Tubular chloride transport and the mode of action of some diuretics.

The renal diluting segment (thick ascending limb of Henle's loop) reabsorbs sodium chloride in excess of water and is responsible for dilution of the urine as well as reabsorption of a large fraction of the salt present in the glomerular ultrafiltrate. In this segment, there is active reabsorption of chloride which causes the voltage to be positive in the tubule lumen. Most, if not all, of the sodium transport is passive, driven by the voltage. Three major diuretic drugs (mersalyl, furosemide and ethacrynic acid) act in the lumen of the diluting segment to inhibit active chloride transport, not sodium transport as previously believed. This specific action on chloride transport may explain how these drugs are able to inhibit salt transport in the kidney while having so little effect on the electrolyte transport processes elsewhere in the body.