Aldosterone by M.Pandian
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Introduction HORMONES OF ADRENAL CORTEX MINERALOCORTICOIDS Aldosterone Life-saving Hormone Actions of aldosterone Aldosterone escape or escape phenomenon Regulation of aldosterone secretion Renin–angiotensin system Applied
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Aldosterone by M.Pandian
- 1. Aldosterone essential for life PANDIAN M DEPARTMENT OF PHYSIOLOGY DYPMCKOP
- 2. • INTRODUCTION • HORMONES OF ADRENAL CORTEX • MINERALOCORTICOIDS • Aldosterone • LIFE-SAVING HORMONE • ACTIONS OF ALDOSTERONE • ALDOSTERONE ESCAPE OR ESCAPE PHENOMENON • REGULATION OF ALDOSTERONE SECRETION • RENIN–ANGIOTENSIN SYSTEM • Applied
- 3. HORMONES OF ADRENAL CORTEX Adrenocortical hormones are steroids in nature, hence the name ‘corticosteroids’. Based on their functions, •CORTICOSTEROIDS ARE CLASSIFIED INTO THREE GROUPS: 1. MINERALOCORTICOIDS 2. GLUCOCORTICOIDS 3. SEXHORMONES.
- 4. MINERALOCORTICOIDS • Mineralocorticoids are the corticosteroids that act on the minerals (electrolytes), particularly promote sodium retention and Potassium excretion from kidneys to maintain Na balance. Mineralocorticoids are: 1. Aldosterone 2. 11-deoxycorticosterone
- 5. Synthesis •Aldosterone, the chief mineralocorticoid, is synthesized by the zona glomerulosa cells. zones of the adrenal cortex
- 6. Chemistry and half-life •Mineralocorticoids are C21 steroids having 21 carbon Atoms. Half-life of mineralocorticoids is 20 minutes. •Approximately 80 % of the cholesterol used for steroid synthesis is provided by low-density lipoproteins (LDL) in the circulating plasma.
- 7. • The LDLs, which have high concentrations of Cholesterol, • It diffuse from the plasma into the interstitial fluid • Attach to specific receptors contained in structures called Coated pits on the adrenocortical cell membranes. • The coated Pits are then internalized by endocytosis, forming vesicles • That Eventually fuse with cell lysosomes and release cholesterol that can be used to synthesize adrenal steroid hormones
- 10. TRANSPORT • In the plasma, 40% aldosterone circulates in free form and 60% in bound form. Aldosterone is weakly bound to the specific aldosterone-binding globulin to transcortin and to albumin.
- 11. LIFE-SAVINGHORMONE • Aldosterone is very essential for life • It maintains the osmolarity and volume of ECF. • It is usually called life-saving hormone because, its absence causes death within 3 days to 2 weeks. • Aldosterone has three important functions. It increases: 1. Reabsorption of sodium from renal tubules 2. Excretion of potassium through renal tubules 3. Secretion of hydrogen into renal tubules.
- 14. ACTIONS OF ALDOSTERONE 1. Conservation of Na+ & excretion of K+ 2. Water excretion & ECF Volume Regulation 3. Relationship with Acid – Base balance 4. Secondary effects of excess of aldosterone
- 15. 1. EFFECTSON RENALTUBULES Aldosterone acts on late distal tubules and collecting ducts of kidney and causes following effects: (I) sodium reabsorption from the tubular fluid into the Renal tubular epithelial cells. (II) potassium excretion. In the kidney, the active reabsorption of Na+ occurs in exchange of K+ and H+. Thus, Aldosterone not only causes reabsorption ofNa+, but Excretion of K+ as well by renal tubular epithelial cells. (III) H+ excretion. Aldosterone also enhances the tubular Secretion of H+ as Na+ is reabsorbed. (IV) Ammonium and Magnesium excretion is also increased by aldosterone.
- 16. • It increases Na+ reabsorption from GIT, salivary gland and sweat glands. • It may also lead to ↑ses in K+ and ↓ses the Na+ in muscle & brain cells • Stimulation of K+ excretion is greatly depends on dietary Na+ proved by Animal studies • Excess aldosterone leads to • ↑sed plasma Na+/K+ Conc. Ratio due to ↑ses K+ excretion • Decline in urine Na+/ K+ Conc. Ratio due to ↓ses Na+ and ↑ses K+ excertion • Removal of adrenal cortex results in • Na+ & Water loss, but Na+ loss excess then water • Result decreased in ECFV produces hypotension, dehydration, circulatory collapse, finally death. • Retention of K+ produces hyperkalemia, dehydration and circulatory collapse. Therefore aldosterone essential for life
- 18. 2. Water excretion & ECF Volume Regulation • Aldosterone has no direct effects on GFR, renal plasma flow or renin production • By stimulating Na+ reabsorption it causes water retention • The result expansion of ECFV then leads to ↑ses in GFR, RPF & ↓ses renin production • High circulating aldosterone level is common finding in cirrhosis of liver, nephrosis, CHF, etc.
