- 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.
- Aldosterone also promotes the secretion of H+ by the intercalated cells of the collecting duct, and K+ secretion by the principal cells.
- The Na+-conserving action of aldosterone is also seen in salivary ducts, sweat glands, and the distal colon.
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.
ANP is the hormone secreted by the right atrium. The stimuli that release ANP (two peptides are released) are:
- Stretch, an action independent of nervous involvement
- Increased salt intake
ANP-mediated constriction of the efferent arteriole
All use cAMP as second messenger except (AIIMS Nov 09)
- 28% of the mass of the adrenal gland
- 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
- Extradrenal sources (sympathetic ganglia): Mainly norepinephrine & 50% of Dopamine
- 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
- Norepinephrine is formed by hydroxylation and decarboxylation of tyrosine, and epinephrine by methylation of norepinephrine.
- Phenylethanolamine-N-methyltransferase (PNMT), catalyzes the formation of epinephrine from norepinephrine, is found in brain & adrenal medulla.
- Adrenal medullary PNMT is induced by glucocorticoids. So, after hypophyrsectomy epinephrine synthesis decreases.
- The catecholamines have a half-life of about 2 min in the circulation. They are methoxylated and then oxidized to vanillylmandelic acid [VMA].
- About 50% of the secreted catecholamines appear in the urine as free or conjugated metanephrine and normetanephrine, and 35% as VMA
- Catecholamines increase alertness, epinephrine usually evokes more anxiety and fear.
- Epinephrine and norepinephrine both cause glycogenolysis.
- Norepinephrine and epinephrine also produce a prompt rise in the metabolic rate
- 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
- Adrenalectomy: Free E in plasma becomes zero NE unchanged
Hormones involved in Calcium Homeostasis
- 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:
- Complexed with organic acids
- Protein bound
- 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.