Pathways Involving Cell Membrane Hormone Receptors For both steroid and thyroid hormones, binding of the hormone-receptor complex with DNA triggers transcription of a target gene to mRNA, which moves to the cytosol and directs protein synthesis by ribosomes. In contrast, thyroid hormones bind to receptors already bound to DNA. In either case, this binding generates a hormone-receptor complex that moves toward the chromatin in the cell nucleus and binds to a particular segment of the cell’s DNA. The location of steroid and thyroid hormone binding differs slightly: a steroid hormone may bind to its receptor within the cytosol or within the nucleus. Thyroid hormones, which contain benzene rings studded with iodine, are also lipid-soluble and can enter the cell. Steroid hormones are derived from cholesterol and therefore can readily diffuse through the lipid bilayer of the cell membrane to reach the intracellular receptor ( (Figure)). Hormones that bind to this type of receptor must be able to cross the cell membrane. Intracellular hormone receptors are located inside the cell. Pathways Involving Intracellular Hormone Receptors Moreover, a single hormone may be capable of inducing different responses in a given cell. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Once the target cell receives the hormone signal, it can respond in a variety of ways. Thus, the response triggered by a hormone depends not only on the hormone, but also on the target cell. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Hormone receptors recognize molecules with specific shapes and side groups, and respond only to those hormones that are recognized. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane.
In contrast, the amino acid–derived hormone epinephrine has a half-life of approximately one minute. For example, the lipid-derived hormone cortisol has a half-life of approximately 60 to 90 minutes. A hormone’s half-life is the time required for half the concentration of the hormone to be degraded. This more complex structure extends the half-life of steroid hormones much longer than that of hormones derived from amino acids. Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein. Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). The adrenal glands produce the steroid hormone aldosterone, which is involved in osmoregulation, and cortisol, which plays a role in metabolism. For example, the reproductive hormones testosterone and the estrogens-which are produced by the gonads (testes and ovaries)-are steroid hormones. Steroid hormones are derived from the lipid cholesterol. The primary hormones derived from lipids are steroids. Stimulate development of female secondary sex characteristics and prepare the body for childbirth Stimulates development of male secondary sex characteristics and sperm production Stimulates uterine contractions during childbirth Stimulates hormone release by adrenal cortex Endocrine Glands and Their Major Hormones The major hormones of the human body and their effects are identified in (Figure). These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions.
Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. Discuss the role of feedback loops and humoral, hormonal, and neural stimuli in hormone controlĪlthough a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells that is, cells with receptors for that particular hormone.Identify several factors that influence a target cell’s response.Describe signaling pathways that involve cAMP and IP3.Compare and contrast intracellular and cell membrane hormone receptors.Identify the three major classes of hormones on the basis of chemical structure.By the end of this section, you will be able to: