Alveolar epithelium transport of albumin and sucrose: concentration difference effect

Isolated Ringer-perfused rabbit lungs were used to study albumin and sucrose transport across the blood-gas barrier. Lungs were filled to about 20% total lung capacity (TLC) ith Ringer solution containing radioactive albumin and sucrose, and their rate of appearance in the recirculating perfusate was monitored. From this the product permeability x area (PA) was calculated. In the middle of the 180-min experiments, some alveolar fluid was removed. In control experiments, the same fluid was reinstilled, in other experiments new fluid with higher test molecule concentrations was infused. In all experiments the results for both molecules were similar: PA for the second half of the experiment was 80% of that in the first half. The reduction was probably due to a decrease in exchange area. We thus find the albumin and sucrose permeabilities to be proportional to their concentration difference. In addition, the PA for sucrose was roughly four times that of albumin. These results can possibly be explained by a two-compartment model with two parallel pathways across the alveolar epithelium. One pathway would be small pores, whereas the other would involve a bulk flow or pinocytosis process.