INVITRO PERFUSION OF CHINCHILLA THIN LIMB SEGMENTS - UREA AND NACL PERMEABILITIES

We measured the urea and NaCl permeabilities (P(urea) and P(NaCl), respectively) of the following nephron segments from chinchilla: the upper part of the long-loop descending limb (from outer medulla, LDL(u)), the middle part of the long-loop descending limb (from outer 30% of the inner medulla, LDL(m)), the lower part of the long-loop descending limb (from deep inner medulla, LDL(l)), and the thin ascending limb (from deep inner medulla, ATL). We found that P(urea) (X 10(-5) CM/s) was relatively low in the LDL(u) (3.3), but that the value was larger in the inner medullary thin descending limb (16.8 for LDL(m) and 47.6 for LDL(l)). The ATL had an even higher value (I 70). Phloretin, 0.25 mM, added to the peritubular bath had no effect on P(urea) of these segments, suggesting that the rapid transport rate is not due to a phloretin-sensitive facilitated transport pathway like that seen in the inner medullary collecting duct. P(NaCl) (x 10(-5) CM/s) also increased with distance along the length of the thin descending limb (LDL(u), 11.7; LDL(m), 41.2; LDL(l), 98.4; and ATL, 321). Calculations from NaCl dilution potential measurements showed that LDL(u) was Na+ permselective, whereas LDL(l) and ATL were Cl- permselective. High solute permeabilities in the inner medullary thin descending limb contradict a major requirement of the passive model of urinary concentration developed previously (J. P. Kokko and F. C. Rector, Jr. Kidney Int. 2: 214-223, 1972; and J. L. Stepohenson. Kidney Int. 2: 85-94, 1972).