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

Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

  • 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.