Angiotensin 1-7 has a biphasic effect on fluid absorption in the proximal straight tubule

The effect of angiotensin 1-7 (Ang 1-7) on the proximal tubule has not been well studied. It was hypothesized that Ang 1-7 has a biphasic effect on fluid absorption in the isolated rat proximal straight tubule. Proximal straight tubules were perfused at a rate of 5.81 +/- 0.44 nL/mm per minute and absorbed fluid at 0.98 +/- 0.10 nL/mm per minute. Bicarbonate absorption was 80.1 +/- 11.6 pmol/mm per minute. When 10(-12) M Ang 1-7 was added to the bath, fluid absorption increased to 1.47 +/- 0.10 nL/mm per minute (P < 0.013) and bicarbonate increased to 115.0 +/- 12.8 pmol/mm per minute (P < 0.004). Ang 1-7 had no effect on either the maximum rate of bicarbonate absorption (P > 0.90) or bicarbonate permeability (P > 0.60). Next, 10(-8) M Ang 1-7 was used. During the control period, fluid absorption was 0.90 +/- 0.09 nL/mm per minute. When 10(-8) M Ang 1-7 was added, fluid absorption decreased to 0.62 +/- 0.04 nL/mm per minute (P < 0.05). DuP 753, an AT1 receptor antagonist, blocked both effects induced by Ang 1-7, whereas PD 123319, an AT2 receptor antagonist, did not block the stimulatory effect. From these data, it was concluded that Ang 1-7 binds AT1 receptors and has a biphasic effect on fluid absorption, and at physiologic levels, the heptapeptide induces the stimulation of bicarbonate absorption.