The Endocrine System: Part A

The Endocrine System: Part A
16 The Endocrine System: Part A

Endocrine System: Overview*
Acts with the nervous system to coordinate and integrate the activity of body cells Influences metabolic activities by means of hormones transported in the blood Responses occur more slowly but tend to last longer than those of the nervous system Endocrine glands: pituitary, thyroid, parathyroid, adrenal, and pineal glands

Endocrine System: Overview*
Some organs produce both hormones and exocrine products (e.g., pancreas and gonads) The hypothalamus has both neural and endocrine functions Other tissues and organs that produce hormones include adipose cells, thymus, cells in the walls of the small intestine, stomach, kidneys, and heart

Pineal gland Hypothalamus Pituitary gland Thyroid gland
Parathyroid glands (on dorsal aspect of thyroid gland) Thymus Adrenal glands Pancreas Ovary (female) Testis (male) Figure 16.1

Chemical Messengers* Hormones: long-distance chemical signals that travel in the blood or lymph Autocrines: chemicals that exert effects on the same cells that secrete them Paracrines: locally acting chemicals that affect cells other than those that secrete them Autocrines and paracrines are local chemical messengers and will not be considered part of the endocrine system

Electrical signal triggers release of neurotransmitter.
Figure 11.5 Local signaling Target cells Electrical signal triggers release of neurotransmitter. Neurotransmitter diffuses across synapse. Secreting cell Secretory vesicles Target cell Local regulator (a) Paracrine signaling (b) Synaptic signaling Long-distance signaling Endocrine cell Target cell specifically binds hormone. Figure 11.5 Local and long-distance cell signaling by secreted molecules in animals Hormone travels in bloodstream. Blood vessel (c) Endocrine (hormonal) signaling

Chemistry of Hormones*
Two main classes 1. Amino acid-based hormones Amines, thyroxine, peptides, and proteins 2. Steroids Synthesized from cholesterol Gonadal and adrenocortical hormones

Mechanisms of Hormone Action*
Hormone action on target cells Alter plasma membrane permeability of membrane potential by opening or closing ion channels Stimulate synthesis of proteins or regulatory molecules Activate or deactivate enzyme systems Induce secretory activity Stimulate mitosis

Two mechanisms, depending on their chemical nature Water-soluble hormones (all amino acid–based hormones except thyroid hormone) Cannot enter the target cells Act on plasma membrane receptors Coupled by G proteins to intracellular second messengers that mediate the target cell’s response

Lipid-soluble hormones (steroid and thyroid hormones) Act on intracellular receptors that directly activate genes

Couple of things to remember so the following slides don’t sound like Greek
-ase on a word means it is an enzyme To phosphorylate a molecule means to add a phosphate group to it (PO4) A kinase is an enzyme that phosphorylates something, i.e. adds a phosphate group.

Plasma Membrane Receptors and Second-Messenger Systems*
cAMP signaling mechanism Hormone (first messenger) binds to receptor Receptor activates G protein G protein activates adenylate cyclase Adenylate cyclase converts ATP to cAMP (second messenger) cAMP activates protein kinases

Plasma Membrane Receptors and Second-Messenger Systems
Plasma Membrane Receptors and Second-Messenger Systems* (For water based) 6. cAMP signaling mechanism Activated kinases phosphorylate various proteins, activating some and inactivating others cAMP is rapidly degraded by the enzyme phosphodiesterase Intracellular enzymatic cascades have a huge amplification effect

Hormone (1st messenger) binds receptor. Extracellular fluid
Adenylate cyclase G protein (GS) 5 cAMP activates protein kinases. Receptor GDP Inactive protein kinase Active protein kinase 2 Receptor activates G protein (GS). 3 G protein activates adenylate cyclase. Adenylate cyclase converts ATP to cAMP (2nd messenger). 4 Hormones that act via cAMP mechanisms: Triggers responses of target cell (activates enzymes, stimulates cellular secretion, opens ion channel, etc.) Epinephrine ACTH FSH LH Glucagon PTH TSH Calcitonin Cytoplasm Figure 16.2

