Week 20 Physiology

Adrenal morphology / architecture (MCQs).

• Adrenal medulla surrounded by cortex
• Medulla is 28% of total mass – has adrenaline secreting cells(larger with less dense granules) and noradrenaline secreting cells (smaller with very dense granules)
• Cortex has three sections, from outer – zona glomerulosa (15%), zona fasciculata(50%), zona reticularis (7%)
• All 3 secrete corticosteroid, only zona Glomerulosa secretes aldosterone, the other 2 secrete sex hormone
• Venous sinus exists interlaced between all sections

 

Adrenal medulla:

• Substances secreted and their effects.
  • Adrenaline & Noradrenaline
    • Glycogenolysis in liver and skeletal muscle
    • Mobilisation of FFA
    • Stimulate metabolic rate
    • Positive ionotrope and chronotrope via B1 receptor
    • Increase myocardial excitability
    • Increase alertness
    • NA –> peripheral vasoconstriction via A1 receptor
      • Increase systolic and diastolic BP –> baroreceptors –> bradycardia –> decrease CO
    • Adrenaline dilate blood vessels in liver and skeletal muscle via B2 receptor
      • Drop peripheral resistance –> widening pulse pressure –> HR and CO increase
  •  Dopamine
    • Renal and mesenteric vasodilation
    • Elsewhere vasoconstriction
    • Positive ionotrope via B1
    • Increase SBP
    • Natriuresis via ? Inhibiting NaK ATPase

• Regulation of medullary secretion.
  • Stimulation by sympathetic preganglionic nerves
  • Increased when standing
  • After adrenalectomy, NA levels unchanged but adrenaline levels drop to near zero.

• Adrenal cortex and Substances secreted.
  • Cortisol/corticosterone
  • Testosterone/oestrogens – DHEA, androstenedione
  • Mineralocorticoid – aldosterone

 

ACTH.

• From anterior pituitary
• Binds G protein receptor on adrenocortical cells –> activate adenyl cyclase –> increase cAMP –> increased cortisol and aldosterone formation
• Controls glucocorticoid, mineralocoticoid and sex hormone release from adrenal cortex
• Increases sensitivity of adrenal to subsequent catecholamines.
• Secreted in bursts (Circadian rhythm) so high in early morning.

 

Steroids

• Glucocorticoids are pretty important (use Cortisol as prototype).
• When bound glucocorticoid+steroid compound becomes a transcription factor that promote transcription of parts of the DNA to cause its effects

• Physiological effects
  • Increase protein catabolism
  • Increase glycogenolysis and gluconeogenesis –> increase BSL
  • Insulin resistance
  • Permissive action – low amount of Glucocorticoid needed for catecholamines to exert their calorigenic effects, vasopressor effects, lipolytic effects. Very important to have vascular reactivity.
  • Needed for water excretion
  • Decrease circulating lymphocytes, eosinophils and basophils
  • Inhibit ACTH secretion(neg feedback)

• Pathological effects
  • Immunosuppressant
  • HTN
  • Hyperglycaemia
  • Central adiposity
  • Proximal myopathy
  • Buffalo hump, moon facies,
  • Striae
  • Bruising, skin fragility
  • Poor wound healing
  • Irritability
  • Insomnia
  • Inability to concetrate
  • osteoporosis

 

Aldosterone actions and factors affecting secretion.

• Increases Na reabsorption in kidney and increase H2O retention as a result. Also increase Na absorption from sweat, saliva and colon contents.
• Act on principal cells to increase activity of NaK counter transport and on intercalated cells to increase H+ ATPase –> high K and acidic urine.
• Release stimulated by ACTH, Angiotensin II(renin) by another G protein receptor and activation of protein kinase C, and high plasma K+.
• Aldosterone itself bind to its receptor and transcribes DNA to exert its effect.

 

Pituitary:

Know which hormones come from where and the functions of each.

• Anterior pituitary
  • F: FSH – basophil
  • L: LH – basophil
  • A: ACTH- BASOPHILS
  • T: TSH -basophil
  • P: PROLACTIN – acid
  • i: ignore ACIDOPHILS (“acid pig”)
  • G: GH – acid
• Posterior pituitary
  • Vassopressin
  • oxytosin

 

Overview of growth hormone.

• 50% of active growth hormone is plasma protein bound
• Promotes long bone growth when ephiphyseal plate is not fused
• When in excess causes acromegaly – most viscera enlarged, high plasma protein levels.
• Lipolysis
• Na retention
• Decreased insulin sensitivity
• IGF -1 – pro growth but is antilipolytic and has insulin like activity

• Increase GH secretion
  • Hypoglycaemia
  • Starvation
  • Exercise
  • Stress
  • Increased amino acids
  • Protein meal
  • Glucagon
  • Vasopressin
  • Androgens and oestrogens

• Decrease GH secretion
  • REM sleep
  • Cortisol
  • Glucose
  • FFA
  • Medroxyprogesterone
  • GH and IGF-1 (neg feedback)

 

Viva questions:

1. What substances are secreted in the adrenal medulla ?
2. How is their secretion controlled ?
3. What substances are secreted in the adrenal cortex ?
4. What do glucocorticoids do ?
5. Tell me about Aldosterone.