Summary

Introduction to Endocrine Principles

• Hormone molecules are synthesized, stored, and released by nonneural endocrine cells or neurons, circulate in the blood at low concentrations, exert their effects on target tissues, and are metabolically destroyed or directly excreted from the body.
• The magnitude of a hormone’s effect on target cells depends on both the abundance of receptor molecules with which it can bind and its concentration in the blood. Blood concentration of a hormone depends on a balance between the rate of synthesis and the rate of degradation or excretion. The rate of synthesis and secretion of a particular hormone is often governed by another hormone; some endocrine cells also receive neural input.
• Hormones are categorized into three main classes: steroids, peptides and proteins, and amines. The same chemical messenger may function as a hormone in one context and as a different type of chemical messenger in another.
• The half-lives of hormones vary depending on their chemical class, ranging from seconds to hours or days. Carrier proteins in the blood transport all lipid-soluble, nonpolar hormones and many water-soluble, polar ones. Only free hormone molecules are able to bind to receptor molecules in or on target cells.
• Both lipid-soluble and water-soluble hormones initiate biochemical changes in their target cells by binding to receptor molecules. Measurable responses to water-soluble and lipid-soluble hormones that bind cell-surface receptor molecules occur with a shorter delay than do responses to lipid-soluble hormones that bind intracellular receptors to initiate genomic actions.

Synthesis, Storage, and Release of Hormones

• Peptide hormones are synthesized by transcription of DNA, translation, and posttranslational processing. They are stored in vesicles and secreted on demand by exocytosis.
• Steroid hormones are synthesized from cholesterol. Steroidogenic cells use different biochemical pathways and sets of enzymes to produce different steroid hormones. Steroid hormones are synthesized on demand and secreted by diffusion through the cell membrane.

Types of Endocrine Cells and Glands

• Epithelial (nonneural) endocrine cells are generally controlled by hormones. (Some, such as the β cells of the pancreas, also receive neural input.)
• Neurosecretory cells are always controlled by synaptic input from neurons. Neurons and neurosecretory cells are thought to be related evolutionarily, but their origins are not known.
• Endocrine glands may be discrete, diffuse, or intermediate. Certain discrete glands appear to have evolved from diffusely distributed cells.

Control of Endocrine Secretion: The Vertebrate Pituitary Gland

• The vertebrate pituitary gland consists of the adenohypophysis (anterior pituitary) and the neurohypophysis (posterior pituitary). Posterior pituitary hormones (vasopressin and oxytocin) are secreted in response to neural activity. Secretions of anterior pituitary hormones are controlled by releasing hormones (RHs) and inhibiting hormones (IHs) from the hypothalamus, which are transported to the anterior pituitary through the hypothalamo–hypophysial portal system.
• The rate of and pattern of hormone secretion are influenced by a combination of hormonal modulation (such as feedback mechanisms, synergism, permissiveness, and antagonism) and neural modulation (such as sensory input and clock mechanisms).

The Mammalian Stress Response

• The stress response is a generalized constellation of physiological changes aimed at ensuring survival when an animal is exposed to real or perceived hostile or challenging conditions.
• The major physiological actions in the stress response include mobilizing stored energy and inhibiting energy storage; enhancing cardiovascular and respiratory functions; increasing alertness and cognition; inhibiting feeding, digestion, and reproduction; and modulating immune function. If an animal experiences loss of blood, hormones are secreted that promote the retention of water and solutes.
• The same effects of the stress response that are essential for survival of an animal exposed to an acute stressor can be deleterious during periods of prolonged stress.
• Wild animals in their natural environments experience seasonal variations in blood concentrations of glucocorticoids.

Endocrine Control of Nutrient Metabolism in Mammals

• Insulin is secreted when nutrient molecules are abundant in the blood. It exerts a hypoglycemic effect by promoting uptake and storage of nutrients and inhibiting degradation of glycogen, lipids, and proteins. In the absence of insulin, nutrient molecules are mobilized to enter the blood from storage sites.
• Glucagon is secreted when blood glucose levels are low. It exerts a hyperglycemic effect by stimulating the breakdown of glycogen (glycogenolysis), the breakdown of triglyceride molecules, and the formation of glucose from noncarbohydrate sources (gluconeogenesis).
• Growth hormone, glucocorticoids, epinephrine, thyroid hormones, and androgens play permissive and synergistic roles in nutrient metabolism.