Hormones that act via cAMP mechanisms: Epinephrine ACTH FSH LH Glucagon PTH TSH Calcitonin Cytoplasm Figure 16.2, step 1

G protein (GS) Receptor GDP 2 Receptor activates G protein (GS). Hormones that act via cAMP mechanisms: Epinephrine ACTH FSH LH Glucagon PTH TSH Calcitonin Cytoplasm Figure 16.2, step 2

Adenylate cyclase G protein (GS) Receptor GDP 2 Receptor activates G protein (GS). 3 G protein activates adenylate cyclase. Hormones that act via cAMP mechanisms: Epinephrine ACTH FSH LH Glucagon PTH TSH Calcitonin Cytoplasm Figure 16.2, step 3

Adenylate cyclase G protein (GS) Receptor GDP 2 Receptor activates G protein (GS). 3 G protein activates adenylate cyclase. 4 Adenylate cyclase converts ATP to cAMP (2nd messenger). Hormones that act via cAMP mechanisms: Epinephrine ACTH FSH LH Glucagon PTH TSH Calcitonin Cytoplasm Figure 16.2, step 4

Adenylate cyclase G protein (GS) 5 cAMP activates protein kinases. Receptor GDP Inactive protein kinase Active protein kinase 2 Receptor activates G protein (GS). 3 G protein activates adenylate cyclase. Adenylate cyclase converts ATP to cAMP (2nd messenger). 4 Hormones that act via cAMP mechanisms: Triggers responses of target cell (activates enzymes, stimulates cellular secretion, opens ion channel, etc.) Epinephrine ACTH FSH LH Glucagon PTH TSH Calcitonin Cytoplasm Figure 16.2, step 5

The Three Stages of Cell Signaling: A Preview
Earl W. Sutherland discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals went through three processes Reception Transduction Response

In reception, the target cell detects a signaling molecule that binds to a receptor protein on the cell surface In transduction, the binding of the signaling molecule alters the receptor and initiates a signal transduction pathway; transduction often occurs in a series of steps In response, the transduced signal triggers a specific response in the target cell

EXTRACELLULAR FLUID CYTOPLASM Plasma membrane Reception Receptor
Figure EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception Receptor Figure Overview of cell signaling (step 1) Signaling molecule

EXTRACELLULAR FLUID CYTOPLASM Plasma membrane Reception Transduction
Figure EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction Receptor 1 2 3 Relay molecules Figure Overview of cell signaling (step 2) Signaling molecule

Activation of cellular response 1 2 3
Figure EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response 1 2 3 Relay molecules Figure Overview of cell signaling (step 3) Signaling molecule

Concept 11.2: Reception: A signaling molecule binds to a receptor protein, causing it to change shape The binding between a signal molecule (ligand) and receptor is highly specific A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins

Receptors in the Plasma Membrane
G protein-coupled receptors (GPCRs) are the largest family of cell- surface receptors Most water-soluble signal molecules bind to specific sites on receptor proteins that span the plasma membrane

There are three main types of membrane receptors
G protein-coupled receptors Receptor tyrosine kinases Ion channel receptors

G protein-coupled receptors (GPCRs) are cell surface transmembrane receptors that work with the help of a G protein G proteins bind the energy-rich GTP G proteins are all very similar in structure GPCR systems are extremely widespread and diverse in their functions

Receptor tyrosine kinases (RTKs) are membrane receptors that attach phosphates to tyrosines
A receptor tyrosine kinase can trigger multiple signal transduction pathways at once Abnormal functioning of RTKs is associated with many types of cancers

A ligand-gated ion channel receptor acts as a gate when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor

Signaling molecule binding site
Figure 11.8a Signaling molecule binding site Segment that interacts with G proteins Figure 11.8a Exploring cell-surface transmembrane receptors (part 1: GPCR ribbon model) G protein-coupled receptor

