Endocrine System Introduction (PDF)

**An Introduction to the Endocrine System** Endocrine cells and tissues produce about 30 different **[hormones]** (chemical messengers). - Hormones control and coordinate body processes. 18-1 Homeostasis is preserved through intercellular communication by the nervous and endocrine systems. **M...

**An Introduction to the Endocrine System** Endocrine cells and tissues produce about 30 different **[hormones]** (chemical messengers). - Hormones control and coordinate body processes. 18-1 Homeostasis is preserved through intercellular communication by the nervous and endocrine systems. **Mechanisms of Intercellular Communication** There are several mechanisms of intercellular communication... **[Direct communication:]** exchange of ions and molecules between adjacent cells across gap junctions between **two cells of the same type.** - Highly specialized and relatively rare. **[Paracrine communication:]** chemical signals transfer information from **cell to cell within a single tissue.** **[Autocrine communication:]** messages affect the **same cells that secrete them.** - **[Autocrine:]** the chemicals involved in autocrine communication. - Ex: *prostaglandins secreted by smooth muscle cells cause the same cells to contract.* **[Endocrine communication:]** cells release hormones that are transported in bloodstream. - Alters metabolic activities of many organs. **[Target cells:]** have receptors needed to bind and "read" hormonal messages. **[Hormones:]** chemical messengers. - Change types, quantities, or activities of enzymes and structural proteins in target cells. - Can alter metabolic activities of multiple tissues and organs at the same time. - Affect long-term processes like growth and development. **[Synaptic communication:]** neurons release neurotransmitters at a synapse. - Leads to action potentials that are propagated along axons. - Allows for high-speed "messages" to reach specific destinations. - Ideal for crisis management. **Comparisons of Endocrine and Nervous Communication** **BOTH** endocrine and nervous systems... - Rely on release of chemicals that bind to specific receptors on target cells. - Share chemical messengers such as, norepinephrine and epinephrine. - Regulated mainly by negative feedback. - Function to preserve homeostasis by coordinating and regulating activities. The endocrine system includes all endocrine cells and tissues that produce hormones or paracrines. - Endocrine cells release secretions into extracellular fluid. - Unlike exocrine cells. - Endocrine organs are scattered throughout body. 18-2 The endocrine systemphysiological procoesses by releasing bloodborne hormones that bind to receptors on remote target organs. **[Classes of Hormones]** There are 3 classes of homrones: amino acid derivatives, peptide hormones, and lipid derivatives. **[1.Amino Acid Derivatives (Biogenic Amines)]** - Small molecules structurally related to amino acids. Amino acid derivative hormones can either be synthesized from tyrosine or tryptophan. **Derivatives of [tyrosine]** - Thyroid hormones. - Catecholamines (epinephrine, norepinephrine, and dopamine). **Derivatives of [tryptophan]** - Serotonin and melatonin. **[2. Peptide Hormones]** - Chains of amino acids. - Most are synthesized as **[prohormones]**. - Inactive molecules converted to active hormones before or after they are secreted. We can divide peptide hormones into 2 groups: glycoproteins and short polypeptides/small proteins. **Glycoproteins** - Proteins more than 200 amino acids long that have carbohydrate side chains Ex. *TSH, LH, FSH.* **Short Polypeptides** - ADH and OXT are each 9 amino acids long. **Small Proteins** - Insulin (51 amino acids). - GH (191 amino acids). - PRL (198 amino acids). Includes all hormones secreted by hypothalamus, heart, thymus, digestive tract, pancreas, posterior lobe of the pituitary gland, etc. **[3.Lipid Derivatives]** There are 2 groups of lipid derivatives: eicosanoids and steroid hormones. **Eicosanoids** **[Eicosanoids:]** a signaling molecule, derived from **[arachidonic acid]**, a 20-carbon fatty acid. Eicosanoids include... - Leukotrienes. - **[Prostaglandins:]** coordinate local cellular activities. - Thromboxanes. - Prostacyclins. **Steroid Hormones** **[Steroid hormones:]** derived from cholesterol. - Bound to specific transport proteins in the plasma. - Remain in circulation longer than peptide hormones. Steroid hormones