Endocrinology/Hormones

THIS IS NOTE IS LARGE AND HAS LOTS OF PICTURES; MOST PICS ARE TAKEN FROM SILVERTHORN TEXTBOOK


 What are the different types of hormones? How are hormones released and controlled? What are the major endocrine organs (glands) and endocrine pathogenesis as well as the interactions between hormones (synergistic or antagonistic)

What are hormones?

Hormones: derived from greek verb ormao = 'to excite or arouse', they are chemical messengers secreted into the blood by specialised cells and generally act on remote organ sites and alter rates of processes in target cells. They act at very low concnetrations nano to picomolar range (10^-9 to 10^-12). They control long-term homeostatic processes:
 Grow, development, metabolism, reproduction and internal environment regulation.
Hormones are produced by endocrine cells and organs - released from endocrine glands. Endocrinology is the study of the endocrine system and hormone action
Hormones act by binding receptors on or in target cells:
- Controlling the rates of enzymatic reactions.
- Controlling the movement of ions or molecules across membranes
- Controlling gene expression and protein synthesis

   Hormones have half-lives; i.e. they act for specific period of time before becoming inactivated.

Abit of history of endocrinology:
   First experiments performed in mid 1800s. 1849 Berthold and his roosters. 1889 C. Brown-Sequard's Viagra. 1889 discovery of insulin by Minkowski. 1891 thyroid hormone replacement. 1922 purified insulin was used in clinical trials

Types of Hormones
 Many different classification schemes:
a. Protein/peptide
b. Steroid (cholesterol derivatives)
c. Amine (tryptophan or tyrosine derivatives)
 OR
i. Lipophilic: penetrate cell membrane readily (steroid hormones and thyroid hormones)
ii. Lipophobic: do not penetrate cell membrane (peptides and catecholamines)

Look at the figure to the right to see the different hormones - must know ALL (espcially the most important - thyroid, parathyroid, adrenal, pancreas, hypothalamus-pituitary, gonads)









A. PROTEIN AND PEPTIDE HORMONES
These are the most common types of hormones.
- preprohormone - large inactive
- prohormone - posttranslational modification
- peptide hormone-receptor complex - signal transduction system
Stored in secretory vesicles and secreted by exocytosis. Dissolved in blood for transport. Act on cell surface receptors - do not penetrate target cell by diffusion.
Rapid onset, short duration and short half-life in the blood - usually inactivated by proteases.
Not active orally. This is because they are proteins/peptides and are digested in the gut (so administration is other than oral cavity - injections!?)

More is discussed below about hormone receptors


B. STEROID HORMONES
- Cholesterol-derived synthesized in the smooth ER. Synthesize as needed from precursors. Lipophilic and can diffuse across membranes and are not stored in secretory cell.

They transported through the blood with carrier proteins. They bind on intracellular receptor (cytoplasmic or nuclear receptors (mostly)) which recognize specific steroids. Steroid hormones induce their effects by altering transcription - they activate or repress gene expression for protein synthesis. They are slower acting and have long duration and longer half-life. May be active orally  Examples: cortisol, estrogen, testosterone

The pineal gland, the thalamus and corpus callosum

HORMONE RECEPTORS (some things are mentioned below again; this area must be cleaned!)
    Hormones work by binding to specific receptors on or in target cells. Receptors are specific for hormone(s). Presence of receptor is necessary for response in cell. Cells have multiple receptors for different hormones. Some hormones have more than one type of receptor; e.g. multiple types adrenergic receptor.
They can get up or down regulation of receptors. Little exposure to hormone - upregulation; e.g. alloxan diabetes. Lot of exposure to a hormone - down regulation - type 2 diabetes?
Agonists: substances that bind to receptor and mimic effect of a hormone. Antagonists are substances that bind to receptor but do not stimulate, so block the effect of a natural hormone. Partial agonists bind and havve low activity, phytoestrogens in soya. Many drugs act as hormone agonists/antagonists

Types of hormone receptors
1.  Intracellular receptors that affect mRNA transcription
2.  G-protein coupled receptors
3.  Tyrosine Kinase associated receptors (RTK)

1. Intracellular receptors
     Lipophilic hormone enters cell by simple diffusion. It binds to receptor in nucleus or cytoplasm (translocated to nucleus). Binding induces conformational change and form receptor dimers. The activated receptor has a DNA binding region that binds to ''hormone-responsive elements'' of DNA. Receptor binds to specific base sequences on DNA so affects specific genes. Binding of receptor to DNA switches genes for proteins in that area on or off. Get induction or repression of synthesis of key proteins. Calcitriol (from vitamin D) induces synthesis of several key proteins involved in calcium transport in the gut.

2.  G-protein coupled receptors - GPCR
     Proteins in membrane with three subunits α,β and γ. They are called G proteins because they are associated with GDP/GTP. G protein complex in membrane with GDP bound to a region. Hormone binds to GPCR increases its affinity for αβγ-trimer. When GPCR and αβγ unit combine, the GDP on the α unit is replaced by GTP. The α unit when bound to GTP dissociates from βγ. The α unit binds to a membrane enzyme (or ion channel) and alters its activity. The α unit is a weak GTPase and as GTP is converted to GDP so it rebinds to form αβγ - so terminating its effects.
G-proteins
 There are several types of G-proteins. Proteins Gs and Gi stimulate or inhibit the membrane enzyme adenylate cyclase. This catalyses ATP to cyclic AMP which is a secondary messenger which brings about intracellular hormone effects. There are other second messengers like cGMP, inositol triphosphate, diacyglycerol and calcium.

Sorry for the informal diagram!

Glucagon and cAMP
   Glucagon binds to its membrane receptor. The αβγ G protein attaches to the receptor. The α unit's GDP is replaced by GTP; this α unit detaches and binds to and activates membrane adenylate cyclase. Increased cAMP produced inside the cell. α-unit converts GTP to GDP and it rejoins to βγ-unit - cessation of effect. Cyclic AMP binds to and activates cAMP dependent protein kinase (PKA). This protein kinase adds phosphate residues to specific protein sites which activate or inhibit it. Glycogen synthase becomes inactive when phosphorylated - less glycogen synthesis. PKA phosphorylates phosphorylase kinase to make it more active and this in turn phoshorylate phosphorylate b (inactive) to an (active) increased glycogen breakdown. cAMP broken down to AMP by enzyme phosphodiesterase which stops effect. Phosphodiesterase inhibited by caffeine, theophylline and sidenafil (viagra - omg). Phosphatases within cell remove phosphate groups from protein - counteract protein kinases. Enzyme activity depends upon proportion phosphorylated - balance of kinase and phosphatase effects!

cAMP
 A number of hormones work via cAMP as a secondary messenger. Precise effect of rise or fall in cAMP depends upon tissue enzymes. Catabolic enzymes are more active when phosphorylated i.e. by increased cAMP - anabolic less active. Theophylline potentiates effects of cAMP and is a derivative of cAMP that penetrates cell and mimics effect of cAMP (hormone?)
Phosphotidyl inositol system
   When hormone binds to its receptor it activates a G-protein. The α subunit binds to and activates phospholipase C in membrane. This enzyme converts phospholipid phosphotidyl inositol biphosphate (PIP2) to diacylglycerol (DAG) and inositol triphosphate (IP3) - both are secondary messengers.
Diacylglycerol (DAG)
   This activates protein kinase C in the membrane. This causes protein phosphorylation which brings about a response in the cell.
Inositol Triphosphate (IP3)
   Enters cytoplasm. Triggers calcium release from endoplasmic reticulum. This calcium binds to a protein called calmodulin which activates a calmodulin sensitive protein kinase. Calcium can also trigger secretion.
Guanylate cyclase
  The receptor(s) for atrial natriuretic factor (ANF) have a guanylate cyclase component. The ANF binds to the receptor which increases guanylate cyclase activity. Cyclic GMP is produced in the cell - the secondary messenger.


