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 p roduced angiotensin II on proximal tubule transport in the male Spragu e-Dawley rat is regulated by acute changes in extracellular volume. We measured the magnitude of endogenous angiotensin II-mediated stimulat ion of transport by sequentially perfusing proximal tubules in vivo, f irst with an ultrafiltrate-like solution, then by reperfusion of the s ame tubule with an ultrafiltrate-like solution containing 10(-8) M los artan (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 decre ased 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 transpor t. 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 n l.mm(-1).min(-1) (P < 0.005). These data are consistent with endogenou sly produced angiotensin II augmenting proximal tubule transport to a greater degree during volume contraction than after volume expansion.