G protein-coupled receptor Plasma membrane Activated receptor
Figure 11.8b Signaling molecule Inactive enzyme G protein-coupled receptor Plasma membrane Activated receptor GTP GDP GDP G protein (inactive) GTP CYTOPLASM Enzyme GDP 1 2 Activated enzyme GTP Figure 11.8b Exploring cell-surface transmembrane receptors (part 2: GPCR signaling) GDP P i Cellular response 3 4

G protein-coupled receptor
Figure 11.8ba G protein-coupled receptor Plasma membrane GDP G protein (inactive) CYTOPLASM Enzyme 1 Signaling molecule Inactive enzyme Activated receptor Figure 11.8ba Exploring cell-surface transmembrane receptors (part 2a: GPCR signaling, activation) GTP GDP GTP GDP 2

3 4 Activated enzyme Cellular response GTP GDP P Figure 11.8bb
Figure 11.8bb Exploring cell-surface transmembrane receptors (part 2b: GPCR signaling, cellular response and hydrolysis) GDP P i 4

 helix in the membrane CYTOPLASM 1 2 3 4 Signaling molecule (ligand)
Figure 11.8c Signaling molecule (ligand) Signaling molecule Ligand-binding site  helix in the membrane Tyr Tyr Tyr Tyr Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosine kinase proteins (inactive monomers) Dimer CYTOPLASM 1 2 Activated relay proteins Figure 11.8c Exploring cell-surface transmembrane receptors (part 3: RTKs) Cellular response 1 Tyr Tyr P Tyr P Tyr P Tyr P Tyr Tyr Tyr P Tyr Tyr P Tyr P Tyr P Cellular response 2 Tyr Tyr P P Tyr Tyr P 6 ATP 6 ADP Tyr Tyr P Activated tyrosine kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine kinase (phos- phorylated dimer) Inactive relay proteins 3 4

Signaling molecule (ligand)
Figure 11.8ca Signaling molecule (ligand) Ligand-binding site  helix in the membrane Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Figure 11.8ca Exploring cell-surface transmembrane receptors (part 3a: RTKs, inactive) Receptor tyrosine kinase proteins (inactive monomers) CYTOPLASM 1

Signaling molecule Dimer 2 Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr
Figure 11.8cb Signaling molecule Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Figure 11.8cb Exploring cell-surface transmembrane receptors (part 3b: RTKs, dimerization) Dimer 2

Activated tyrosine kinase regions (unphosphorylated dimer)
Figure 11.8cc Tyr Tyr P Tyr Tyr P Tyr Tyr P Tyr Tyr P Tyr Tyr P Tyr Tyr P 6 ATP 6 ADP Activated tyrosine kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine kinase (phos- phorylated dimer) Figure 11.8cc Exploring cell-surface transmembrane receptors (part 3c: RTKs, phosphorylation) 3

Activated relay proteins
Figure 11.8cd Activated relay proteins Cellular response 1 P Tyr Tyr P P Tyr Tyr P P Cellular response 2 Tyr Tyr P Figure 11.8cd Exploring cell-surface transmembrane receptors (part 3d: RTKs, cellular response) Inactive relay proteins 4

Figure 11.8d-1 1 Gate closed Ions Signaling molecule (ligand) Plasma membrane Ligand-gated ion channel receptor Figure 11.8d-1 Exploring cell-surface transmembrane receptors (part 4: ion channel receptors, step 1)

Figure 11.8d-2 1 2 Gate open Gate closed Ions Signaling molecule (ligand) Cellular response Plasma membrane Ligand-gated ion channel receptor Figure 11.8d-2 Exploring cell-surface transmembrane receptors (part 4: ion channel receptors, step 2)

Figure 11.8d-3 1 2 Gate open Gate closed Ions Signaling molecule (ligand) Cellular response Plasma membrane Ligand-gated ion channel receptor 3 Gate closed Figure 11.8d-3 Exploring cell-surface transmembrane receptors (part 4: ion channel receptors, step 3)

Signal Transduction Pathways
The binding of a signaling molecule to a receptor triggers the first step in a chain of molecular interactions Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated At each step, the signal is transduced into a different form, usually a shape change in a protein

