Studies on isolated perfused tubules of the tiger salamander (Ambystoma tigrinum) have shown that the distal nephron is heterogeneous with respect to function (Stoner, 1977). In this study, the initial portion of the distal tubule (diluting segment) exhibited a voltage, positive in the lumen, and a net absorption of chloride. Since the chloride was transported against an electrochemical gradient, its transport was active. More distad, the junctional segment exhibited a lumen-negative voltage and sodium, rather than chloride, was transported actively. More recently Delaney and Stoner (1981) have demonstrated in vitro that the collecting duct of this species also has a lumen-negative voltage which is probably associated with active sodium reabsorption.
The primary objective of the present paper was to correlate the morphology of the diluting and junctional segments of the Ambystoma distal tubules with the physiologic data from the same isolated perfused tubules. The results indicate that the morphological heterogeneity previously demonstrated in distal tubules of Necturus exists with respect to both structure and function in Ambystoma. The cell types found in the amphibian distal nephron appear to be homologous to those seen in the mammalian nephron.