Endothelin inhibits fluid and bicarbonate transport in part by reducing Na+/K+ ATPase activity in the rat proximal straight tubule.

The effects of endothelin on fluid and bicarbonate absorption and Na+/K+ ATPase activity in the proximal straight tubule were investigated. During the control period, fluid absorption was 0.59 +/- 0.03 nL/mm.min when tubules were perfused at 4.76 nL/mm.min. After treatment with 10(-9) M endothelin, fluid absorption fell to 0.36 +/- 0.06 nL/mm.min when perfused at a similar flow rate. During the control period, bicarbonate absorption was 67.9 +/- 3.6 pmol/mm.min. After treatment with endothelin, it fell to 42.8 +/- 4.3 pmol/mm.min, an inhibition of 38%. To test whether inhibition of fluid and bicarbonate absorption was due to suppression of Na+/K+ ATPase activity, the effect of endothelin on pump activity was investigated. In control tubules, Na+/K+ ATPase activity was 85 +/- 5 pmol/mm.min. In endothelin-treated tubules, Na+/K+ ATPase activity was 68 +/- 4 pmol/mm.min, a reduction of 20%. From these data, it was concluded that endothelin inhibits fluid and bicarbonate transport in the proximal tubule and that this inhibition is in part due to suppression of Na+/K+ ATPase activity.