Protein Phosphorylation and Dephosphorylation
Phosphorylation and dephosphorylation of proteins is a widespread cellular mechanism for regulating protein activity Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation Many relay molecules in signal transduction pathways are protein kinases, creating a phosphorylation cascade

Phosphorylation cascade
Figure 11.10 Signaling molecule Receptor Activated relay molecule Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 ATP Phosphorylation cascade ADP Active protein kinase 2 P PP P i Inactive protein kinase 3 Figure A phosphorylation cascade ATP ADP Active protein kinase 3 P PP P i Inactive protein ATP ADP P Active protein Cellular response PP P i

Inactive protein kinase 1
Figure 11.10a Signaling molecule Activated relay molecule Receptor Inactive protein kinase 1 Active protein kinase 1 Figure 11.10a A phosphorylation cascade (part 1: cascade initiation)

Phosphorylation cascade
Figure 11.10b Phosphorylation cascade Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 ATP ADP P Active protein kinase 2 PP P i Inactive protein kinase 3 Figure 11.10b A phosphorylation cascade (part 2: phosphorylation cascade) ATP ADP P Active protein kinase 3 PP P i

Inactive protein kinase 3
Figure 11.10c Inactive protein kinase 3 ATP ADP P Active protein kinase 3 PP P i Inactive protein ATP P ADP Active protein Cellular response Figure 11.10c A phosphorylation cascade (part 3: cellular response) PP P i

Protein phosphatases rapidly remove the phosphates from proteins, a process called dephosphorylation
This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off or up or down, as required

Small Molecules and Ions as Second Messengers
Many signaling pathways involve second messengers Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion Second messengers participate in pathways initiated by GPCRs and RTKs Cyclic AMP and calcium ions are common second messengers

Cyclic AMP Cyclic AMP (cAMP) is one of the most widely used second messengers Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal

Understanding of the role of cAMP in G protein signaling pathways helps explain how certain microbes cause disease The cholera bacterium, Vibrio cholerae, produces a toxin that modifies a G protein so that it is stuck in its active form This modified G protein continually makes cAMP, causing intestinal cells to secrete large amounts of salt into the intestines Water follows by osmosis and an untreated person can soon die from loss of water and salt

Other pathways regulate the activity of enzymes rather than their synthesis
For example, a signal could cause opening or closing of an ion channel in the plasma membrane, or a change in cell metabolism

Glucose 1-phosphate (108 molecules)
Figure 11.16 Reception Transduction Binding of epinephrine to G protein-coupled receptor (1 molecule) Inactive G protein Active G protein (102 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (102) ATP Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104) Inactive phosphorylase kinase Figure Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine (adrenaline) Response Active phosphorylase kinase (105) Inactive glycogen phosphorylase Glycogen Glucose 1-phosphate (108 molecules) Active glycogen phosphorylase (106)

Glucose 1-phosphate (108 molecules)
Figure 11.16a Reception Binding of epinephrine to G protein-coupled receptor (1 molecule) Response Glycogen Figure 11.16a Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine (adrenaline) (part 1: reception and response) Glucose 1-phosphate (108 molecules)

Active G protein (102 molecules)
Figure 11.16b Transduction Inactive G protein Active G protein (102 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (102) ATP Figure 11.16b Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine (adrenaline) (part 2: transduction, active protein kinase A) Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104)

Inactive protein kinase A
Figure 11.16c Transduction Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104) Inactive phosphorylase kinase Active phosphorylase kinase (105) Figure 11.16c Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine (adrenaline) (part 3: transduction, active glycogen phosphorylase) Inactive glycogen phosphorylase Active glycogen phosphorylase (106)

Signaling pathways can also affect the overall behavior of a cell, for example, a signal could lead to cell division

Regulation of the Response
A response to a signal may not be simply “on” or “off” There are four aspects of signal regulation to consider Amplification of the signal (and thus the response) Specificity of the response Overall efficiency of response, enhanced by scaffolding proteins Termination of the signal

