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Endocrine SystemHormones

Chapter 45.

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Regulation Why are hormones needed?

homeostasis animals = fine-tuned, complex, multi-

functioning systems communication needed to coordinate

whole body functions metabolic rate growth maturation reproduction

growth hormonesprogesterone estradiol

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Regulation & Communication Animals rely on 2 systems for regulation

endocrine system ductless gland which secrete

chemical signals directly into blood chemical travels to target tissue slow, long-lasting response

nervous system system of neurons, central

nerve system transmits “electrical” signal to

target tissue fast, short-lasting response

Hormones coordinate slower but longer–acting responses to stimulisuch as stress, dehydration, and low blood glucose levels.Hormones also regulate long–term developmental processes byinforming different parts of the body how fast to grow or when todevelop the characteristics that distinguish male from female orjuvenile from adult. Hormone–secreting organs, called endocrineglands, are referred to as ductless glands because they secrete theirchemical messengers directly into extracellular fluid. From there, thechemicals diffuse into the circulation.

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Neurosecretory cells Overlap between endocrine & nervous

system regulation specialized nerve cells which release

hormones into blood hypothalamus adrenal gland

epinephrine

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Receptor detects change in internal orexternal stimulus & informs control center

Control center sends out signal hormone or neurohormone

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Homeostasis Negative feedback

stimulus triggerscontrol mechanismthat counteractsfurther change body temperature sugar metabolism

Positive feedback stimulus triggers

control mechanismthat amplifies effect lactation

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Modes of hormone action ontarget cells Hormone binds to

cell membrane protein hormones signal

transduction Hormone enters

cell & binds toreceptors within cell lipid hormones

steroids

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Protein hormones Communication from the outside

hormone binds to cell membrane receptor triggers multi-step signal-transduction

pathway initiates cellular response

activation of enzyme, change in uptake or secretionof specific molecules, rearrangement of cytoskeleton

regulates transcription

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Melanocyte stimulatinghormone (MSH)Cell-surface receptorsrespond to changing lighttrigger movement ofmelanocytes in frog skinhelping to camouflage thefrog

Triggering internal cellular activity

Another demonstration of the role of cell–surface receptors involveschanges in a frog’s skin color, an adaptation that helps camouflagethe frog in changing light. Skin cells called melanocytes contain thedark brown pigment melanin in cytoplasmic organelles calledmelanosomes. The frog’s skin appears light when melanosomescluster tightly around the cell nuclei and darker when melanosomesspread throughout the cytoplasm. A peptide hormone calledmelanocyte–stimulating hormone controls the arrangement ofmelanosomes and thus the frog’s skin color. Addingmelanocyte–stimulating hormone to the interstitial fluid surroundingthe pigment–containing cells causes the melanosomes to disperse.However, direct microinjection of melanocyte–stimulating hormoneinto individual melanocytes does not induce melanosomedispersion—evidence that interaction between the hormone and asurface receptor is required for hormone action.

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Lipid hormones Communication from the inside

hormone enters through cell membrane steroids hormones / sex hormones

binds to receptor in cytoplasm or nucleus receptor triggers transduction of signal

directly regulates transcription

change in gene expression

Intracellular receptors usually perform the entire task of transducing asignal within a target cell. The chemical signal activates the receptor,which then directly triggers the cell’s response. In almost all cases,the intracellular receptor activated by a lipid–soluble hormone is atranscription factor, and the response is a change in geneexpression.Most intracellular receptors are already located in the nucleus whenthey bind hormone molecules, which have diffused in from thebloodstream. The resulting hormone–receptor complexes bind, inturn, to specific sites in the cell’s DNA and stimulate the transcriptionof specific genes. Some steroid hormone receptors, however, aretrapped in the cytoplasm when no hormone is present. Binding of asteroid hormone to its cytoplasmic receptor forms ahormone–receptor complex that can move into the nucleus andstimulate transcription of specific genes. In both cases, mRNAproduced in response to hormone stimulation is translated into newprotein in the cytoplasm. For example, estrogen induces cells in thereproductive system of a female bird to synthesize large amounts ofovalbumin, the main protein of egg white.

