, and secretin depend on the cAMP version of the two-messenger model of hormonal control. Insulin, however, relies on cyclic guanine monophosphate (cGMP) instead of on cAMP.
Thyroxin and the steroid hormones depend on a one-messenger model of hormonal control. Thyroxin enters target cells directly and alters the genes in the nucleus and mitochondrial enzyme activity to exact its effect on target cells. The steroid hormones, being lipid-soluble, also penetrate the plasma membrane and cytoplasm of their target cells. In the cytoplasm, steroidal hormones bind with specific receptor molecules and move into the nucleus where they influence the transcription of RNA and, thus, influence what proteins (including, especially, enzymes) the cell produces.
Hormones can diffuse from one cell to another, but the main mechanism for hormone transport in higher animals is the circulatory system. Hormones are secreted directly into the blood, entering the capillaries serving the endocrine tissues without the involvement of special ducts or tubes. In the blood, hormones form weak bonds with plasma proteins and are transported in this form to their target tissues.