Endocrine Control of Salt and Water Balance in Vertebrates

• Hormones continuously regulate the balance of salt and water in vertebrates.
• Vasopressins are peptide neurohormones that stimulate the conservation of water.
• Aldosterone is a steroid hormone that stimulates the excretion of K⁺ and conservation of Na⁺. It is part of the renin–angiotensin–aldosterone system that is set in motion under conditions of low arterial blood pressure.
• Atrial natriuretic peptide (ANP) exerts many different actions, all of which stimulate the excretion of Na⁺ and water.

Endocrine Control of Calcium Metabolism in Mammals

• Three hormones finely regulate the extracellular concentration of calcium ions in mammals: parathyroid hormone (PTH), active vitamin D (1,25-dihydroxycholecalciferol, or 1,25[OH]₂D₃), and calcitonin.
• The peptide PTH is secreted by chief cells of the parathyroid glands when extracellular Ca²⁺ is low. PTH stimulates cells in the nephron of the kidney to reabsorb Ca²⁺ and excrete phosphate and also to increase the conversion of inactive vitamin D to active vitamin D. In concert with active vitamin D, PTH stimulates bone resorption. These functions all contribute to increased Ca²⁺ in the extracellular fluid.
• Active vitamin D is formed when extracellular Ca²⁺ is low. It is a steroid and binds to intracellular receptors to influence gene transcription in its target tissues. It promotes absorption of dietary Ca²⁺ across the intestinal epithelium, the resorption of bone, and the reabsorption of both Ca²⁺ and phosphate in the nephron. Its actions increase Ca²⁺ and phosphate in the extracellular fluid.
• The peptide calcitonin targets osteoclasts in bone to inhibit bone resorption. At the kidney nephron, calcitonin increases the excretion of calcium and phosphate ions. These actions decrease calcium and phosphate in the extracellular fluid.

Chemical Signals along a Distance Continuum

• Chemical signals fall along a “distance spectrum” ranging from molecules that signal between individual cells and over short distances, to hormones and neurohormones that travel long distances in the blood, to chemical signals released by animals into the environment. Receptor molecules of target cells in tissues or sense organs detect these signals to trigger a functional response.
• Locally acting paracrines and autocrines include neuromodulators and cytokines.
• Pheromones are chemical signals released into the environment that convey information to animals of the same species. Kairomones, allomones, and synomones are chemicals released into the environment that convey information between members of different species.

Summary Insect Metamorphosis

• Insect metamorphosis illustrates the convergent evolution of endocrine and neuroendocrine functions between vertebrates and invertebrates.
• Insects change form in the course of their life cycles. Hemimetabolous insects go through gradual metamorphosis, and holometabolous insects go through complete metamorphosis.
• Environmental and behavioral signals mediated by the nervous system initiate molting by providing synaptic input to the PTTH neuroendocrine cells in the brain. These cells secrete PTTH, which stimulates secretion of ecdysone from the prothoracic glands. Ecdysone is converted to 20-hydroxyecdysone (20E) by peripheral activation.
• 20E stimulates the epidermis to secrete enzymes required for the molting process. At each molt, the epidermis lays down a new cuticle beneath the old one.
• Under the control of PETH and ETH, the insect performs stereotyped muscle contractions in order to shed the old cuticle.
• JH, secreted by nonneural endocrine cells in the corpora allata, prevents metamorphosis into the adult form. The relative amounts of JH and 20E in the hemolymph determine whether the epidermis will produce juvenile, pupal (in holometabolous forms), or adult structures.
• In adults, JH functions as a gonadotropin, stimulates the production of sex-attractant pheromones, and stimulates the secretion of ecdysone, which promotes incorporation of yolk into eggs.

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