Intracellular Receptors
Intracellular receptor proteins are found in the cytoplasm or nucleus of target cells Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors Examples of hydrophobic messengers are the steroid and thyroid hormones of animals An activated hormone-receptor complex can act as a transcription factor, turning on specific genes

Intracellular Receptors and Direct Gene Activation*
Steroid hormones and thyroid hormone Diffuse into their target cells and bind with intracellular receptors Receptor-hormone complex enters the nucleus Receptor-hormone complex binds to a specific region of DNA This prompts DNA transcription to produce mRNA The mRNA directs protein synthesis

Receptor- hormone complex
Steroid hormone Plasma membrane Extracellular fluid 1 The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor. Cytoplasm Receptor protein Receptor- hormone complex 2 The receptor- hormone complex enters the nucleus. Hormone response elements Nucleus 3 The receptor- hormone complex binds a hormone response element (a specific DNA sequence). DNA 4 Binding initiates transcription of the gene to mRNA. mRNA The mRNA directs protein synthesis. 5 New protein Figure 16.3

Steroid hormone Plasma membrane Extracellular fluid 1 The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor. Cytoplasm Receptor protein Receptor- hormone complex Nucleus Figure 16.3, step 1

Steroid hormone Plasma membrane Extracellular fluid 1 The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor. Cytoplasm Receptor protein Receptor- hormone complex 2 The receptor- hormone complex enters the nucleus. Nucleus Figure 16.3, step 2

Steroid hormone Plasma membrane Extracellular fluid 1 The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor. Cytoplasm Receptor protein Receptor- hormone complex 2 The receptor- hormone complex enters the nucleus. Hormone response elements Nucleus 3 The receptor- hormone complex binds a hormone response element (a specific DNA sequence). DNA Figure 16.3, step 3

Steroid hormone Plasma membrane Extracellular fluid The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor. 1 Cytoplasm Receptor protein Receptor- hormone complex The receptor- hormone complex enters the nucleus. 2 Hormone response elements Nucleus 3 The receptor- hormone complex binds a hormone response element (a specific DNA sequence). DNA 4 Binding initiates transcription of the gene to mRNA. mRNA Figure 16.3, step 4

Steroid hormone Plasma membrane Extracellular fluid 1 The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor. Cytoplasm Receptor protein Receptor- hormone complex 2 The receptor- hormone complex enters the nucleus. Hormone response elements Nucleus 3 The receptor- hormone complex binds a hormone response element (a specific DNA sequence). DNA 4 Binding initiates transcription of the gene to mRNA. mRNA The mRNA directs protein synthesis. 5 New protein Figure 16.3, step 5

Target Cell Specificity*
Target cells must have specific receptors to which the hormone binds ACTH receptors are only found on certain cells of the adrenal cortex Thyroxin receptors are found on nearly all cells of the body

Target Cell Activation*
Target cell activation depends on three factors Blood levels of the hormone Relative number of receptors on or in the target cell Affinity of binding between receptor and hormone

Target Cell Activation*
Hormones influence the number of their receptors Up-regulation—target cells form more receptors in response to the hormone Down-regulation—target cells lose receptors in response to the hormone

Hormones in the Blood* Hormones circulate in the blood either free or bound Steroids and thyroid hormone are attached to plasma proteins All others circulate without carriers The concentration of a circulating hormone reflects: Rate of release Speed of inactivation and removal from the body

Hormones in the Blood* Hormones are removed from the blood by
Degrading enzymes Kidneys Liver Half-life—the time required for a hormone’s blood level to decrease by half

Control of Hormone Release*
Blood levels of hormones Are controlled by negative feedback systems Vary only within a narrow desirable range Hormones are synthesized and released in response to Humoral stimuli (blood) Neural stimuli (nerves) Hormonal stimuli (other hormones)

Humoral Stimuli* Changing blood levels of ions and nutrients directly stimulates secretion of hormones Example: Ca2+ in the blood Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone) PTH causes Ca2+ concentrations to rise and the stimulus is removed

