Endocrine System Overview

Endocrine System

• Endocrine and nervous systems communicate in the body
• Nervous system is faster than the endocrine system
• Endocrine response is not always slower; examples include norepinephrine and insulin
• Endocrine system functions include homeostasis, growth regulation, reproduction regulation, and maturation regulation

Hormone Classification

• Steroid Hormone: Derived from cholesterol, insoluble in blood, requiring protein transporters
• Protein Hormone: Protein in nature, soluble in blood, does not need transporters, hydrophobic-lipophilic/hydrophilic-lipophilic, act on intracellular or extracellular receptors.

Types of Hormones

• Thyroid Hormone: Crosses cell membranes, stored in follicles, transported in blood via proteins, act on intracellular receptors, long half-life

• Catecholamines: Do not cross cell membranes, stored in granules, transported in blood freely or loosely bound to proteins, act on extracellular receptors, short half-life

• Amines: Hormones derived from tyrosine; act on intracellular (nuclear) or extracellular receptors

Hormone Receptors

• Function as glycoprotein structures that bind with hormones (lock-and-key theory)
• Physiological conditions, other hormones, and hormones acting on receptors affect hormone quality, quantity, and sensitivity

Hormone Regulation

• Example of Up-regulation: During parturition, oxytocin hormone leads to an increase in oxytocin receptors as estrogen hormone also increases receptor sensitivity.
• Example of Down-regulation: Insulin regulation decreases receptor synthesis and decreases receptor numbers when hormone levels are high

Hormone Action

• Autocrine action: Hormone acts within the organ of synthesis (e.g., testosterone influencing testes)
• Paracrine action: Hormone acts on nearby cells (e.g., somatostatin affecting nearby cells)
• Endocrine action: Hormone is produced by ductless endocrine glands and released into the blood (e.g., insulin and glucagon secretion)

Mechanism of Action of Protein Hormones

• Hormone is the first messenger, binding with receptors to start biochemical reactions
• Second messengers form, e.g., cAMP (adenyl cyclase), phosphatidyl inositol system, calcium calmodulin system, and tyrosine kinase system
• Examples of hormones using this pathway include epinephrine, norepinephrine, glucagon, and calcitonin

Mechanism of Action of Steroid and Thyroid Hormones

• Hormone binds with intracellular receptors, forming a hormone-receptor complex (HRC) to cause DNA sequence changes
• Receptors have ligand-binding, DNA-binding, and N-terminal domains
• This affects the transcription of specific genes, producing a specific protein.

Phosphatidyl Inositol System

• Hormone binding triggers conformational changes in HAC
• This activates phospholipase C, hydrolyzing PIP₂ into IP₃ and DAG
• IP₃ mobilizes Ca²⁺ from intracellular stores
• DAG stimulates protein kinase C (PKC)
• PKC phosphorylates enzymes, impacting transcription and causing changes

Calcium Calmodulin System

• In muscle and nerve cells, calcium ions (Ca²⁺) act as second messengers
• Ca²⁺ is released from the ER, and activation of Ca-calmodulin-dependent protein kinase
• Kinase phosphorylates target enzymes.