Functional heterogeneity of the descending limbs of Henle's loop

By using the in vitro microperfusion technique, were examined functions of the descending limbs of Henle's loop obtained from either the short-loop nephron (SDL) or the upper portion of the long-loop nephron of hamsters (LDLu). Morphological distinctions between these segments were confirmed by light and electron microscopic observations. Both segments were highly permeable to water. The LDLu was highly permeable to sodium and to chloride: efflux coefficients (10^–7 cm²·s^–1) for²²Na and³⁶Cl were 41.0±5.4 and 3.8±0.6, respectively. The SDL were less permeable to sodium and to chloride: efflux coefficient for²²Na and³⁶Cl were 2.9±1.4 and 0.9±0.2, respectively. In contrast, the SDL was more permeable to urea as compared to the LDL, efflux coefficients for urea being 5.1±1.4 vs 1.4±0.3, respectively. When composition of the perfusate was identical to that of the bathing fluid, no transepithelial voltage was demonstrated. The volume flux was very small or undetectable. From these observations, we propose that the internephron heterogencity must be taken into consideration for constructing a model of countercurrent system in the renal medulla.