Diuretic and Saluretic Activity

Abstract

Purpose and Rationale Acetazolamide (Diamox) was one of the first synthetic nonmercurial diuretics. The mode of action was found to be inhibition of carbonic anhydrase. Carbonic anhydrase is a zinccontaining enzyme that catalyzes the reversible hydration (or hydroxylation) of CO2 to form H2CO3 which dissociates nonenzymatically into HCO3 and H. The enzyme is located within the cytoplasm and at the apical and basolateral membranes of proximal tubules as well as on the apical (lumenal) surface of distal tubules and in the thick ascending limb of the loop of Henle. Its primary function is to enhance H secretion into the urine. At least three isoenzymes, designated as I, II, and II or A, B, and C, are known to exist. Carbonic anhydrase inhibitors were originally used as diuretics but are no longer used for this purpose except in limited circumstances (metabolic alkalosis accompanied by edema). Chronic inhibition of carbonic anhydrase results in significant hypokalemia and metabolic acidosis due to bicarbonate wasting, and thus inhibition of carbonic anhydrase may be mechanistically important if these effects are noted in in vivo studies or clinically (Okusa and Ellison 2000). Numerous isoenzymes exist, of which CA II is the most abundant in proximal tubules and erythrocytes. The activity of carbonic anhydrase in erythrocytes is significantly higher than that in the kidney, and the enzyme is easily harvested by lysing mammalian whole blood. Thus, examination of the degree of inhibition of erythrocyte carbonic anhydrase may serve as a suitable surrogate for examination of effects on renal activity (Maren 1967; Armstrong et al. 1966). In spite of the fact that newer diuretics are based on other modes of action, the test for inhibition of carbonic anhydrase should be performed for evaluation of a new compound. Moreover, the specific use of carbonic anhydrase inhibitors as antiglaucoma drugs has been described (Friedland and Maren 1984; Caprioli 1985). The mechanism by which carbonic anhydrase inhibitors lower intraocular pressure is through a reduction in aqueous humor formation, by affecting electrolyte and water balance in the nonpigmented ciliary epithelium (Friedland and Maren 1984; Caprioli 1985). Although many methods to measure carbonic anhydrase activity have been developed (Philpot and Philpot 1936), the micro method described by Maren (1960) is relatively simple, sensitive, and reliable. The enzyme sources are red cells, a rich source of the same isoenzymes found in the eye (Maren 1967; Armstrong et al. 1966; Wistrand et al. 1986; Wistrand and Knuuttila 1980).