Three kidney systems appear during vertebrate development - the pronephros, mesonephros and metanephros. A characteristic of vertebrate organogenesis is the development of a primary ureter in association with the pronephros. This duct forms the collecting duct system of the latter kidneys and it is furthermore a key player in the induction of these kidney generations. Whether the ureter participates in urine modification, remains to be elucidated. In amphibians the pronephros is a large organ, which is functional for a considerable time before it degenerates.

The aim of this study was to investigate putative ion transport mechanisms in the primary ureter of the freshwater amphibian Ambystoma mexicanum (axolotl). Primary ureters isolated from axolotl larvae were perfused in vitro and single cells were impaled across the basal cell membrane with glass microelectrodes. In 42 cells the membrane potential (V_m) was - 79 ± 11 mV (mean ± sd). V_m depolarized by 35 ± 8 mV (n = 21) upon a bath [K⁺] step from 3 to 20 mmol/l, demonstrating the presence of a large K⁺ conductance in the basolateral cell membrane. Luminal fluid exchange experiments in 4 cells revealed a depolarization of V_m in response to [K⁺] steps from 3 to 20 mmol/l and a hyperpolarization of V_m upon lowering [Na⁺] from 102 to 2 mmol/l, indicating the presence of luminal K⁺ and Na⁺ conductances. This study provides the first functional data on the vertebrate primary ureter.

The data show that the primary ureter of axolotl larvae participates in final urine modification and indicate that the ureter reabsorbs Na⁺ and secretes K⁺ via channels present in the apical cell membrane.