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Cell specific response Interaction of hormones & receptors

based on chemical properties

One chemical signal, different effects. Epinephrine, the primary“fight–or–flight” hormone, produces different responses in differenttarget cells. Responses of target cells may differ if they have differentreceptors for a hormone [compare (a) with (b)]. Target cells with thesame receptor exhibit different responses if they have different signaltransduction pathways and/or effector proteins [compare (b) with (c)].

As with hormones that bind to cell–surface receptors, hormones thatbind to intracellular receptors may exert different effects on differenttarget cells. The estrogen that stimulates a bird’s reproductivesystem to make ovalbumin causes the bird’s liver to make otherproteins. The same hormone also may have different effects indifferent species. For instance, thyroxine produced by the thyroidgland regulates metabolism in humans and other vertebrates. But infrogs, thyroxine has additional effects: it triggers the metamorphosisof a tadpole into an adult, stimulating resorption of the tadpole’s tailand other changes.

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Ductlessglands releasehormones intoblood

Endocrine system

Duct glands = exocrine(tears, salivary)

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Endocrine & Nervous system links Hypothalamus = “master control center”

nervous system receives information from nerves around body

about internal conditions regulates release of hormones from pituitary

Pituitary gland = “master gland” endocrine system secretes broad range of

hormones regulatingother glands

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Hypothalamus& Pituitaryglands Posterior Pituitary

gland extension of

hypothalamus stores & secretes

hypothalamushormones

Nervous system

Production and release of posterior pituitary hormones. Theposterior pituitary gland is an extension of the hypothalamus. Certainneurosecretory cells in the hypothalamus make antidiuretic hormone(ADH) and oxytocin, which are transported to the posterior pituitarywhere they are stored. Nervous signals from the brain trigger releaseof these neurohormones.

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lowosmolarity

highosmolarity

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Hypothalamus & Pituitary glands Anterior Pituitary gland synthesizes

own hormones

Endocrine system

Production and release of anterior pituitary hormones. The releaseof hormones synthesized in the anterior pituitary gland is controlledby hypothalamic tropic hormones. The hypothalamic releasing andinhibiting hormones are secreted by neurosecretory cells into acapillary network within the hypothalamus. These capillaries draininto portal vessels that connect with a second capillary network in theanterior pituitary. Each hormone made in the anterior pituitary issecreted in response to a specific releasing hormone.

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Homology

prolactin

mammals birds amphibiansfish

milkproduction

fatmetabolism

metamorphosis& maturation

salt &water

balance

growthhormone

growth& development

What does this tell you about these hormones?same gene family

The most remarkable characteristic of prolactin (PRL) is the greatdiversity of effects it produces in different vertebrate species. Forexample, prolactin stimulates mammary gland growth and milksynthesis in mammals; regulates fat metabolism and reproduction inbirds; delays metamorphosis in amphibians, where it may alsofunction as a larval growth hormone; and regulates salt and waterbalance in freshwater fishes. This list suggests that prolactin is anancient hormone whose functions have diversified during theevolution of the various vertebrate groups.

Growth hormone (GH) is so similar structurally to prolactin thatscientists hypothesize that the genes directing their productionevolved from the same ancestral gene.

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Thyroid regulates metabolism Hypothalamus

TRH = TSH-releasing hormone Anterior Pituitary

TSH = thyroid stimulating hormone Thyroid

produces T3 & T4 hormones metabolism & development

bone growth mental development metabolic use of energy blood pressure & heart rate muscle tone digestion reproduction

tyrosineiodine

The thyroid gland produces two very similar hormones derived fromthe amino acid tyrosine: triiodothyronine (T3), which contains threeiodine atoms, and tetraiodothyronine, or thyroxine (T4), whichcontains four iodine atoms. In mammals, the thyroid secretes mainlyT4, but target cells convert most of it to T3 by removing one iodineatom. Although both hormones are bound by the same receptorprotein located in the cell nucleus, the receptor has greater affinity forT3 than for T4. Thus, it is mostly T3 that brings about responses intarget cells.

