R. Fransen et al., EFFECTS OF LOW-DOSE ANGIOTENSIN-II INFUSION ON LOOP SEGMENT REABSORPTION - A FREE-FLOW MICROPUNCTURE STUDY IN RATS, Clinical science, 88(3), 1995, pp. 351-358
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.