3.  Tyrosine Kinase associated receptors (RTK)
            Insulin and receptor tyrosine kinase (RTK) 
  Insulin receptor is tetramer α2β2. Insulin binds to external binding sites on a subunits. The β subunits are large transmembrane proteins with tyrosine kinase activity. The α units suppress the TK activity but when insulin binds this removes inhibition. The TK units phosphorylate each other and other cellular proteins.
TK effects:  activates a protein phosphatase which dephosphorylates glycogen synthase - activated. In cells like muscle and fat releases the glucose transporter GLUT-4 from vesicles to membrane. Other effects also... The hormone receptor complexes aggregate and internalised into vesicles and insulin is degraded.

Feedback loop and Response loop

Now lets talk abit about the how hormones are controlled

Afferent: incoming signal:           a. Stimulus -> b. Sensor or Receptor -> -> d. Afferent Pathway ->        

Efferent: outgoing signal:     -> e. Integrated center -> f. Efferent pathway -> g. Target or Effector -> h. Response
(this was the response loop: from stimulus to response)    
(Feedback loop: from response back to stimulus is the feedback loop mechanism)......Stimulus -> Sensor or receptor......etc.    We gone see how this control mechanism (stimulus then sensor or receptor then afferent pathway to integrated center to efferent pathway then target or effector and response) occurs in other hormones and in different examples. Look (click) at the diagrams!!!

Example 1

Example 2

Major endocrine organs
Brain:
- Hypothalamus (trophic and neurohormones)
- Pituitary Gland (8 major hormones)
Other organ sites:
- Thyroid gland (thyroid hormones and calcitonin)
- Adrenal cortex: cortisol and other steroids
- Adrenal medulla: catecholamines
- Liver: IGFs
- Pancreas: insulin and glucagon
- Gonads: sex hormones
Neurohormone is any hormone synthesized and released by a neuron
Trophic hormone is a hormone that controls the secretion of another hormone.

Posterior Pituitary

                                                 Summary of hormones
 Endocrine Control
Three levels:
Hypothalamic stimulation <-- from CNS
Pituitary stimulation <-- from hypothalamic trophic hormones
Endocrine gland stimulation <--from pituitary trophic hormones

What about feedback loops?

Example 3

Adrenal Gland: major site of steroid hormone synthesis

Other endocrine organs: Thyroid

Types of Endocrine disorders:
- Hyposecretion: genetic (dwarfism); damage to gland (type 1 diabetes); dietary (thyroid - goiter, low thyroxine); lack of pituitary stimulation
- Hypersecretion:
Genetic (gigantism); overstimulation (hyperthyroidism or overactive pituitary); tumour or cancer; grave's disease or immune mediated
- Abnormal tissue response/Signal transduction abnormalities:
Lack of receptors (testicular feminisation syndrome - mutant adrogen receptor); down regulation receptors (type 2 diabetes); growth hormone abnormalities (mutant GHRH receptor)

Thyrotrophin releasing hormone




Thyrotrophin

 Growth Hormone:
- Growth is a continuous process. In adolescence largest amount of growth in rapid spurts of growth and development. Requires production of other hormones as well as GH. Adequate diet. Abscence of stress i.e. cortisol driven failure to thrive.

Somatotrophin -------------------->>>

Hormones are synergistic and antagonistic such as the antagonism between insulin and glucagon, thyroid and parathyroid, etc.. The diagram below shows the antagonisms of hormones. Proper balance between them is extremely important for health

 Summary:
Hormones are chemical messengers. They are synthesized, released and act at remote sites. Three main groups of hormones (proteins/peptides, steroids, amines). Each group has a different mechanism of action. Hormone release and action is controlled by response feedback loops; these loops can have multiple input and output points. Each endocrine organ makes a specific set of hormones. Hypothalamus and Pituitary gland (hormone central). Steroid hormone synthesis mainly in adrenal gland. Endocrine disorders from hormone imbalances also occur producing hyper/hypo secretion. Like the thyroid gland hyper/hypo, growth hormone production and the pancreas and insulin/glucagon balance. Also remember the importance of hormone synergy and antagonism (look at the hormone wheel diagram!)

