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  1. The primary target tissue for aldosterone is the kidney, where its most important action is to increase Na+ reabsorption by the principal cells (P cells) of the kidney's collecting ducts.
  2. Aldosterone also promotes the secretion of H+ by the intercalated cells of the collecting duct, and K+ secretion by the principal cells.
  3. The Na+-conserving action of aldosterone is also seen in salivary ducts, sweat glands, and the distal colon.
Transport, Metabolism, Exretion
Aldosterone is bound to protein to only a slight extent, and its half-life is short (about 20 min). The amount secreted is small and the total plasma aldosterone level in humans is normally about 0.006 g/dL (0.17 nmol/L). Much of the aldosterone is converted in the liver to the tetrahydro glucuronide derivative, but some is changed in the liver and in the kidneys to an 18-glucuronide.
  • Increased Aldosterone Secretion: Decreased pressure in the renal artery (e.g., hemorrhage, shock, dehydration, sweating) will activate the renin-angiotensin system, increase aldosterone secretion, and increase sympathetic stimulation to return blood pressure toward normal.
  • Decreased Aldosterone Secretion: Any condition that increases blood pressure in the renal artery, eg HT, weightlessness.
  • Escape Phenomenon: Odema due to Na* and water retention is prevented by "Na+ escape" in primary hyperaldosteronism. It is because of atrial natriuretic peptide.
Atrial Natriuretic Peptidev(ANP)
ANP is the hormone secreted by the right atrium. The stimuli that release ANP (two peptides are released) are:
  1. Stretch, an action independent of nervous involvement
  2. Increased salt intake
ANP increases sodium loss (natriuresis) and water loss by the kidney because of, in part, an increase in glomerular filtration rate due to: ANP-mediated dilation of the afferent arteriole
ANP-mediated constriction of the efferent arteriole

All use cAMP as second messenger except (AIIMS Nov 09)

A. Corticotropin                  
B. Dopamine
C. Glucagon                       
D. Vasopressin


D. Vasopressin

Adrenal Medulla

  1. 28% of the mass of the adrenal gland
  2. In normal individuals 90% of output from adrenal medulla is epinephrine & only 10% is norepinephrine. Adrenal Medulla also secretes Dopamine (50%), Chromogranin A, Opioid peptides & Adrenomedullin
  3. Extradrenal sources (sympathetic ganglia): Mainly norepinephrine & 50% of Dopamine
  4. Pheochromocytoma are catecholamine secreting tumour of chromaffin cells derived from adrenal medulla or extradrenal sources (chromaffin cells in sympathetic ganglia).Output in Pheochromocytoma is mainly norepinephrine
  5. Norepinephrine is formed by hydroxylation and decarboxylation of tyrosine, and epinephrine by methylation of norepinephrine.
  6. Phenylethanolamine-N-methyltransferase (PNMT), catalyzes the formation of epinephrine from norepinephrine, is found in brain & adrenal medulla.
  7. Adrenal medullary PNMT is induced by glucocorticoids. So, after hypophyrsectomy epinephrine synthesis decreases.
  8. The catecholamines have a half-life of about 2 min in the circulation. They are methoxylated and then oxidized to vanillylmandelic acid [VMA].
  9. About 50% of the secreted catecholamines appear in the urine as free or conjugated metanephrine and normetanephrine, and 35% as VMA
  10. Catecholamines increase alertness, epinephrine usually evokes more anxiety and fear.
  11. Epinephrine and norepinephrine both cause glycogenolysis.
  12. Norepinephrine and epinephrine also produce a prompt rise in the metabolic rate
  13. When injected, epinephrine and norepinephrine cause an initial rise in plasma K+ because of release of K+ from the liver and then a prolonged fall in plasma K+ because of an increased entry of K+ into skeletal muscle
  14. Adrenalectomy:  Free E in plasma becomes zero NE unchanged

Hormones involved in Calcium Homeostasis

  1. There is approximately 1 kg of calcium in the human body. About 99% exists in the bone and 1% in the extra cellular fluid. Plasma calcium exists in 3 forms:
  2. Complexed with organic acids
  3. Protein bound
  4. Ionized
  5. The ionized calcium is maintained at a concentration between 1.1 and 1.3 mmol/L. If the ionic calcium levels fall the organism develops hyper excitability and develops tetanic convulsions. A marked elevation may result in death owing to muscle paralysis and coma.

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