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Physiology: Endocrine > Introduction To Endocrinology > Flashcards

Introduction To Endocrinology Flashcards

1
Q

Nervous system

A
  • Rapid control system
  • Rapid response
  • Effect lasts briefer
  • Acts on specific targets
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2
Q

Endocrine system

A

Slower control system

  • slower response
  • effect lasts longer
  • broader influence (acts also on distant organs)

Is a chemical messenger system, circulatory system= mean of propagation

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3
Q

Endocrine system: functions

A
  • Metabolism
  • Growth= longitudinal and development of mature tissues (GH VS dwarfism)
  • Development
  • Water and electrolyte balance
  • Reproduction (Sex hormones VS lack of sexual development)
  • Behavior

Thyroxine and triiodothyronine VS sluggish reactions body
Insulin VS lack of energy taken up from carbohydrates

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4
Q

Neurotransmitters

A

Are released by the axon terminals of neurons into the synaptic junctions and act locally to control nerve cell functions

Extra: It is a type of chemical messenger which transmits signals across a chemical synapse, such as a neuromuscular junction, from one neuron (nerve cell) to another “target” neuron, muscle cell, or gland cell.

Examples: acetylcholine, GABA, dopamine, serotonin, glutamate, norepinephrine

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5
Q

Endocrine

A

Are released by glands or specialized cells into the circulating blood. Influence the function of target cells at another location in the body

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6
Q

Neuroendocrine

A

Are released by neurons into the circulating blood and influence the function of target cells at another location in the body

Neuroendocrine cells are cells that receive neuronal input (neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release message molecules (hormones) into the blood.

Examples: somatostatin, vasopressin, corticotropin- , growth hormone and thyrotropin- releasing hormones

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7
Q

Paracrine

A

Are released by cells into the extracellular fluid and affect neighboring cells which can be of the same or of a different cell types

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8
Q

Autocrine

A

are secreted by cells into the extracellular fluid and act on the same cell that secretes them

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9
Q

Cytokines

A

Are peptides secreted by cells into the extracellular fluid
Can function as paracrines, autocrines or endocrine hormones

Ex: interleukins secreted often by CD4 T lymphocytes
Adipokines secreted by adipose tissue

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10
Q

Endocrine glands

A
  • Ductless
  • Products are secreted into the blood
  • Can reach distant tissues

Examples: Langerhans cells in the pancreas that secrete insulin and glucagon and somatostatin

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11
Q

Examples of peptide and protein hormones (1), steroid hormones (2) and hormones derivative of the amino acid tyrosine (3)

A

1.Anterior and posterior pituitary gland
Pancreas= insulin and glucagon
Parathyroid gland = parathyroid hormone

2.Adrenal cortex= cortisol and aldosterone
Ovaries= estrogen and progesterone
Testis= testosterone

  1. Thyroid= thyroxine and triiodothyronine
    Adrenal medullas= epinephrine and norepinephrine
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12
Q

Peptide and protein hormones

A

Peptide hormones <100 amino acids
Proteins >= 100 amino acids
Water soluble

Synthesized in the rough end of the ER as larger proteins which are not active (preprohormones)
Then cleaved into the ER to form pro hormones
Transferred to the Golgi apparatus where packed into vesicles (in which enzymes cleave them into a biologically active and and inactive fragment)
Secretion of the contents of the vesicles through exocytosis

Stimulus : increase in cytosolic calcium concentration caused by depolarization of the plasma membrane
Stimulation of an endocrine cell surface receptor—> increased cAMP and activation of protein kinases —> secretion of hormone

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13
Q

Steroid hormones

A

Structure similar to cholesterol
Formed by three cyclohexyl rings and one cyclopentyl ring
Lipid soluble - diffuse through the plasma membrane

Steroid producing endocrine cells have few storage for steroid hormones
BUT large storage of cholesterol esters in cytoplasm vacuoles which after a stimulus are mobilized for steroid synthesis

Cholesterol

  • comes from plasma
  • de novo synthesis
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14
Q

Amine hormones derived from tyrosine

A

Both synthesized by enzymes in the cytoplasmic compartments of glandular cells

Thyroid hormones: synthesized and stored in the thyroid gland, incorporated into thyroglobulin, stored in the thyroid follicles.
When amines are split from thyroglobulin and free hormones are released in the cytoplasm—> hormones enter in the blood. Most of the thyroid hormones combine with plasma proteins especially thyroxine-binding globulin.

