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Its structural formula Post Date: Tue, 14 Oct 2008 16:50:14 +0000 SPIRONOLACTONEChemistry. A number of 17-spirolactone steroids have been employed, of which spironolactone ap-pears to have the greatest selectivity and efficacy. Its structural formula is as follows: Mechanism of Diuretic Action. Autor of the post: Undefined This concept is based Post Date: Tue, 14 Oct 2008 16:30:20 +0000 Com-pounds of this type are considered as com-petitive antagonists of aldosterone, the most potent endogenous mineralocorticoid. Pre-sumably they compete for receptor sites because they are homologs of the natural hormone, but they fail to evoke an active response. This concept is based on two general types of indirect evidence obtained from experiments in vivo: first, that the antagonist drug is effective only in the presence of either endogenous or exogenous aldosterone; second, that the action of the antagonist may be overcome by increasing the concentration of aldosterone (Kagawa et al, 1959; Liddle, 1961). Autor of the post: Undefined Assay of aldosterone antagonists Post Date: Tue, 14 Oct 2008 16:15:15 +0000 The secretion of aldosterone is increased primarily by alterations in electrolyte balance, most importantly by a reduction in effective blood volume, hyponatremia, or hyperkalemia. In addition, angiotensin resulting from the release of renin by the kidney stimulates aldosterone production (see Chapter 30). Assay of aldosterone antagonists is usually undertaken experimentally following sodium chloride restriction. Autor of the post: Undefined The mineralocorticoids normally act to augment Post Date: Tue, 14 Oct 2008 15:59:13 +0000 In clinical disorders, augmented aldosterone production is commonly associated with cirrhosis and less consistently with cardiac or nephrotic edema. Advanced renal hypertensive disease is accompanied by very high rates of aldosterone secretion.The mineralocorticoids normally act to augment the renal tubular reabsorption of sodium and chloride, and to increase the excretion of potassium by facilitation ofNa+-K+ exchange. Autor of the post: Undefined In the absence of adrenal Post Date: Tue, 14 Oct 2008 15:40:08 +0000 Excessive mineralocorticoid activity leads to metabolic alkalosis in which potassium depletion plays a major role. Aldosterone has little direct action on the Na+-H+ exchange mechanism of the renal tubule; that is, its effect on acid-base balance is secondary to alterations in potassium balance. In the absence of adrenal function, there is an increased excretion of sodium and chloride, or, stated more exactly, sodium and chloride excretion persists even in the presence of depleted body stores, whereas in the normal subject such a condition is associated with extremely low excretory rates. Autor of the post: Undefined In addition, potassium excretion Post Date: Tue, 14 Oct 2008 15:22:39 +0000 On this basis, one would anticipate that the antagonism of the action of the steroids would result in only a slight increase in electrolyte and water excretion. This is in contrast to the much higher excretion rates evoked by many other diuretics that act on other aspects of the tubular transport mechanisms. In addition, potassium excretion is increased by aldosterone and decreased by the antagonism of its action. Autor of the post: Undefined Agents that inhibit the synthesis Post Date: Tue, 14 Oct 2008 15:06:50 +0000 Hence, under controlled conditions the urinary Na+:K+ ratio serves as an indirect index of aldosterone activity. Spironolactone increases calcium excretion through a direct effect on tubular transport (Wills et al, 1969).Agents that inhibit the synthesis of aldos-terone by the adrenal cortex theoretically have potential diuretic activity, but these are not of practical usefulness. Autor of the post: Undefined It is effective Post Date: Tue, 14 Oct 2008 14:53:50 +0000 Preparations. Spironolactone, USP (ALDAC-TONE), a microcrystalline preparation, is available in official 25-mg oral tablets. It is effective in an average daily dose of 100 mg, given in divided doses. Autor of the post: Undefined Frequently, they have been employed Post Date: Tue, 14 Oct 2008 14:41:24 +0000 Clinical Use. The aldosterone antagonists have been used mostly in the management of refractory edema. Frequently, they have been employed in conjunction with other diuretic agents rather than as the sole drug. Autor of the post: Undefined However, the quantitative effects Post Date: Tue, 14 Oct 2008 14:22:01 +0000 On theoretical grounds, the potassium loss that occurs secondary to the use of other diuretics may be decreased by the coadministration of aldosterone antagonists. In general this has been substantiated by clinical experience. However, the quantitative effects are not exactly predictable, due to the complex interactions of the primary disease, the degree of secondary hyperaldosteronism, and the actions of the diuretics given concomitantly. 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