There are three general classes of hormones:
1. Proteins and polypeptides, including hormones secreted by the anterior and posterior pituitary gland, the pancreas (insulin and glucagon), the parathyroid gland (parathyroid hormone), and many others (see Table 74-1).
2. Steroids secreted by the adrenal cortex (cortisol and aldosterone), the ovaries (estrogen and progesterone), the testes (testosterone), and the placenta (estrogen and progesterone).
3. Derivatives of the amino acid tyrosine, secreted by the thyroid (thyroxine and triiodothyronine) and the adrenal medullae (epinephrine and norepinephrine). There are no known polysaccharides or nucleic acid hormones.
Polypeptide and Protein Hormones Are Stored in Secretory Vesicles Until Needed. Most of the hormones in the body are polypeptides and proteins. These hormones range in size from small peptides with as few as 3 amino acids (thyrotropin-releasing hormone) to proteins with almost 200 amino acids (growth hormone and prolactin). In general, polypeptides with 100 or more amino acids are called proteins, and those with fewer than 100 amino acids are referred to as peptides.
Protein and peptide hormones are synthesized on the rough end of the endoplasmic reticulum of the different endocrine cells, in the same fashion as most other proteins (Figure 74-2). They are usually synthesized first as larger proteins that are not biologically active (preprohormones) and are cleaved to form smaller prohormones in the endoplasmic reticulum. These are then transferred to the Golgi apparatus for packaging into secretory vesicles. In this process, enzymes in the vesicles cleave the prohormones to produce smaller, biologically active hormones and inactive fragments. The vesicles are stored within the cytoplasm, and many are bound to the cell membrane until their secretion is needed. Secretion of the hormones (as well as the inactive fragments) occurs when the secretory vesicles fuse with the cell membrane and the granular contents are extruded into the interstitial fluid or directly into the blood stream by exocytosis.
In many cases, the stimulus for exocytosis is an increase in cytosolic calcium concentration caused by depolarization of the plasma membrane. In other instances, stimulation of an endocrine cell surface receptor causes increased cyclic adenosine monophosphate (cAMP) and subsequently activation of protein kinases that initiate secretion of the hormone. The peptide hormones are water soluble, allowing them to enter the circulatory system easily, where they are carried to their target tissues.
Steroid Hormones Are Usually Synthesized from Cholesterol and Are Not Stored. The chemical structure of steroid hormones is similar to that of cholesterol, and in most instances they are synthesized from cholesterol itself.
Endocrine Glands, Hormones, and Their Functions and Structure
Hypothalamus (Chapter 75)
Anterior pituitary (Chapter 75)
Posterior pituitary (Chapter 75)
Adrenal cortex (Chapter 77)
(Chapter 60) Pancreas (Chapter 78)
Parathyroid (Chapter 79)
(Chapter 80) Ovaries
Kidney (Chapter 26)
(Chapter 22) Stomach
(Chapter 64) Small intestine (Chapter 64)
Adipocytes (Chapter 71)
Thyrotropin-releasing hormone (TRH) Corticotropin-releasing hormone (CRH) Growth hormone-releasing hormone (GHRH) Growth hormone inhibitory hormone (GHIH)
(somatostatin) Gonadotropin-releasing hormone (GnRH) Dopamine or prolactin-inhibiting factor (PIF) Growth hormone
Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Prolactin
Follicle-stimulating hormone (FSH) Luteinizing hormone (LH)
Antidiuretic hormone (ADH) (also called vasopressin)
Thyroxine (T4) and triiodothyronine (T3) Calcitonin
Norepinephrine, epinephrine Insulin (b cells) Glucagon (a cells) Parathyroid hormone (PTH)
Human chorionic gonadotropin (HCG)
Atrial natriuretic peptide (ANP)
Cholecystokinin (CCK) Leptin
Major Functions Structure
Stimulates secretion of TSH and prolactin Peptide
Causes release of ACTH Peptide
Causes release of growth hormone Peptide
Inhibits release of growth hormone Peptide
Causes release of LH and FSH
Stimulates protein synthesis and overall growth Peptide of most cells and tissues Stimulates synthesis and secretion of thyroid Peptide hormones (thyroxine and triiodothyronine) Stimulates synthesis and secretion of adrenocortical Peptide hormones (cortisol, androgens, and aldosterone) Promotes development of the female breasts and Peptide secretion of milk Causes growth of follicles in the ovaries and sperm Peptide maturation in Sertoli cells of testes Stimulates testosterone synthesis in Leydig cells of Peptide testes;stimulates ovulation, formation of corpus luteum, and estrogen and progesterone synthesis in ovaries
Increases water reabsorption by the kidneys and Peptide causes vasoconstriction and increased blood pressure
Stimulates milk ejection from breasts and uterine Peptide contractions
Increases the rates of chemical reactions in most Amine cells, thus increasing body metabolic rate Promotes deposition of calcium in the bones and Peptide decreases extracellular fluid calcium ion concentration
Has multiple metabolic functions for controlling Steroid metabolism of proteins, carbohydrates, and fats; also has anti-inflammatory effects Increases renal sodium reabsorption, potassium Steroid secretion, and hydrogen ion secretion Same effects as sympathetic stimulation Amine
Promotes glucose entry in many cells, and in Peptide this way controls carbohydrate metabolism Increases synthesis and release of glucose from Peptide the liver into the body fluids Controls serum calcium ion concentration by Peptide increasing calcium absorption by the gut and kidneys and releasing calcium from bones Promotes development of male reproductive Steroid system and male secondary sexual characteristics Promotes growth and development of female Steroid reproductive system, female breasts, and female secondary sexual characteristics Stimulates secretion of "uterine milk" by the uterine Steroid endometrial glands and promotes development of secretory apparatus of breasts Promotes growth of corpus luteum and secretion of Peptide estrogens and progesterone by corpus luteum Probably helps promote development of some Peptide fetal tissues as well as the mother's breasts See actions of estrogens from ovaries Steroid
Catalyzes conversion of angiotensinogen to Peptide angiotensin I (acts as an enzyme) Increases intestinal absorption of calcium and bone Steroid mineralization
Increases erythrocyte production Peptide
Increases sodium excretion by kidneys, reduces Peptide blood pressure
Stimulates pancreatic acinar cells to release Peptide bicarbonate and water Stimulates gallbladder contraction and release of Peptide pancreatic enzymes Inhibits appetite, stimulates thermogenesis Peptide
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