Adrenal medullary hormones= epinephrine and norepinephrine
They are taken up into preformed vesicles and stored until secreted.
Released by exocytosis
Can exist in plasma in free form or in conjugation to other substances

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15
Q

Negative feedback

A

After a stimulus causes release of a hormone, conditions or products resulting from the action of the hormone tend to suppress its further release. The controlled variable is sometimes not the secretory rate of the hormone itself but the degree of activity of the target tissue.
Feedback regulation can occur at all levels, including gene transcription and translation involved in synthesis of hormones, steps involved in processing and releasing stored hormones

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16
Q

Negative feedback: examples

Insulin and glucose

A

Eat—> stimulus: rising blood glucose level—> high blood glucose level is detected by insulin- secreting cells of pancreas—>pancreas secretes insulin—> liver cells take up glucose and store it as glycogen and also most body cells take up more glucose—> glucose levels in the blood decrease—> insulin release stops—> return to homeostatic blood glucose level

17
Q

Negative feedback example: TSH and TRH

A

TRH is secreted by hypothalamus—> it is detected by some specific cells of the anterior pituitary—> stimulated by TRH they produce TSH—> TSH is detected by TSH receptors in the thyroid gland—> production of thyroid hormones T3 and T4.
Thyroid hormones exert a negative feedback effect on the hypothalamus and on the pituitary—> reduce production of TSH and TRH (VS hyperthyroidism = The term hyperthyroidism refers to any condition in which there is too much thyroid hormone produced in the body. In other words, the thyroid gland is overactive)

18
Q

Positive feedback

A

The biological action of the hormone causes additional secretion of the hormone

19
Q

Positive feedback: examples

Uterus dilation

A

Baby’s growth takes up all the space in the uterus—> stretch receptors in the uterus detect the dilation—> brain signals for release of oxytocin—> contractions of uterine muscles (they get stronger until the baby is delivered)

20
Q

Positive feedback: examples

Milk release

A

Stimulus: baby suckles at nipples—> suckling sends impulses to the hypothalamus—> hypothalamus signals posterior pituitary to release oxytocin—> stimulates milk ejection—> milk is released and the baby continues to feed.

21
Q

Cyclical variations

A

Superimposed on the negative and positive feedback are periodic variations in hormone release that are influenced by seasonal changes, stages of development and aging, daily cycle and sleep.
In many cases these changes are due to changes in neural pathways which control hormone release

22
Q

Pulsatility

A

Pulsatile secretion is a biochemical phenomenon in which a chemical, such as a hormone, is secreted in a burst-like or episodic manner rather than constantly. Examples of hormones that are secreted pulsatilely include gonadotropin-releasing hormone (GnRH) and growth hormone (GH).

Pulsatility is a fundamental property of the majority of hormone secretion and denotes the recurrence of individual punctuated events interrupting a more or less constant baseline process.

23
Q

Hormone transport

A

The majority of hormones are peptides= hydrophilic
Cathecolamines= hydrophilic
—> they can travel through the bloodstream easily since they are water soluble
They are usually degraded by enzymes in the blood and tissues and rapidly excreted by kidneys and liver

Steroid and thyroid hormones are hydrophobic= lipid soluble
—> to travel through the bloodstream they bind to transport proteins = become water soluble
Only unbound hormones can leave the blood capillary and get to the target cell
Bound hormones are cleared at much slower rates

24
Q

Concentration of a hormone in the blood

A

Two factors can increase or decrease the concentration of a hormone in the blood.

1) Rate of hormone secretion in the blood
2) Rate of removal of hormone in the blood a.k.a metabolic clearance rate

25
Q

Metabolic clearance rate= milliliters of plasma cleared of the hormone per minute

A

Rate of disappearance of the hormone from the plasma/ plasma concentration of the hormone

26
Q

Clearance ways

A

Hormones are cleared from the plasma

1) metabolic destruction by the tissues
2) binding with the tissues
3) excretion by the liver into the bile
4) excretion by the kidneys into the urine
5) sometimes degraded at target cells by endocytosis of cell membrane hormone- receptor complex: the hormone is metabolized in the cell while the receptor is recycled back to the cell membrane

27
Q

Non lipid soluble hormonal action

A

Peptides and cathecolamines have receptors in or on the cell plasma membrane. Therefore they cannot have a direct effect inside the target cell. They use intracellular intermediaries to exert effects.
CASCADE MECHANISM

Among the second messengers ( the intracellular intermediaries) there is cAMP; calcium ions and associated calmodulin; products of membrane phospholipid breakdown

