Mineralocorticoids are the corticosteroids that act on the minerals (electrolytes), particularly sodium and potassium. Mineralocorticoids are:
SOURCE OF SECRETION
Mineralocorticoids are secreted by zona glomerulosa of adrenal cortex.
CHEMISTRY AND HALF-LIFE
Mineralocorticoids are C21 steroids having 21 carbon atoms. Half-life of mineralocorticoids is 20 minutes.
FUNCTIONS OF MINERALOCORTICOIDS
Ninety percent of mineralocorticoid activity is provided by aldosterone.
- On Sodium Ions
Aldosterone acts on the distal convoluted tubule and the collecting duct and increases the reabsorption of sodium. During hypersecretion of aldosterone, the loss of sodium through urine is only few milligram per day. But during hyposecretion of aldosterone, the loss of sodium through urine increases (hypernatriuria) up to about 20g/day. It proves the importance of aldosterone in regulation of sodium ion concentration and osmolality in the body.
- On Extracellular Fluid Volume
When sodium ions are reabsorbed from the renal tubules, simultaneously water is also reabsorbed. Water reabsorption is almost equal to sodium reabsorption; so the net result is the increase in ECF volume. Even though aldosterone increases the sodium reabsorption from renal tubules, the concentration of sodium in the body does not increase very much because water is also reabsorbed simultaneously. But still, there is a possibility for mild increase in concentration of sodium in blood (mild hypernatremia). It induces thirst, leading to intake of water which again increases the ECF volume and blood volume.
- On Blood Pressure
Increase in ECF volume and the blood volume finally leads to increase in blood pressure. 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.
- On Potassium Ions
Aldosterone increases the potassium excretion through the renal tubules. When aldosterone is deficient, the potassium ion concentration in ECF increases leading to hyperkalemia. Hyperkalemia results in serious cardiac toxicity, with weak contractions of heart and development of arrhythmia. In very severe conditions, it may cause cardiac death. When aldosterone secretion increases, it leads to hypokalemia and muscular weakness.