The Mode of Action of Diuretics

Summary

This review will address several recent findings regarding the interaction of so called loop diuretics of the furosemide type with the Na⁺2Cl⁻K⁺ cotransporter in the thick ascending limb of the loop of Henle (TAL): (i) The organ selectivity of these transport inhibitors is caused by their secretion by the proximal tubule leading to an increase in their luminal concentration. We have examined whether probenecid, a selective inhibitor of proximal organic anion secretion, reduces the diuretic effect of known loop diuretics. We show that all compounds tested: furosemide (FUR), piretanide (PIR), bumetanide (BUM), and azosemide (AZO) lose part of their diuretic effect when administered in the presence of probenecid. (ii) Previously, we have shown that all loop diuretics of the furosemide type require an acidic group for binding to the Na⁺2Cl⁻K⁺ cotransporter. This is a carboxylate group in the case of FUR, PIR, and BUM; a tetrazolic acid group in the case of AZO; and a sulfonylurea group in the case of torasemide (TOR). Now we show that TOR-derivates with an even less acidic sulfonylurea group (pK_a > 8) are still very potent inhibitors of the Na⁺2C1⁻K⁺ cotransporter. Experiments at various pH values indicate that, even for these substances, it is only the anionic form which inhibits the Na⁺2C1⁻K⁺ cotransporter. (iii) Most of the loop diuretics, with the exception of TOR, are rather hydrophilic (FUR, PIR, BUM, AZO). Now we have examined whether one can design compounds which sustain their inhibitory effect on the Na⁺2C1⁻K⁺ cotransporter, although they are highly lipophilic. We found that cyclo-alkyl substitutions at the tolyl-and sulfonylurea-moieties of TOR led to highly lipophilic compounds with very high affinity for the Na⁺2C1⁻K⁺ cotransporter. (iv) Previous data suggested that loop diuretics bind to the Na⁺2Cl⁻K⁺ cotransporter at the extracellular side of the luminal membrane. Now we have designed impermeable macromolecular derivatives of PIR and we show that these macromolecular probes inhibit the Na⁺2Cl⁻K⁺ cotransporter as well as PIR. (v) Previous data indicated that loop diuretics interrupt the macula densa feedback mechanism, and circumstantial evidence suggested that the uptake of Cl⁻ and/or Na⁺ by macula densa cells may occur via the Na⁺2Cl⁻K⁺ cotransporter. Now we show that macula densa cells sense the luminal NaCl concentration via FUR sensitive Na⁺2C1⁻K⁺ cotransport.