28
Q

The number and sensitivity of hormone receptors are regulated

Downregulation

A

Receptors can be destroyed, inactivated vs manufactured, activated
In general this effect is due to binding of the hormone

The down-regulation can result from
1) inactivation of receptors
2) inactivation of intracellular protein signaling molecules
3) sequestration of the receptor inside the cell (cell membrane receptors)
4) destruction of receptors by lysosomes after being internalized
5) decreased production of receptors
—> tissue’s responsiveness to hormone decreases

The up-regulation can result from
1) activation of receptors
2) manufacture of receptors
—> tissue becomes more sensitive to hormone

30
Q

G protein-linked hormone receptors

A

Receptors that indirectly regulate activity of other proteins (enzymes or ion channels) by coupling with groups of cell membrane proteins (G proteins= heterotrimeric GTP binding proteins)

7 transmembrane segments

STEPS:
Ligand binds to receptor—> change of conformation of GPCR—> activation of G protein through replacement of GDP with GTP in alpha subunit (stimulatory or inhibitory)—> alpha subunit associates to other target proteins and regulates them

31
Q

Ion channel-linked receptors

A

Virtually all the neurotransmitter substances such as acetylcholine and norepinephrine combine with receptors in the post synaptic membrane. This causes a change int eh structure of the receptors either opening or closing the ion channels. The movement of the ions causes the subsequent effects.
Sodium, calcium and potassium ions channels.

32
Q

Enzyme-linked hormone receptors

A

Some receptors, when activated, function directly as enzymes or are closely associated with enzymes that they activate.

Example: leptin
Leptin binds to leptin receptor—> conformational change enables phosphorylation and activation of JAK2 molecules—> phosphorylation of tyrosine residues by JAK2—> phosphorylation of STAT—> transcription by leptin target genes to initiate protein synthesis

Phosphorylation of JAK2 also leads to activation of other intracellular enzyme pathways such as MAPK and PI3K

33
Q

Adenylyl cyclase-cAMP second messenger system

A

Binding of the hormone to the GPCR allows coupling of the receptor to the G protein.

Gs protein= stimulatory G protein
G protein (activated) couples to GPCR—> stimulates adenylyl cyclase—> conversion of ATP to cAMP inside the cell—> activation of cAMP protein dependent kinase—> phosphorylation of specific proteins triggering biochemical reactions in the cells
CASCADE OF ENZYMES

Inhibitory G proteins—> adenylyl cyclase will be inhibited reducing the formation of cAMP—> inhibitory action in the cell

34
Q

Cell membrane phospholipid second messenger system

A

Some hormones activate transmembrane receptors that activate the enzyme phospholipase C—> catalyzes the breakdown of phospholipids in the cell membrane especially PIP2 into inositol triphosphate (IP3) and diacylglycerol (DAG)

IP3 mobilizes calcium ions from mitochondria and ER. Calcium ions have their own second messenger effect

DAG activates PKC. PKC phosphorylates proteins and its lipid portion is the precursor for prostaglandins and other hormones

35
Q

Calcium- calmodulin second messenger system

A

Calcium entry through change in membrane potential that opens calcium channels or through hormone interacting with membrane receptors that open calcium channels

As they enter calcium ions bind to calmodulin (3 out of 4) —> change in conformation—> activation or inhibition of protein kinases—> activation or inhibition of proteins involved in the cell’s response.

Example: myosin light chain kinase- acts on myosin—> contraction of smooth muscle

Normal calcium concentration 10^-8.. when it rises to 10^-6 enough binding occurs to see the action of calmodulin and troponin C (skeletal muscle contraction)

36
Q

Steroid hormones

A

Steps:

1) steroid hormone diffuses across cell membrane and enters the cytoplasm
2) steroid hormone binds to receptor
3) combined receptor protein-hormone diffuses into or is transported in nucleus
4) the combination binds at specific points of DNA strands in the chromosomes which activates the transcription process of specific genes to form mRNA
5) The mRNA diffuses into the cytoplasm, where it promotes the translation promotes the translation

37
Q

Thyroid hormones

A

Thyroxine and Triiodothyronine bind with the receptor proteins in the nucleus ( activated transcription factors) located within the chromosomal complex and they control the function of the gene promoters

Activate genetic mechanisms for the formation of many types of intracellular proteins (enhanced intracellular metabolic activity)

Once bound to their receptors the thyroid hormones can continue to express their control functions for days or even weeks

Physiology: Endocrine (1 decks)

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