Angiotensin stimulates bicarbonate transport and Na+/K+ ATPase in rat proximal straight tubules.

The effect of angiotensin on HCO3- absorption, fluid absorption, and Na+/K+ ATPase activity in isolated rat proximal straight tubules was investigated. During the control period, tubules absorbed fluid at 0.66 +/- 0.12 nL/mm.min and bicarbonate at 60.2 +/- 10.7 pmol/mm.min. After 10(-10) M angiotensin was added to the bath, tubules absorbed fluid at 0.93 +/- 0.19 nL/mm.min and bicarbonate at 77.4 +/- 15.2 pmol/mm.min, indicating stimulation of both parameters. Time controls showed no significant change in the rate of bicarbonate or fluid absorption. To determine whether this stimulation was due to an increase in the maximum rate of transport, tubules were perfused at greater than or equal to 20 nL/mm.min. During the control period, tubules absorbed bicarbonate at 82.5 +/- 13.0 pmol/mm.min. After 10(-10) M angiotensin was added to the bath, these same tubules absorbed bicarbonate at 75.9 +/- 11.9 pmol/mm.min. Thus, angiotensin did not alter the maximum rate of transport. Angiotensin also had no effect on bicarbonate permeability, which was 1.1 +/- 0.2 x 10(-4) cm/s before treatment and 1.3 +/- 0.3 x 10(-4) cm/s afterward. Finally, the effect of angiotensin on Na+/K+ ATPase activity was measured in paired experiments. Na+/K+ ATPase activity of control tubules was 36 +/- 6 pmol of ADP/mm.min; after angiotensin treatment, it was 47 +/- 6 pmol ADP/mm.min. From these data it was concluded that: (1) angiotensin stimulates bicarbonate absorption in the rat proximal straight tubule; (2) this stimulation is the result of a change in Km rather than of an increase in the maximum rate of transport or permeability; and (3) angiotensin directly stimulates Na+/K+ ATPase activity in the proximal nephron.