Capillary blood contains low concentration of Ca2+, which stimulates…
(a) Humoral Stimulus Capillary blood contains low concentration of Ca2+, which stimulates… 1 Capillary (low Ca2+ in blood) Thyroid gland (posterior view) Parathyroid glands Parathyroid glands PTH …secretion of parathyroid hormone (PTH) by parathyroid glands* 2 Figure 16.4a

Neural Stimuli* Nerve fibers stimulate hormone release
Sympathetic nervous system fibers stimulate the adrenal medulla to secrete catecholamines

Preganglionic sympathetic fibers stimulate adrenal medulla cells…
(b) Neural Stimulus Preganglionic sympathetic fibers stimulate adrenal medulla cells… 1 CNS (spinal cord) Preganglionic sympathetic fibers Medulla of adrenal gland Capillary …to secrete catecholamines (epinephrine and norepinephrine) 2 Figure 16.4b

Hormonal Stimuli* Hormones stimulate other endocrine organs to release their hormones Hypothalamic hormones stimulate the release of most anterior pituitary hormones Anterior pituitary hormones stimulate targets to secrete still more hormones Hypothalamic-pituitary-target endocrine organ feedback loop: hormones from the final target organs inhibit the release of the anterior pituitary hormones

The hypothalamus secretes hormones that…
(c) Hormonal Stimulus The hypothalamus secretes hormones that… 1 Hypothalamus …stimulate the anterior pituitary gland to secrete hormones that… 2 Pituitary gland Thyroid gland Adrenal cortex Gonad (Testis) …stimulate other endocrine glands to secrete hormones 3 Figure 16.4c

Nervous System Modulation*
The nervous system modifies the stimulation of endocrine glands and their negative feedback mechanisms Example: under severe stress, the hypothalamus and the sympathetic nervous system are activated As a result, body glucose levels rise

The Pituitary Gland and Hypothalamus
The pituitary gland (hypophysis) has two major lobes Posterior pituitary (lobe): Anterior pituitary (lobe) (adenohypophysis)

Pituitary-Hypothalamic Relationships*
Posterior lobe synthesizes the neurohormones oxytocin and antidiuretic hormone (ADH) Neurohormones are transported to the posterior pituitary

Inferior hypophyseal artery
Hypothalamic neurons synthesize oxytocin and ADH. 1 Paraventricular nucleus Hypothalamus Supraoptic nucleus Oxytocin and ADH are transported along the hypothalamic-hypophyseal tract to the posterior pituitary. 2 Optic chiasma Infundibulum (connecting stalk) Inferior hypophyseal artery Hypothalamic- hypophyseal tract Oxytocin and ADH are stored in axon terminals in the posterior pituitary. 3 Axon terminals Oxytocin and ADH are released into the blood when hypothalamic neurons fire. 4 Posterior lobe of pituitary Oxytocin ADH (a) Relationship between the posterior pituitary and the hypothalamus Figure 16.5a

Hypothalamic hormones travel through the portal
Hypothalamus When appropriately stimulated, hypothalamic neurons secrete releasing and inhibiting hormones into the primary capillary plexus. 1 Hypothalamic neuron cell bodies Superior hypophyseal artery Hypophyseal portal system Hypothalamic hormones travel through the portal veins to the anterior pituitary where they stimulate or inhibit release of hormones from the anterior pituitary. 2 • Primary capillary plexus • Hypophyseal portal veins • Secondary capillary plexus Anterior lobe of pituitary Anterior pituitary hormones are secreted into the secondary capillary plexus. 3 TSH, FSH, LH, ACTH, GH, PRL (b) Relationship between the anterior pituitary and the hypothalamus Figure 16.5b

Anterior Pituitary Hormones
Growth hormone (GH) Thyroid-stimulating hormone (TSH) or thyrotropin Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Prolactin (PRL)

Anterior Pituitary Hormones
All are proteins All except GH activate cyclic AMP second-messenger systems at their targets TSH, ACTH, FSH, and LH are all tropic hormones (regulate the secretory action of other endocrine glands)

Growth Hormone (GH)* Stimulates most cells, but targets bone and skeletal muscle Promotes protein synthesis and encourages use of fats for fuel

The Endocrine System: Part A

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