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Now lets talk abit about glucose and calcium metabolism problems

 Insulin and Diabetes: An overview

 Table of Contents:
   History and discovery of insulin
   Nature of insulin and implications
   Actions of insulin
   Control of insulin secretion
   Glucagon actions
   Control of glucagon release
   Integrated pancreatic control of blood glucose
   Diabetes mellitus and its types
   Symptoms and long term complications
   Causes
   Pathophysiology
   Diagnosis
   Treatment
   Prevention

Introduction
 - Islets of Langerhans in pancreas:
      α cells secrete glucagon
      β cells secrete insulin
- Diabetes mellitus (lack of insulin) most common endocrine disorder
- 1.8 million diabetics in UK (3%) 1.55 million type 2 and may be 1 million more undiagnosed (5% of NHS spending)
- Pima Indians (Arizona) half population over 35 years diabetic - also very prevalent in South Asians in UK.

Historical Landmarks
 - 150-200AD term Diabetes (syphon) applied to condition in which ''flesh and bones melt down into urine''
 - 1674 the term Mellitus (honeyed) was applied because of urine's sweet taste
 - 1889 - Pancreatectomy in dogs produces diabetic-like state postulated that pancreas produces an antidiabetic factor.
 - Ligation of pancreatic duct causes degeneration of pancreas but not islets - no diabetes
 - 1909 the term insulin was coined but could not be isolated
 - 1921 Banting and Best extracted insulin from dog pancreas.
 - 1953 Sanger - complete sequence of insulin.1st protein to be sequenced

Nature of Insulin
- Protein hormone of 51 amino acids - not active by mouth. A chain is 21 aa and the B chain is 30 aa. Linkages are S-S links between cysteine residues. Pig and beef insulin are active in humans despite slight differences. First treatments were with impure animal insulin;  now use ''monocomponent'' human insulin.
- 1969: Proinsulin extracted from pancreas. Single chain of 84aa and C-peptide (33aa) cut out to leave insulin i.e. 1 gene. C-peptide used as a marker for endogenous insulin production in diabetes.

Actions of Insulin
- Increased synthesis glycogen, fat and protein.
- Increased glucose uptake into muscle and adipose tissue but not brain, liver, red cells, etc.
- Increased glycogen formation and glucose use
- Increased fat synthesis
- Increased protein synthesis in muscle
- Reduced ketone production
- So lack of insulin.........


Control of insulin secretion
  Increases in blood glucose (and blood amino acids) have direct effect on β-cell. GIP produced by gut in response to eating increases insulin release - anticipatory. Parasympathetic stimulation (vagus) increases release but sympathetic reduces it.

Glucagon
 29 amino acid peptide from α-cells. Increases glycogen breakdown and gluconeogenesis in liver. Release is stimulated by low blood glucose. High blood amino acids increases release but this is suppressed by high blood glucose. Increases by sympathetic or parasympathetic activity,

 DIABETES


 Diagnosis
- Symptoms plus casual plasma glucose 11.1mmol/l.
- Fasting (8h+) plasma glucose 7.0mmol/l (6.1 for whole blood).
- 2h plasma glucose 11.1mmol/l during oral glucose (75g) tolerance test.
- 6.1-7mmol fasting impaired - half will become diabetic within 10yrs
- No symptoms - need to repeat

Type 1 diabetes

Type 1 diabetes
- Less than 10% of total
- Characterized by β-cell destruction and almost total failure of insulin supply. Fairly rapid onset usually in childhood. Need isnulin injections and prone to ketoacidosis
- Often thin at diagnosis. Inherited predisposition (linked to tissue-type) - environmental trigger(s) e.g. infection
- Most have autoantibodies to β-cells at diagnosis

