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Introduction to the Endocrine System: Hormones, Regulation, and Pathologies

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Chapter 7: Introduction to the Endocrine System

7.1 Hormones

The endocrine system is responsible for regulating long-term, ongoing functions of the body through chemical messengers called hormones. Endocrinology is the study of hormones and their effects on metabolism, growth, development, reproduction, and the regulation of the internal environment.

  • Hormones act in three basic ways:

    1. Regulating rates of enzymatic reactions

    2. Controlling transport of ions or molecules across cell membranes

    3. Influencing gene expression and protein synthesis

  • Diseases of the endocrine system have been documented since ancient times.

  • Classic steps to identify an endocrine gland and its hormone:

    1. Remove the suspected gland

    2. Replace the hormone

    3. Create hormone excess

What Makes a Chemical a Hormone?

  • A hormone is a chemical signal secreted by a cell or group of cells into the blood, transported to a distant target, and exerts its effect at very low concentrations.

  • Hormones act by binding to specific receptors on or in target cells, initiating biochemical responses.

  • Hormone action must be terminated; the half-life indicates the duration of activity.

Table 7.1: Comparison of Peptide, Steroid, and Amino Acid-Derived Hormones

This table compares the synthesis, storage, transport, and mechanism of action of the three major hormone classes.

Feature

Peptide Hormones

Steroid Hormones

Amine Hormones (Catecholamines)

Amine Hormones (Thyroid Hormones)

Synthesis & Storage

Synthesized in advance; stored in vesicles

Made on demand; not stored

Synthesized in advance; stored in vesicles

Synthesized in advance; stored in vesicles

Release from Parent Cell

Exocytosis

Simple diffusion

Exocytosis

Transport protein

Transport in Blood

Dissolved in plasma

Bound to carrier proteins

Dissolved in plasma

Bound to carrier proteins

Half-Life

Short

Long

Short

Long

Location of Receptor

Cell membrane

Cytoplasm or nucleus

Cell membrane

Nucleus

Response to Receptor-Ligand Binding

Activation of second messenger systems; may activate genes

Activation of genes for transcription and translation

Activation of second messenger systems

Activation of genes for transcription and translation

Examples

Insulin, parathyroid hormone

Estrogen, cortisol

Epinephrine, norepinephrine

Thyroxine (T4)

7.2 The Classification of Hormones

Hormones are classified based on their chemical structure and synthesis.

  • Peptide hormones are the most common. They are synthesized as large, inactive precursors called preprohormones, processed to prohormones, and then to active hormones stored in vesicles.

  • Peptide hormones bind to surface membrane receptors and initiate cellular responses via signal transduction systems.

  • Steroid hormones are derived from cholesterol, synthesized on demand in the adrenal cortex and gonads, and bind to carrier proteins in the blood. They act on cytoplasmic or nuclear receptors to produce genomic effects, or on membrane receptors for nongenomic responses.

  • Amine hormones are derived from single amino acids:

    • Tryptophan (e.g., melatonin)

    • Tyrosine (e.g., catecholamines like epinephrine, norepinephrine, dopamine; thyroid hormones)

    Catecholamines behave like peptide hormones; thyroid hormones behave like steroid hormones.

7.3 Control of Hormone Release

Hormone release is regulated by reflex pathways involving stimulus, sensor, input signal, integration, output signal, target(s), and response.

  • In simple endocrine reflexes, the endocrine cell is the sensor (e.g., parathyroid hormone).

  • Many endocrine reflexes involve the nervous system; neurohormones are secreted into the blood by neurons (catecholamines, hypothalamic nuclei, hypothalamic neurohormones to anterior pituitary).

The Pituitary Gland

The pituitary gland consists of two fused glands:

  • Posterior pituitary stores and releases two neurohormones:

    • Antidiuretic hormone (ADH, vasopressin)

    • Oxytocin

  • Anterior pituitary secretes six hormones:

    • Prolactin (PRL)

    • Thyrotropin (TSH)

    • Adrenocorticotropin (ACTH)

    • Growth hormone (GH)

    • Follicle-stimulating hormone (FSH)

    • Luteinizing hormone (LH)

    These are regulated by hypothalamic hormones via a portal system.

Portal System

The hypothalamic-hypophyseal portal system consists of two sets of capillaries connected in series by a vein, ensuring that small amounts of concentrated hormone are directed to their target tissues.

