Transition of permeability properties along the descending limb of long-loop nephron

The isolated segments of the hamster descending limb of the long-loop nephron (LDL) were perfused in vitro to demonstrate the axial heterogeneity with respect to permeability properties. When a NaCl gradient from the lumen to bath was present, the lumen-negative diffusion voltage (VD) was generated in the upper portion (LDLU). When the VD was measured stepwise along the axis of tubules, the magnitude of the VD decreased in the portion within 0.5 mm before the border between the outer and inner medulla in most cases, indicating that a gradual functional transition to the lower portion (LDLL) occurs along the descending limb. The lumen-to-bath flux coefficients (K1----b) for Na+ were 14.3 +/- 3.7 X 10(-7) and 2.4 +/- 0.8 X 10(-7) cm2/s in the LDLU and LDLL, respectively. The K1----b for urea were 1.7 +/- 0.6 X 10(-7) and 7.9 +/- 4.1 X 10(-7) cm2/s, respectively. The LDLL was highly permeable to water, with osmotic permeability coefficient being 1,693 +/- 517 X 10(-9) cm2.s-1.atm-1. The reflection coefficients for NaCl and urea were not different from unity. From these observations, we conclude that the functional transition occurs along the LDL from the segment with a high Na+ permeability to that with a low Na+ permeability.