Type 2 diabetes

Type 2 Diabetes

- No initial β-cell degeneration. Gradual onset traditionally in people over 40 but starting to occur in young - why?
- Usually overweight at diagnosis
- Symptoms relatively mild at start and ketoacidosis uncommon  (undiagnosed)
- Most do not need insulin unless advanced
- Runs in families but not linked to tissue type and no islet cell antibodies present
- Prevalence increasing as population ages and gets fatter

Causes of type 2 diabetes:
- No initial β-cell pathology and insulin still produced (absolute level may be high)
- Reduced sensitivity to insulin - need more to get same effect
- Genetic susceptibility - racial differences
- Environmental triggers - ''western diet and lifestyle''
       high fat diet; inactivity; overweight and abdominal obesity

Metabolic syndrome (syndrome X)
- Insulin resistance causes problems even without diabetes: high blood insulin, moderate hyperglycaemia, hypertension, raised blood TAg and lowered HDL.
- Many later become diabetic and all at increased risk of CHD

Diagnosis

High waist (102cm men or 88cm women) or WHR 0.95/0.85

Moderate fasting hyperglycaemia

Elevated BP

Elevated TAG

Low HDL

ANY THREE suggests diagnosis

Symptoms of Diabetes Mellitus

Symptoms of diabetes mellitus

High blood glucose (diagnosis) – why??

Glucose in urine – why??

Diuresis – why??

Prone to dehydration despite increased thirst and drinking

In type 1 get ketoacidosis:

Fatty acids →Acetyl CoA →                 Ketones

Rapid weight loss (may also occur in type 2 so underestimate link with obesity)

Hypoglycaemic coma?

Long term complication

Diabetics tend to die prematurely despite effective short term treatment

Very high rates of cardiovascular disease – insulin resistance (pre-diabetes) is a risk factor for heart disease

Also prone to retinopathy and cataracts – blindness

Diabetic nephropathy (renal failure)

Gangrene and risk of amputation

Causes of long term problems

High levels of blood lipoproteins causing increased risk of atherosclerosis in arteries

Changes in functioning of small blood vessels due to persistent hyperglycaemia and glycosylation of protein in membranes

Latter thought to be important factor in retinopathy and renal failure

To improve long term prospects

Normalise blood lipoprotein profile

Normalise body weight

Minimise the hyperglycaemia without repeated bouts of hypoglycaemia

 Diagnose and treat hidden cases because long term damage is ongoing

Implementation

Low (saturated) fat diet – prior to 1970 diabetic diet was high fat low carbohydrate. sSubstitution of complex for simple carbohydrates, increase in dietary fibre, restriction of energy intake (when necessary)

This helps to normalise blood lipoproteins but also improves insulin sensitivity

Rapid self monitoring of blood glucose

Long term check – glycosylated haemoglobin

Use of rapid soluble insulin and prolonged action depot insulin

Oral hypoglycaemic agents

Sulponylureas act to stimulate insulin release (e.g. tolbutamide and chlorpropamide) – only work in type 2 where there are functioning β-cells

Biguanides – reduces hepatic gluconeogenesis, slows absorption from gut and increases uptake by muscle

2 Trials

Diabetes Control and Complications Trial DCCT) for type 1

UK Prospective Diabetes Survey - type 2

Development renal, retinal & neuropathy delayed by intensified therapy

Higher glycated Hb – more complications & deaths

 

 Calcium and Osteo

 Calcium Balance

- Overview of typical calcium fluxes

- Vitamin D – nature and role

- Role of parathyroid hormone

- Role of calcitonin

- Integrated control of blood calcium

- Osteoporosis

- What is it?

- What causes it?

- What can be done about it?