Table: Hormones of the Hypothalamic-Anterior Pituitary Pathway

Anterior Pituitary Hormone

Hypothalamic Releasing Hormone

Hypothalamic Inhibiting Hormone

Prolactin (PRL)

None

Dopamine (PIH)

Thyrotropin (TSH)

Thyrotropin-releasing hormone (TRH)

Adrenocorticotropin (ACTH)

Corticotropin-releasing hormone (CRH)

Growth hormone (GH)

GHRH (somatotropin)

Somatostatin (GHIH)

Follicle-stimulating hormone (FSH)

Gonadotropin-releasing hormone (GnRH)

Luteinizing hormone (LH)

Gonadotropin-releasing hormone (GnRH)

Anterior Pituitary Hormones: Functions

  • Prolactin (PRL): Controls milk production (lactation) in the female breast.

  • Growth hormone (GH): Affects metabolism and stimulates hormone production in the liver.

  • Follicle-stimulating hormone (FSH): Stimulates growth of ovarian follicles.

  • Luteinizing hormone (LH): Stimulates ovulation, corpus luteum formation, and synthesis of estrogen/progesterone.

  • Thyroid-stimulating hormone (TSH): Controls hormone synthesis and secretion in the thyroid.

  • Adrenocorticotropic hormone (ACTH): Controls hormone synthesis and secretion in the adrenal cortex (e.g., cortisol).

22.6 Homeostatic Control of Metabolism: Insulin & Glucagon

The pancreas regulates metabolism through the secretion of insulin and glucagon from the islets of Langerhans:

  • Beta cells secrete insulin

  • Alpha cells secrete glucagon

  • D cells secrete somatostatin

  • PP (F) cells secrete pancreatic polypeptide

The insulin-to-glucagon ratio regulates metabolism:

  • In the fed state, insulin dominates

  • In the fasting state, glucagon dominates

Insulin Promotes Anabolism

  • Insulin binds to tyrosine kinase receptor, activating insulin-receptor substrates (IRS).

  • Insulin lowers plasma glucose by:

    1. Increasing glucose transport into most insulin-sensitive cells

    2. Enhancing utilization and storage of glucose

    3. Enhancing utilization of amino acids

    4. Promoting fat synthesis

Glucagon Is Dominant in the Fasted State

  • Glucagon is generally antagonistic to insulin.

  • Prevents hypoglycemia by stimulating glycogenolysis and gluconeogenesis in the liver.

  • Release is stimulated by low blood glucose and plasma amino acids.

Diabetes Mellitus

  • Diabetes mellitus is characterized by abnormally elevated plasma glucose concentrations (hyperglycemia).

  • Type 1 diabetes: Insulin deficiency due to autoimmune destruction of beta cells.

  • Type 2 diabetes: Insulin-resistant diabetes.

  • Diagnosis:

    • Fasting blood glucose: Prediabetes (100-125 mg/dL), Diabetes (>125 mg/dL)

    • Glucose tolerance test (after 2 hrs): Prediabetes (140-199 mg/dL), Diabetes (>200 mg/dL)

7.4 Hormone Interactions

  • Permissive hormones allow another hormone to exert its full effect.

  • Antagonistic hormones have opposing effects (competitive inhibitors vs. functional antagonists).

  • Synergism: Combined effect of hormones is greater than the sum of individual effects.

7.5 Endocrine Pathologies

  • Hypersecretion: Excess hormone exaggerates effect, often due to tumors or exogenous treatment; negative feedback may lead to atrophy of gland.

  • Hyposecretion: Deficient hormone diminishes or eliminates effect, caused by decreased synthesis or atrophy; absence of negative feedback leads to overproduction of trophic hormones.

  • Abnormal tissue responsiveness: Down-regulation (decreased receptor number), receptor/signal transduction abnormalities (missing or nonfunctional receptors).

  • Diagnosis:

    • Primary pathology: Last endocrine gland in pathway

    • Secondary pathology: Pituitary gland

    • Tertiary pathology: Hypothalamus

Feedback Loops in the Hypothalamic-Pituitary Pathway

  • Long-loop negative feedback: Dominant mechanism; hormone itself is the feedback signal.

  • Short-loop negative feedback: Pituitary hormone suppresses hypothalamic trophic hormone production.

  • Ultra-short-loop negative feedback: Occurs in hypothalamus and pituitary via autocrine or paracrine signals.

Summary

  • Hormones regulate essential physiological processes through specific mechanisms of action and feedback regulation.

  • Classification, synthesis, and release of hormones are fundamental to understanding endocrine system function and pathology.

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