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Diseases of the thyroid Hyperthyroid

Graves disease Hypothyroid

Cretinism

The thyroid gland also has important homeostatic functions. In adultmammals, for instance, thyroid hormones help maintain normalblood pressure, heart rate, muscle tone, digestion, and reproductivefunctions. Throughout the body, T3 and T4are important inbioenergetics, generally increasing the rate of oxygen consumptionand cellular metabolism. Too much or too little of these hormones inthe blood can result in serious metabolic disorders. In humans,excessive secretion of thyroid hormones, known as hyperthyroidism,can lead to high body temperature, profuse sweating, weight loss,irritability, and high blood pressure. The most common form ofhyperthyroidism is Graves’ disease; protruding eyes, caused by fluidaccumulation behind the eyes, are a typical symptom

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GoiterIodine deficiency& feedbackcauses thyroidto enlarge

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Thyroid &Parathyroid Regulating

calciummetabolism

When blood Ca2+ level falls below this set point, parathyroidhormone (PTH) is released. PTH is produced by four smallstructures, the parathyroid glands, that are embedded in the surfaceof the thyroid.PTH raises the level of blood Ca2+ by direct and indirect effects. Inbone, PTH induces specialized cells called osteoclasts todecompose the mineralized matrix of bone and release Ca2+ into theblood. In the kidneys, it directly stimulates reabsorption of Ca2+through the renal tubules. PTH also has an indirect effect on thekidneys, promoting the conversion of vitamin D to its active hormonalform. An inactive form of vitamin D , a steroid–derived molecule, isobtained from food or synthesized in the skin. Activation of vitamin Dbegins in the liver and is completed in the kidneys, a processstimulated by PTH. The active form of vitamin D acts directly on theintestines, stimulating the uptake of Ca2+ from food and thusaugmenting the effect of PTH.A rise in blood Ca2+ level above the set point promotes release ofcalcitonin from the thyroid gland. Calcitonin exerts effects on boneand kidneys opposite to those of PTH and thus lowers the bloodCa2+ level.

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Pancreas Regulates

blood sugar insulin

beta cells reduces

blood sugar glucagon

alpha cells increases

blood sugar

Maintenance of glucose homeostasis by insulin and glucagon. Theantagonistic effects of insulin and glucagon help maintain the bloodglucose level near its set point. A rise in blood glucose level abovethe set point promotes insulin release from the pancreas, leading toremoval of excess glucose from the blood and its storage asglycogen. A fall in blood glucose level below the set point stimulatesthe pancreas to secrete glucagon, which acts on the liver to raise theblood glucose level.Clusters of endocrine cells, the islets of Langerhans , are scatteredthroughout the exocrine tissue of the pancreas. Each islet has apopulation of alpha cells, which produce the hormone glucagon , anda population of beta cells, which produce the hormone insulin. Bothof these protein hormones, like all endocrine signals, are secretedinto the circulatory system.

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Stress response

Stress and the adrenal gland. Stressful stimuli cause thehypothalamus to activate the adrenal medulla via nerve impulses (a)and the adrenal cortex via hormonal signals (b). The adrenal medullamediates short–term responses to stress by secreting thecatecholamine hormones epinephrine and norepinephrine. Theadrenal cortex controls more prolonged responses by secretingcorticosteroids.

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Hormonalregulationof insectdevelopment

The hormonal regulation of insect development has been studiedextensively. Three hormones play major roles in molting andmetamorphosis into the adult form.Brain hormone, produced by neurosecretory cells in the insect brain,stimulates the release of ecdysone from the prothoracic glands, apair of endocrine glands just behind the head. Ecdysone promotesmolting and the development of adult characteristics, as in thechange from a caterpillar to a butterfly. Brain hormone and ecdysoneare balanced by the third hormone in this system, juvenile hormone.Juvenile hormone is secreted by a pair of small endocrine glandsjust behind the brain, the corpora allata (singular, corpus allatum),which are somewhat analogous to the anterior pituitary gland invertebrates. As its name suggests, juvenile hormone promotes theretention of larval (juvenile) characteristics.In the presence of a relatively high concentration of juvenile hormone,ecdysone can still stimulate molting, but the product is simply alarger larva. Only when the level of juvenile hormone wanes canecdysone–induced molting produce a developmental stage called apupa. Within the pupa, metamorphosis replaces larval anatomy withthe insect’s adult form. Synthetic versions of juvenile hormone arenow being used as insecticides to prevent insects from maturing intoreproducing adults.