Effects of Low-dose Angiotensin II Infusion on Loop Segment Reabsorption: A Free-Flow Micropuncture Study in Rats

Abstract

1. Little direct information is available on the actions of angiotensin II beyond the proximal tubule. We therefore studied the effect of a mildly vasoconstrictive dose of angiotensin II on tubular handling of water, sodium (Na+) and lithium (Li+) in rats by means of free-flow micropuncture at the late proximal tubule and the early distal tubule. 2. Endogenous angiotensin II was suppressed by pretreament with enalapril. Compared with a control group, angiotensin II increased mean arterial pressure by 15 mmHg. Glomerular filtration rate decreased from 1.32 ± 0.05 to 1.10 ± 0.05 ml/min, Na+ excretion from 0.43 ± 0.09 to 0.13 ± 0.03 μmol/min, fractional delivery of water at the late proximal tubule from 50.1 ± 1.7 to 42.9 ± 3.2%, fractional delivery of Na+ at the late proximal tubule from 46.5 ± 1.3 to 39.1 ± 3.5% and fractional delivery of water at the early distal tubule from 26.4 ± 1.4 to 21.9 ± 1.0% (P < 0.05 for each variable). Fractional delivery of Na+ at the early distal tubule did not change significantly. 3. Similar experiments were performed during partial aortic constriction to exclude the effects of increased perfusion pressure. The data obtained were similar, except that in this group the fractional delivery of Na+ at the early distal tubule decreased from 8.6 ± 0.7 to 6.8 ± 0.9% (P < 0.05). 4. The relation between late proximal tubule Na+ delivery and Na+ reabsorption between late proximal and early distal tubule was not disturbed by angiotensin II. For water, however, this relation tended to shift to a higher reabsorption rate. 5. The decrease in fractional excretion of Li+ followed the decrease in proximal reabsorption as measured directly by micropuncture. Angiotensin II infusion did not appear to affect Li+ reabsorption beyond the proximal tubule. Distal fractional Na+ reabsorption estimated by the Li+ clearance increased significantly during angiotensin II infusion. 6. In conclusion, our data indicate that a Na+-retaining dose of angiotensin II increases Na+ reabsorption in the proximal tubule, an effect correctly indicated by the change in the fractional excretion of Li+, does not influence Na+ reabsorption in the loop of Henle beyond the proximal tubule, but may increase Na+ reabsorption in more distal segments.