- 21. 3. Relationship with Acid – Base balance
- 22. 4. Secondary effects of excess of aldosterone • Increased secretion of K+ in DCT increases its excretion in urine causing marked hypokalemia • Characterized by – muscular weakness, ↑ses H+ secretion – acidic urine etc • Affects on BP – • Increase in ECF volume and the blood volume finally leads to increase in blood pressure.
- 23. Aldosterone escape or escape phenomenon Aldosterone escape refers to escape of the kidney from salt-retaining effects of excess administration or secretion of aldosterone, as in the case of primary hyperaldosteronism
- 24. REGULATION OF ALDOSTERONE SECRETION
- 25. RENIN–ANGIOTENSIN SYSTEM • The secretion of aldosterone is influenced by changes in the circulating fluid volume, which are sensed in the kidney. Conditions associated with decreased ecf are: • Sodium deprivation (e.g. Dietary restriction), • Haemorrhage, • Upright posture for several hours and • Acute diuresis.
- 29. APPLIED a. CONN’S SYNDROME b. SECONDARY HYPERALDOSTERONISM c. ADDISONIAN CRISIS d. ADDISON’S DISEASE e. SECONDARY AND TERTIARY ADRENOCORTICAL INSUFFICIENCY
- 35. REFERENCES • TEXT BOOK OF MEDICAL PHYSIOLOGY • GUYTON & HALL • HUMAN PHYSIOLOGY • VANDER • TEXT BOOK OF MEDICAL PHYSIOLOGY • INDUKURANA • PRINCIPLES OF ANATOMY AND PHYSIOLOGY • TOTORA • NET SOURCE
- 36. THANK YOU .THANK YOU . . .. .
Editor's Notes
- The zona glomerulosa, a thin layer of cells, constitutes about 15 percent of the adrenal cortex. These cells are the only ones in the adrenal gland capable of secreting significant amounts of aldosterone because they contain the enzyme aldosterone synthase, which is necessary for synthesis of aldosterone. The secretion of these cells is controlled mainly by the extracellular fluid concentrations of angiotensin II and potassium, both of which stimulate aldosterone secretion.
- The general process of steroid hormone synthesis is illustrated in Figure (vander book ). step 1 – the gonads and adrenal cortex cells are stimulated by the binding of a pituitary gland hormone to its receptor Step – 2 These receptors are linked to G-proteins, which activate adenylyl cyclase and thus cAMP production step – 3 The subsequent activation of protein kinase A results in phosphorylation of numerous cytosolic and membrane proteins step -4 which facilitate the subsequent steps in the process by endocytosis. free cholesterol is released from the lipid droplet by the action of the enzyme cholesterol esterase, Step – 5 Carrier proteins then transport the free cholesterol to the mitochondria . the cholesterol must be transported across the outer membrane to the inner mitochondrial membrane, which contains the enzymes required to process cholesterol into steroid hormones. These enzymes, called cytochrome P450s step – 6 These modifications, which also include a dehydrogenation reaction mediated by an enzyme different from the P450s, occur in both the mitochondria and in the smooth endoplasmic reticulum. Thus, the intermediates are shuttled back and forth between the two organelles The final product depends upon the cell type and the types and amounts of the enzymes it expresses. Cells in the ovary, for example, express large amounts of the enzyme needed to convert testosterone to estradiol, whereas cells in the testes do not express signifi cant amounts of this enzyme and thus make primarily testosterone. All of the steroid hormones are derived from cholesterol from molecules of acetate.
- Aldosterone-responsive epithelial cell signaling pathways. ENaC, epithelial sodium channel proteins; MR, mineralocorticoid receptor. Activation of the MR by aldosterone can be antagonized with spironolactone. Amiloride is a drug that can be used to block ENaC.
- It stimulates secretion of atrial natriuretic peptide (ANP) from atrial muscles of the heart: ANP causes excretion of sodium in spite of increase in aldosterone secretion ii. It causes pressure diuresis (excretion of excess salt and water by high blood pressure) through urine. This decreases the salt and water content in ECF, in spite of hypersecretion of aldosterone Besides ANP, two more natriuretic peptides called brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) are also secreted by cardiac muscle BNP and CNP also have similar actions of ANP on sodium excretion. Significance of aldosterone escape Because of aldosterone escape, edema does not occur.