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

Chapter 7: Introduction to the Endocrine System

7.1 Hormones

The endocrine system regulates long-term physiological processes through the secretion of hormones. Endocrinology is the study of hormones and their effects on the body.

• Key Functions: Hormones are responsible for metabolism, regulation of the internal environment, reproduction, growth, and development.

• Mechanisms of Action:

  1. Regulation of enzymatic reaction rates

  2. Control of ion or molecule transport across cell membranes

  3. Gene expression and protein synthesis

• Historical Context: Endocrine disorders have been recognized since ancient times. Classic experimental steps to identify endocrine glands include removal, replacement, and hormone excess studies.

What Makes a Chemical a Hormone?

• Definition: A hormone is a chemical signal secreted by a cell or group of cells into the blood, transported to distant targets, and effective at very low concentrations.

• Transport: Hormones travel via the bloodstream to reach target tissues.

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

• Termination: Hormone action must be terminated, often reflected by the hormone's half-life.

7.2 The Classification of Hormones

Hormones are classified based on their chemical structure and synthesis pathways.

• Peptide/Protein Hormones:

  • Most common type; synthesized as large inactive precursors (preprohormones), processed to prohormones, and stored in vesicles until release.

  • Bind to surface membrane receptors and act via signal transduction pathways.

  • Short half-life in blood.

• Steroid Hormones:

  • Derived from cholesterol; produced in adrenal cortex and gonads.

  • Synthesized on demand, not stored; transported in blood bound to carrier proteins.

  • Longer half-life; act on cytoplasmic or nuclear receptors (genomic effects) or membrane receptors (nongenomic effects).

• Amino Acid-Derived Hormones:

  • Derived from tryptophan (e.g., melatonin) or tyrosine (catecholamines and thyroid hormones).

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

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

Feature	Peptide Hormones	Steroid Hormones	Amine Hormones (Catecholamines)	Amine Hormones (Thyroid Hormones)
Synthesis & Storage	Made in advance; stored in vesicles	Made on demand; not stored	Made in advance; stored in vesicles	Made in advance; stored in vesicles
Release from Parent Cell	Exocytosis	Simple diffusion	Exocytosis	Transport proteins
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; may have nongenomic actions	Activation of second messenger systems	Activation of genes for transcription and translation
Examples	Insulin, parathyroid hormone	Estrogen, cortisol	Epinephrine, norepinephrine	Thyroxine (T4)

7.3 Control of Hormone Release

Hormone release is regulated by reflex pathways involving stimuli, sensors, integration, and responses. The endocrine cell often acts as the sensor in simple reflexes (e.g., parathyroid hormone).

• Neurohormones: Secreted into the blood by neurons (e.g., catecholamines, hypothalamic hormones).

• Simple Endocrine Pathways: Involve direct feedback mechanisms between stimulus and hormone release.

The Pituitary Gland

• Posterior Pituitary: Stores and releases two neurohormones produced in the hypothalamus: antidiuretic hormone (ADH) and oxytocin.

• Anterior Pituitary: True endocrine gland; secretes six hormones:

  • Prolactin (PRL)

  • Thyrotropin (TSH)

  • Adrenocorticotropin (ACTH)

  • Growth hormone (GH)

  • Follicle-stimulating hormone (FSH)

  • Luteinizing hormone (LH)

• Regulation: Controlled by hypothalamic releasing and inhibiting hormones via the hypothalamic-hypophyseal portal system.

Table: Hormones of the Hypothalamic-Anterior Pituitary Pathway

Anterior Pituitary Hormone	Hypothalamic Releasing Hormone	Hypothalamic Inhibiting Hormone
Prolactin (PRL)	Thyrotropin-releasing hormone (TRH)	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): Also called somatotropin; 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.

• Islets of Langerhans:

  • Beta cells: secrete insulin

  • Alpha cells: secrete glucagon

  • D cells: secrete somatostatin

  • PP (F) cells: secrete pancreatic polypeptide

• Insulin-to-Glucagon Ratio: Regulates metabolism

  • Fed state: insulin dominates

  • Fasting state: glucagon dominates

Insulin Promotes Anabolism

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

• 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

• Acts as an antagonist to insulin; prevents hypoglycemia.

• Primary target is the liver, where it stimulates glycogenolysis and gluconeogenesis.

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

7.4 Hormone Interactions

Hormones can interact in complex ways to regulate physiological processes.

• Permissiveness: One hormone allows another to exert its full effect.

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

• Antagonism: One hormone opposes the action of another (e.g., insulin and glucagon).

7.5 Endocrine Pathologies

Disorders of the endocrine system can result from abnormal hormone secretion or tissue responsiveness.

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

• Hyposecretion: Deficient hormone, often due to decreased synthesis or atrophy; absence of negative feedback leads to overproduction of trophic hormones.

• Abnormal Tissue Responsiveness: Can result from down-regulation of receptors or defects in signal transduction pathways.

• Diagnosis: Pathologies are classified as primary (last gland in pathway), secondary (pituitary), or tertiary (hypothalamus).

Summary Table: Types of Endocrine Pathology

Type	Origin	Example
Primary	Last endocrine gland in pathway	Adrenal cortex tumor causing excess cortisol
Secondary	Pituitary gland	Pituitary adenoma causing excess ACTH
Tertiary	Hypothalamus	Hypothalamic dysfunction affecting CRH

Additional info: This summary integrates figures and tables referenced in the slides, providing a comprehensive overview suitable for exam preparation in Anatomy & Physiology.