Effect of luminal angiotensin II on rabbit proximal convoluted tubule bicarbonate absorption

The present in vitro microperfusion study examined the effect of luminal angiotensin II on proximal convoluted tubule (PCT) volume absorption and bicarbonate transport. Neither 10^-11 M, 10^-10 M, nor 2 x 10^-8 M luminal angiotensin II significantly affected PCT transport. When tubules were first perfused with enalaprilat to inhibit endogenous angiotensin II production, addition of 10^-10 M luminal angiotensin II increased volume absorption (0.72 ± 0.08 vs. 0.86 ± 0.07 nl · mm^-1 · min^-1, P < 0.01) and bicarbonate transport (52.3 ± 3.7 vs. 67.9 ± 4.2 pmol · mm^-1 · min^-1, P < 0.01). Addition of 10^-6 M losartan, an AT₁ inhibitor, to the luminal perfusate inhibited volume absorption (0.95 ± 0.14 vs. 0.72 ± 0.11 nl · mm^-1 · min^-1, P < 0.05) and bicarbonate transport (65.0 ± 7.3 vs. 54.7 ± 9.2 pmol · mm^-1 · min^-1, P < 0.05). Addition of 10^-4 M luminal PD-123319, an AT₂ inhibitor, was without effect. In tubules perfused with 10^-4 M luminal enalaprilat and 10^-4 M luminal PD-123319, addition of 10^-10 M luminal angiotensin II in the experimental period resulted in a stimulation in volume absorption (0.61 ± 0.08 vs. 0.81 ± 0.10 nl · mm^-1 · min^-1, P < 0.01) and bicarbonate transport (49.9 ± 6.3 vs. 77.4 ± 14.3 pmol · mm^-1 · min^-1, P < 0.01). In tubules perfused with 10^-6 M losartan and 10^-4 M enalaprilat, addition of luminal 10^-10 M angiotensin II resulted in no change in transport. These data are consistent with endogenous angiotensin II affecting PCT bicarbonate transport in vitro via luminal AT₁ receptors.