include... - Androgens from testes in males. - Estrogens and progesterone from ovaries in females. - Corticosteroids from adrenal cortex. - Calcitriol from kidneys. **Transport and Inactivation of Hormones** Hormones may circulate freely, or travel bound to special carrier proteins. - Free hormones remain functional for less than an hour and are inactivated when they... - Diffuse out of bloodstream and bind to receptors on target cells. - Absorbed and broken down by liver or kidneys. - Broken down by enzymes in blood or interstitial fluids. - Thyroid and steroid hormones. - Remain functional much longer. - More than 99 percent become attached to special transport proteins in blood. - Equilibrium state exists between free and bound forms. - Bloodstream contains a substantial reserve of bound hormones. **Mechanisms of Hormone Action** Binding of a hormone may... - Alter genetic activity. - Alter rate of protein synthesis. - Change membrane permeability. To affect a target cell, a hormone must FIRST interact with an appropriate receptor... **[Hormone receptor:]** protein molecule to which a particular molecule binds strongly. - Different tissues have different combinations of receptors. - Presence or absence of a specific receptor determines hormonal sensitivity of a cell. The presence or absence of a hormone can also affect the nature and number of hormone receptor proteins. **[Down-regulation:]** presence of a hormone triggers a [decrease] in the number of hormone receptors. - When levels of a particular hormone are **[HIGH]**, cells become **[LESS]** sensitive to it. **[Up-regulation:]** absence of a hormone triggers an [increase] in the number of hormone receptors. - When levels of a particular hormone are **[LOW]**, cells become **[MORE]** sensitive to it. Hormone receptors are either located on the plasma membrane (extracellular response) or within target cells (intracellular response). These different types of receptors respond with different mechanisms. **Extracellular Receptors** - Catecholamines and peptide hormones are... - Not lipid soluble. - Unable to penetrate plasma membrane. - Bind to receptor proteins on outer surface of plasma membrane (extracellular receptors). **Intracellular Receptors** - Steroid and thyroid hormones are... - Lipid soluble. - Diffuse across plasma membrane and bind to receptors inside cell (intracellular receptors). **Hormones and Extracellular Receptors: Second Messengers** **[First messenger:]** hormone that binds to extracellular receptor. - Promotes release of second messenger in cell. **[Second messenger:]** intermediary molecule that appears due to hormone--receptor interaction. - May act as enzyme activator, inhibitor, or cofactor. - Results in change in rates of metabolic reactions. - Example: cAMP, cGMP, Ca^2+^. **Process of Amplification** - When a small number of hormone molecules binds to extracellular receptors, thousands of second messengers may appear. - Magnifies effect of hormone on target cell. **G-proteins and cAMP** **[G-protein:]** enzyme complex coupled to membrane receptor. - Protein binds GTP. - Involved in link between first messenger and second messenger. The steps involved in increasing cAMP level, which accelerates metabolic activity of cell... 1. Activated G protein activates **adenylate cyclase**. 2. Adenylate cyclase converts ATP to **cyclic AMP (cAMP)**. 3. cAMP functions as a second messenger. 4. Generally, cAMP activates kinases that phosphorylate proteins. Increase in cAMP level is usually short-lived. - **Phosphodiesterase (PDE)** converts cAMP to AMP. **G-proteins and Ca^2+^** 1. G protein activates **phospholipase C** (**PLC**). 2. Triggers receptor cascade beginning with production of **diacylglycerol** (**DAG**) and **inositol** **triphosphate** (**IP~3~**) from phospholipids. 3. IP**~3~** diffuses into cytoplasm and triggers release of Ca^2+^ from intracellular reserves. 4. Calcium ion channels open due to activation of **protein kinase C** (**PKC**), and Ca^2+^ enters cell. 5. Ca^2+^ binds to **calmodulin**, activating enzymes. **Hormones and Intracellular Receptors** - Steroid hormones can alter rate of DNA transcription in nucleus. - Alterations in synthesis of enzymes or structural proteins. - Directly affect activity and structure of target cell. - Thyroid hormones bind to receptors within nucleus and on mitochondria. - Activate genes or change rate of transcription. - Increase rates of ATP production.