Functional heterogeneity of the descending limbs of Henle's loop

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Abstract
Permeability properties of the descending limbs of Henle's loop were compared among rabbits, hamsters, and rats by measuring transepithelial voltage (Vt) across the isolated renal tubules perfused in vitro. From the deflection of the Vt when the composition of the bathing fluid was varied, the permeabilities of sodium and of potassium relative to chloride (PNa/PCl andPK/PCl, respectively) were determined in either the descending limbs of the short-loop nephron (SDL) or the segments of the upper protion of the long-loop nephron (LDLu). In hamsters and rats, the values ofPNa/PCl of the LDLu (3.98±0.66 and 5.03±0.79) were higher than those of the SDL (0.68±0.03 and 0.61±0.00). In contrast, inrrabbits the value ofPNa/PCl of the LDLu (0.96±0.05) was only slightly higher than that of the SDLu (0.75±0.03). The similar tendency was also noted in the values ofPK/PCl. In hamsters and rats, thePK/PCl ratios were 4.90±0.82 and 6.44±0.90, respectively, in the LDLu and 1.09±0.04 and 1.02±0.0, respectively in the SDL. When a transepithelial osmotic gradient was imposed by adding raffinose to the bath, a lumen-negative streaming voltage of about −8 mV was generated in the hamster and the rat LDLu. Taken together with the findings in the preceding paper, these observations support the view that the descending limbs of rabbits are different from those of hamsters and rats in that internephron heterogeneity is less remarkeble, and that the LDLu of hamsters and rats is highly permeable to sodium and to potassium as well as to water.