Hormones and the Endocrine System: Structure, Function, and Pathology

7.1 Hormones

Basic Ways Hormones Act on Target Cells

Hormones are chemical messengers that regulate physiological processes by acting on specific target cells. They exert their effects through several fundamental mechanisms:

• Endocrine signaling: Hormones are secreted into the bloodstream and travel to distant target cells.

• Paracrine signaling: Hormones act locally by diffusing to nearby cells.

• Autocrine signaling: Hormones act on the same cell that secreted them.

Additional info: Candidate hormones are substances suspected to act as hormones but not yet fully characterized.

Defining Hormones and Their Actions

• Hormone: A chemical substance produced by specialized cells, released into the blood, and transported to distant organs to regulate physiology and behavior.

• Endocrine gland: A group of cells that secrete hormones directly into the bloodstream.

• Types of cells/tissues that secrete hormones: Endocrine glands (e.g., pituitary, thyroid), isolated endocrine cells, neurons (neurohormones), and immune cells.

• Pheromones: Chemical signals released into the environment to affect other individuals of the same species.

Hormone Transport and Concentration

• Hormones are transported via the bloodstream to distant targets.

• They exert effects at very low concentrations (typically nanomolar to picomolar).

Hormone-Receptor Interactions

• Hormones act by binding to specific receptors on or in target cells.

• Cellular mechanism of action: The process by which hormone-receptor binding initiates a cellular response, often involving second messengers.

Termination of Hormone Action

• Hormone action must be terminated to prevent overstimulation; this occurs via enzymatic degradation, uptake by cells, or excretion.

• Half-life: The time required for the concentration of a hormone in the blood to decrease by half.

7.2 The Classification of Hormones

Chemical Classes of Hormones

Hormones are classified based on their chemical structure:

• Peptide/protein hormones

• Steroid hormones

• Amine hormones

Peptide Hormones

• Made from chains of amino acids (building blocks).

• Synthesized as preprohormones, processed to prohormones, and then to active hormones.

• Stored in secretory vesicles and released by exocytosis.

• Undergo posttranslational modification.

• Lipophobic (hydrophilic), dissolve easily in plasma.

• Short half-life (minutes).

• Bind to surface membrane receptors, activating second messenger pathways.

• Target cell response is typically rapid.

Steroid Hormones

• Derived from cholesterol.

• Synthesized in the adrenal cortex, gonads, and placenta.

• Not stored; synthesized on demand.

• Lipophilic, require carrier proteins for transport in blood.

• Longer half-life (hours).

• Bind to intracellular receptors, affecting gene transcription (genomic response).

• Target cell response is slower (hours to days).

• Can also have nongenomic (rapid) effects via membrane receptors.

Amine Hormones

• Derived from amino acids: tyrosine (catecholamines, thyroid hormones) and tryptophan (melatonin).

• Examples: Epinephrine, norepinephrine, dopamine, thyroid hormones (T3, T4), melatonin.

7.3 Control of Hormone Release

Simple Endocrine Reflexes

• Components: sensor (endocrine cell), integrating center, effector (target tissue).

• Example: Parathyroid hormone pathway regulates blood calcium levels.

Neurohormones

• Secreted by neurons into the blood.

• Three major groups: catecholamines (adrenal medulla), hypothalamic neurohormones (posterior pituitary), and hypothalamic releasing/inhibiting hormones (anterior pituitary).

Pituitary Gland Structure and Function

• Composed of two fused glands: anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

• Posterior pituitary stores and releases two neurohormones: oxytocin and vasopressin (antidiuretic hormone, ADH).

• Neurohormones are synthesized in the hypothalamus and transported to the posterior pituitary.

• Anterior pituitary synthesizes and releases six hormones: growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

• Trophic hormone: A hormone that controls the secretion of another hormone.

Portal System

• Connects hypothalamus and anterior pituitary, allowing efficient hormone transport.

• Advantage: Prevents dilution of hypothalamic hormones, ensuring effective signaling.

Feedback Loops in the Hypothalamic-Pituitary Pathway

• Multiple integrating centers involved.

• Negative feedback regulates hormone levels.

• Long-loop feedback: Hormones from peripheral endocrine glands inhibit hypothalamic and pituitary hormone secretion.

• Short-loop feedback: Pituitary hormones inhibit hypothalamic hormone secretion.

7.4 Hormone Interactions

Types of Hormone Interactions

• Synergistic: Two or more hormones produce a greater effect together than individually.

• Permissive: One hormone allows another to exert its full effect (e.g., thyroid hormone permits reproductive hormones to function).

• Antagonistic: Hormones have opposing effects (e.g., insulin lowers blood glucose, glucagon raises it).

7.5 Endocrine Pathologies

Patterns of Endocrine Pathology

• Hypersecretion: Excess hormone production exaggerates effects.

• Hyposecretion: Diminished or absent hormone production reduces effects.

• Abnormal tissue responsiveness: Target cells fail to respond appropriately due to receptor or signal transduction defects.

Atrophy and Exogenous Hormones

• Atrophy: Decrease in size or function of an endocrine gland due to lack of stimulation.

• Exogenous hormones can suppress endogenous hormone production via negative feedback.

Receptor and Signal Transduction Abnormalities

• Abnormal hormone levels can cause up- or down-regulation of receptors.

• Defects in receptors or signaling pathways can result in endocrine disorders.

Diagnosis of Endocrine Pathology

• Primary pathology: Problem originates in the last endocrine gland in the pathway.

• Secondary pathology: Problem originates in the pituitary or hypothalamus.

• Feedback loops help distinguish between primary and secondary pathologies.

Scenario	Diagnosis	Location of Abnormality
High cortisol, high ACTH, low CRH	Secondary hypersecretion	Pituitary
High cortisol, low ACTH, low CRH	Primary hypersecretion	Adrenal cortex

7.6 Hormone Evolution

Pineal Gland Function

• The pineal gland, once thought to have no function, is now known to secrete melatonin.

• Melatonin regulates circadian rhythms and sleep-wake cycles.