05. Adrenal Medulla

• adrenal medulla synthesises catecholamines - not essential though as these are produced in the nervous system
• adrenal medulla is the main site for adrenaline synthesis - noradrenaline also produced but in smaller numbers
• PNMP = enzyme responsible for metabolising noradrenaline into adrenaline
• catecholamine synthesis:

  • tyrosine is hydroxylated by tyrosine hydroxylase (rate limiting)
  • noradrenaline produced by dopamine
  • PNMT turns noradrenaline into adrenaline

  therefore PNMT can regulate adrenaline but not noradrenaline
• glucocorticoids can upregulate PNMT:

  • adrenaline and cortisol are both stress responses
  • cortisol produced in cortex passes through medulla - regulates PNMT
• high catecholamine levels inhibit tyrosine hydroxylase
• adrenaline is a short lived molecule which gets metabolised by various enzymes and then excreted via kidneys as VMA
• adrenaline binds preferentially to beta receptors but also binds to alpha receptors

  • medulla therefore activates beta receptors more than SNS would as it produces more adrenaline rather than noradrenaline
• adrenaline effects:

  • increases heart rate, contraction and broncho and vaso dilation (skeletal muscle)
  • anxiety and muscle tremors
  • glycogen breakdown to increase blood glucose
• phaeochromocytoma = excess catecholamines

  • tumour of chromaffin cells causing chronic over-secretion - only noticeable during sudden stressor
  • causes high bp, headaches and anxiousness
• phaeochromocytoma treatment

  • surgery - has risk of catecholamine crisis
  • excess adrenaline will be activating alpha and beta receptors -> vasoconstriction and high blood pressure
  • can lead to heart struggling to cope
  • surgery causes extra stress leading to more adrenaline
  • must give alpha and beta blockers