Pathways Involving Intracellular Hormone Receptors Moreover, a single hormone may be capable of inducing multiple 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 receptor present 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. Because blood is primarily water, lipid-derived hormones must travel to their target cell bound to a transport protein. Binding to transport proteins extends the half-life of steroid hormones beyond that of hormones derived from amino acids. Like cholesterol, steroid hormones are hydrophobic (not soluble in water). 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. Like other proteins in the body, these hormones result from the transcription and translation of genes. Peptide hormones may be either short chains of amino acids, such as oxytocin, or much longer polypeptides such as insulin. Whereas the amine hormones are derived from a single amino acid, peptide hormones consist of multiple amino acids that link to form an amino acid chain. Epinephrine and norepinephrine are secreted by the adrenal medulla and play a role in the fight-or-flight response, whereas dopamine is secreted by the hypothalamus and inhibits the release of certain anterior pituitary hormones. Tyrosine derivatives include the metabolism-regulating thyroid hormones, as well as the catecholamines, such as epinephrine, norepinephrine, and dopamine. An example of a hormone derived from tryptophan is melatonin, which is secreted by the pineal gland and functions in regulating circadian rhythms. Typically, the original structure of the amino acid is modified such that a –COOH, or carboxyl, group is removed, whereas the −NH 3 +, or amine, group remains.Īmine hormones are synthesized from the amino acids tryptophan or tyrosine. Hormones derived from the modification of amino acids are referred to as amine hormones. Figure 17.2.1: Amine, Peptide, Protein, and Steroid Hormone Structure Amine Hormones These chemical groups affect a hormone’s distribution, the type of receptors it binds to, and other aspects of its function. The hormones of the human body can be structurally divided into three major groups: amino acid derivatives (amines), peptides, and steroids ( Figure 17.2.1). 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 (Table 17.2) The major hormones of the human body and their effects are identified in Table 17.2. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. However, a hormone will only affect the activity of its target cells that is, cells with receptors for that particular hormone. When released into the blood, a hormone circulates freely throughout the body. Understand the various mechanisms for stimulating hormone release.Identify factors that influence a target cell’s response.Compare and contrast intracellular receptor systems and 2nd messenger systems.Identify the three major structural classes of hormones. Explain the chemical composition of hormones and the mechanisms of hormone action.īy the end of this section, you will be able to:
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