Chapter 18a The Endocrine System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Functional Organization of Chapter 18a Functional Organization of Endocrine System

Endocrine System Characteristics Produce hormones In minute amounts secreted into interstitial spaces Enters circulatory system Acts on target tissues at another site Regulate activities of body structures Amplitude modulated signals Frequency-modulated signals Response slower and longer duration than nervous system

Chemical Signals Intercellular: Allow one cell to communicate with other cells as hormones Autocrine Released by cells and have a local effect on same cell type from which chemical signals released as prostaglandin Paracrine Released by cells and affect other cell types locally without being transported in blood as somatostatin Pheromones Secreted into environment that modify behavior and physiology as sex pheromones

Chemical Structure of Hormones

Control of Secretion Rate Most hormones are not secreted at constant rate Patterns of regulation Involves action of substance other than hormone on an endocrine gland Involves neural control of endocrine gland Involves control of secretory activity of one endocrine gland by hormone or neurohormone secreted by another endocrine gland

Nervous System Regulation

Hormonal Regulation

Positive and Negative Feedback

Control of Secretion Rate Negative-feedback Mechanisms that function to maintain homeostasis control most of hormone secretion Mechanisms of hormone secretion from endocrine tissue Nonhormone substance Stimulation by nervous system Hormone from another endocrine tissue

Transport and Distribution Hormones dissolve in blood plasma and are transported in free form or bound to plasma proteins As concentration of free hormone molecules increase, more hormones molecules diffuse from capillaries into interstitial spaces to bind to target cells

Transport and Distribution A large decrease in plasma protein concentration can result in loss of a hormone from the blood because free hormones are rapidly eliminated from circulation through kidney or liver Hormones are distributed quickly because they circulate in the blood

Metabolism and Excretion Half-life: The length of time it takes for half a dose of substance to be eliminated from circulatory system Long half-life Regulate activities that remain at a constant rate through time Short half-life Water-soluble hormones as proteins, epinephrine, norepinephrine These hormones have a rapid onset and short duration Hormones also eliminated by blood by excretion by Kidneys and liver Enzymatic degradation Conjugation Active transport

Hormone Interaction with Target Tissues Hormones Ligands or chemical signals Binding site Receptor site Specificity

Target Cell Response to Hormones Target tissues have receptor molecules specific for particular hormone

Down and Up-Regulation Down-regulation Number of receptors decreases within target cells Up-regulation Some stimulus causes number of receptors for a hormone to increase within target cell

Classes of Hormone Receptors Membrane-bound receptors bind to water-soluble or large-molecular-weight hormones Intracellular receptors bind to lipid-soluble hormones

Membrane-Bound Hormone Receptors Proteins or glycoproteins that have polypeptide chains folded to cross cell several times Results of ligand binding Changes permeability of plasma membrane Alters activity of G proteins Alters activity of intracellular enzymes

Activation of G Proteins

G Proteins and Ca2+ Channels

G Proteins and Synthesis of cAMP

Membrane-Bound Receptors

Cascade Effect

Intracellular Receptors Are proteins in cytoplasm or nucleus Hormones bind with intracellular receptor and receptor-hormone complex activates genes, DNA activated to produce mRNA, mRNA initiates production of certain proteins (enzymes) that produce response of target cell to hormone Slow-acting because time is required to produce mRNA and protein Processes limited by breakdown of receptor-hormone complex

Intracellular Receptor Model Positive and Negative Feedback