Mechanism of distal tubular chloride transport in Amphiuma kidney

To characterize the mechanism of chloride transport across individual cell membranes, experiments were carried out on early distal tubules in the doubly perfused Amphiuma kidney, and net chloride flux, transepithelial and transmembrane cell potentials, and intracellular chloride activity measured. Net chloride flux was evaluated by a modified stationary microperfusion technqiue, and intracellular and intraluminal chloride activities by means of double-barreled liquid ion exchange microelectrodes. Control conditions were characterized by significant net volume and chloride reabsorption, a transepithelial potential difference of +9.0 ± 0.5 mV (lumen positive), and cell chloride activities above electrochemical equilibrium across both luminal and peritubular cell membranes. Following luminal application of furosemide (5 x 10^-5 M) or perfusion with either a sodium- or chloride-free solution, net flux of chloride fell dramatically, the transepithelial potential difference was abolished, and cell chloride activity dropped sharply to approach electrochemical equilibrium. The decrease in transepithelial potential difference was fully accounted for by hyperpolarization of the basolateral cell membrane potential. These results are consistent with a furosemide-sensitive, electrically neutral sodium chloride cotransport mechanism across the luminal cell membrane.