The mode of action of diuretics

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 know loop diuretics and we show that all tested compounds: furosemide (FUR), piretanide (PIR), bumetanide (BUM), azosemide (AZO) loose 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 the binding to the Na+2Cl-K+ cotransporter. This is a carboxylate group in case of FUR, PIR and BUM, a tetrazolic acid group in case of AZO, and a sulfonylurea group in a case of torasemide (TOR). Now we show that TOR-derivatives with an even less acidic sulfonylurea group (pka >8) still are very potent inhibitors of the Na+2Cl-K+ cotransporter. Experiments at various pH-values indicate that, even for these substances, it is only the anionic form which inhibits the Na+2Cl-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+2Cl-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 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 dense feedback mechanism, and circumstantial evidence suggested that the uptake of Cl- and/or Na+ by macula dense cells may occur via the Na+2Cl-K+ cotransporter. Now we show that macula dense cells sense the luminal NaCl concentration via FUR sensitive Na+2Cl-K+ cotransport.