Endogenous angiotensin II modulates rat proximal tubule transport with acute changes in extracellular volume

In the present study, we examined whether the effect of endogenously produced angiotensin II on proximal tubule transport in the male Sprague- Dawley rat is regulated by acute changes in extracellular volume. We measured the magnitude of endogenous angiotensin II-mediated stimulation of transport by sequentially perfusing proximal tubules in vivo, first with an ultrafiltrate-like solution, then by reperfusion of the same tubule with an ultrafiltrate-like solution containing 10^-8 M losartan (angiotensin II receptor antagonist). During volume contraction, 10^-8 M losartan decreased volume reabsorption from 4.20 ± 0.50 to 1.70 ± 0.30 nl · mm^-1 · min^{- 1} (P < 0.05), a decrease of 58.0 ± 7.0%. In contrast, after acute volume expansion, 10^-8 M losartan decreased volume reabsorption from 1.84 ± 0.20 to 1.31 ± 0.20 nl·mm^-1·min^-1 (P < 0.05), a decrease of 29.6 ± 9.0%. In hydropenic rats, addition of exogenous luminaL angiotensin II had no effect on transport. However, in volume-expanded rats, addition of 10^-8 M angiotensin II increased volume reabsorption from 2.10 ± 0.34 to 4.38 ± 0.59 nl·mm^-1· min^-1 (P < 0.005). These data are consistent with endogenously produced angiotensin II augmenting proximal tubule transport to a greater degree during volume contraction than after volume expansion.