Na+-dependent transport of 14C-L-lysine across bullfrog alveolar epithelium

Transepithelial transport of the basic amino acid L-lysine has been studied utilizing the isolated intact bullfrog lung mounted in the Ussing chamber. Lungs were excised from doubly pithed bullfrogs and sandwiched between two hemichambers. 14C-(U)-L-lysine was added to the upstream reservoir of amphibian Ringer solution, while the tissue was short-circuited. Two lungs from the same animal were used simultaneously to determine the two opposite unidirectional fluxes. Downstream and upstream radioactivities were assayed and used to estimate the apparent permeability (P) of the labeled lysine. Results indicate that the apparent P of 14C-L-lysine measured in the alveolar (M) to the pleural (S) direction is 19.06 (+- 2.84) x 10-7 cm/s and P in the S to M direction is 3.29 (+- 0.02) x 10-7 cm/s. When the 100 mM NaCl in the bath was replaced by 110 mM choline chloride, the flux of 14C-L-lysine from the alveolar to the pleural side decreased to the same value as that in the opposite direction. The flux from the pleural to the alveolar direction in the absence of Na+ did not change. These results suggest that the alveolar epithelium exhibits Na+-dependent amino acid (L-lysine) transport in the M->S, but not in the S->M, direction