17.2

Learning Objectives

• Identify the three major classes of hormones based on chemical structure: Understanding these classifications helps in comprehending the mechanisms through which hormones exert their effects.

• Compare and contrast intracellular and cell membrane hormone receptors: Recognizing the distinction between these receptors is crucial for understanding hormone action and response at the cellular level.

• Describe signaling pathways involving cAMP and IP3: Detailed knowledge of these pathways is critical, as they represent key mechanisms by which hormones communicate signals within cells.

• Identify several factors influencing a target cell's response: Recognizing the numerous factors can influence how a target cell responds to hormones is vital for understanding endocrine functions.

• Discuss the role of feedback loops and humoral, hormonal, and neural stimuli in hormone control: Feedback mechanisms are central to maintaining homeostasis in the endocrine system.

Overview of Hormonal Action

• Hormones, the signaling molecules produced by endocrine glands, only affect target cells that have specific receptors, despite their distribution throughout the body's circulatory system.

• Upon binding to their respective receptors, hormones initiate a range of physiological responses critical for various bodily functions including:

  • Human reproduction: Regulating processes like menstrual cycles and pregnancy.

  • Growth and development of body tissues: Promoting cell division and growth.

  • Metabolism: Influencing the rate at which the body converts food into energy.

  • Fluid and electrolyte balance: Maintaining homeostasis of fluids and solutes in the body.

  • Sleep and other bodily functions: Affecting circadian rhythms and overall health.

Major Hormones and Their Effects

Endocrine Glands and Associated Hormones

Types of Hormones

• Hormones can be grouped by chemical structure, which determines how they function:

  • Amino Acid-derived: This category includes amines, peptides, and proteins.

  • Lipid-derived: Comprising steroid hormones, which are derived from cholesterol.

Amine Hormones

• Amine hormones are synthesized from the amino acids tryptophan or tyrosine.

  • Tryptophan example: Melatonin, which helps regulate circadian rhythms.

  • Tyrosine examples: Includes thyroid hormones and catecholamines such as epinephrine, norepinephrine, and dopamine, which have critical roles in the body's response to stress.

Peptide and Protein Hormones

• These hormones are composed of varying numbers of amino acids:

  • Peptide Hormones: Typically consist of short chains of amino acids (e.g., Antidiuretic hormone).

  • Protein Hormones: Longer chains of amino acids/polypeptides (e.g., Growth hormone, Follicle-stimulating hormone).

Steroid Hormones

• Derived from cholesterol, steroid hormones include examples like testosterone, estrogens, aldosterone, and cortisol.

• They are fat-soluble, require transport proteins in the blood to travel, and generally have a longer half-life compared to amino acid-derived hormones.

Pathways of Hormone Action

Intracellular Receptors

• Intracellular receptors are located within the cell, specifically designed to bind lipid-soluble hormones (e.g., steroid hormones) that can diffuse readily through cellular membranes.

• Upon binding, a hormone-receptor complex is formed, which initiates targeted gene transcription, influencing protein synthesis and cellular function.

Cell Membrane Receptors

• Water-soluble hormones cannot diffuse through the cellular membrane; instead, they bind to surface receptors.

• The binding of these hormones as first messengers activates G proteins, which then initiate second messenger signaling cascades, such as the cAMP pathway.

Functions of cAMP Cascade

• The cAMP cascade results in phosphorylation of proteins and can trigger a variety of cellular responses that depend on the specific cell type and physiological context.

Examples of Hormones Using cAMP and Calcium Ions

• cAMP hormones: Examples include calcitonin, glucagon, and thyroid-stimulating hormone; these hormones utilize cAMP as a second messenger to exert their effects.

• Calcium ions: Activated by phospholipase C, generating diacylglycerol (DAG) and inositol triphosphate (IP3), resulting in Ca2+ release which modulates protein kinases involved in diverse cellular processes.

Factors Affecting Target Cell Response

• Responses of target cells to hormones rely on multiple factors:

  • Downregulation: Occurs when there is an excess of hormone, leading to a decrease in receptor numbers to maintain sensitivity.

  • Upregulation: Happens when hormone levels are low, triggering an increase in receptor numbers to enhance sensitivity.

Hormonal Interactions

• Hormonal interactions can notably influence the effects of hormones:

  • Permissive Effect: Occurs when the presence of one hormone enables another's action (e.g., thyroid hormones facilitate reproductive hormones).

  • Synergistic Effect: When two hormones together produce a more significant effect than either hormone alone (e.g., FSH and estrogens).

  • Antagonistic Effect: When hormones have opposing actions (e.g., insulin lowering blood glucose, while glucagon raises it).

Regulation of Hormone Secretion

• Hormone secretion is tightly controlled through feedback loops:

  • Positive Feedback: A mechanism where additional hormone release occurs in response to previous hormone levels (e.g., oxytocin during childbirth).

  • Negative Feedback: A mechanism in which hormone secretion is inhibited in response to sufficient levels of that hormone (e.g., regulation of glucocorticoids).

Role of Endocrine Gland Stimuli

Types of Stimuli

• Hormone secretion can be triggered by various stimuli:

  • Humoral Stimuli: Changes in blood levels of ions or nutrients trigger hormone release (e.g., ADH released in response to increased osmolarity).

  • Hormonal Stimuli: Release of hormones in response to other hormones (e.g., releasing and inhibiting hormones from the hypothalamus).

  • Neural Stimuli: Hormone release can also be activated via neural signals (e.g., the sympathetic nervous system stimulating adrenal glands during stress).