Calcium

For adults:

Gains – losses = 0

Growth & pregnancy – net gain

Gains and losses hormonally controlled

99% of body calcium is in bone mineral as hydroxyapatite – c1kg

Bone is a reserve of calcium – release or uptake of Ca²⁺ hormonally controlled

Functions of non-bone calcium

Muscle contraction – excitation triggers Ca²⁺ release which triggers contraction

Hormone/transmitter release

Intracellular regulator

Co-enzyme function – blood clotting

Etc

So blood calcium finely regulated and must be kept within narrow limits

Osteoporosis

Thinning of bones making them susceptible to fracture e.g. from simple fall

Up to 3 million people in UK may be affected – annual toll of:

70,000 hip fractures

40,000 recorded vertebral fractires

50,000 wrist fractures

c20,000 death each year attributable to hip fractures

Nature of osteoporosis

Osteoclasts – breakdown of bone

Osteoblasts – make new bone

Bone constantly being remodelled but in later life synthesis slightly less than breakdown so gradual erosion

Risk factors for osteoporosis

Bone mass declines with age acceleration at menopause (more elderly women more osteoporosis)

Inactivity – weight bearing exercise increases bone mass and inactivity leads to loss of bone

Lack of sex hormones (even in men)– early menopause, anorexia etc

Lack of vitamin D (sunlight exposure)

What about calcium intake??

Smoking, heavy drinking, being underweight

Osteoporosis – strategies for reducing its effects

Increase peak bone mass – e.g. encourage activity in children and young adults

Slow decline in bone mass – activity, vitamin D, HRT in older people also drugs that block bone breakdown

Reduce risk of falling – building design and maintenance, maintain muscle strength, protect hips with padding?

Increase bone strength – reverse process of oseoporosis

Conventional treatments

Oestrogen therapy – good preventative

Vitamin D/calcium - 40% of old people in residential homes have biochemical evidence of vit. D deficiency

Calcitonin as nasal spray or by injection – blocks bone breakdown

Biphosphonates are analogues of pyrophosphate (etidronate and alendronate) adsorbed onto bone mineral and inhibit resorption

Teriparatide – new treatment

Active parathyroid analogue which is injected

Leads to formation of strong new bone – reverses osteoporosis

Treatment for up to 18 months and benefits last beyond this

Theory:

Pth leads to increased renal Ca²⁺ reabsorption, vit D activation and bone breakdown – but latter only at high doses

At low doses Pth stimulates osteoblasts (bone makers) before osteoclasts (bone breakers)

HRT

HRT has been first line preventative for osteoporosis

Endometrial cancer is a clear risk of oestrogen alone so must give with synthetic progestin if uterus present

Conclusively show to reduce bone loss in postmenopausal women

Case-control studies – up to 50% reduction in hip fractures in menopausal women taking oestrogen

More problems with HRT

Nelson et al (2002) – HRT increased risk of heart disease in elderly women

Beral et al (2003) Million women study – HRT small increase in breast cancer risk

2003 MHRA advised GPs that HRT should not be used for long periods to prevent osteoporosis in women over 50

Phyto-oestrogens

Are these an alternative to HRT?

Substances from plants not steroids but have oestrogenic activity

Isoflavins (genistein, daidzein) from soy and other legumes

Lignans in whole grains

Coumestins in clover and alfalfa sprouts

Soy foods and supplements, blach cohosh, red clover

2-4 mg isoflavins per mg soy protein

Supplements c40mg/day

Some diets high in soy 100mg/day

Babies soy formula 4mg/kg body weight

Act as partial agonists so can increase or decrease oestrogenic activity depending upon circumstances

c1:10,000 potency of oestrogen

But HRT c50μg/day whereas supps 1000X more by weight of isoflavins

So significant total activity

Branca 2003

In vitro genistein reduces osteoclast activity but increases osteoblasts

Soybean feeding increases bone density in ovariectomised rats

Women in SE Asia (where soy intakes are high) – women with highest intakes have higher bone density

Not seen where soy intakes are lower (need high dose)

Review of 7 studies suggest phyto-oestrogens over 6 months positive influence on bone density in lumbar spine

Phyto-oestrogens – substantial report published by Food Standards Agency

Other benefits?

Possible risks?