EFFECTS OF LOW-DOSE ANGIOTENSIN-II INFUSION ON LOOP SEGMENT REABSORPTION - A FREE-FLOW MICROPUNCTURE STUDY IN RATS

1. Little direct information is available on the actions of angiotensi n 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 mi cropuncture at the late proximal tubule and the early distal tubule. 2 . Endogenous angiotensin II was suppressed by pretreatment with enalap ril. Compared with a control group, angiotensin II increased mean arte rial 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 0.1 3+/-0.03 mu mol/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 e arly distal tubule did not change significantly. 3. Similar experiment s were performed during partial aortic constriction to exclude the eff ects 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+ reabsorptio n between late proximal and early distal tubule was not disturbed by a ngiotensin II. For water, however, this relation tended to shift to a higher reabsorption rate. 5. The decrease in fractional excretion of L i+ 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+ reabs orption 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 p roximal tubule, an effect correctly indicated by the change in the fra ctional excretion of Li+, does not influence Na+ reabsorption in the l oop of Henle beyond the proximal tubule, but may increase Na+ reabsorp tion in more distal segments.