In vivo protein secretion by lung. Evidence for active secretion and interspecies differences

Abstract

The present study is an attempt to determine (a) if the lung actively secretes protein into the surface-active fraction of lung lavage returns; (b) if there are interspecies differences in this secretory activity; and (c) if the amount of nonradioactive protein in the lavage surface-active fraction shows interspecies variation. I found that pilocarpine stimulates the release of radioactive protein into the lavage surface-active fraction of rabbits and that this pilocarpine effect is completely blocked by atropine. Inhibition of lung oxygen consumption by iodoacetate is associaged with a dose-dependent inhibition of the pilocarpine-induced secretion. Microtubules may be involved in this secretory process because colchicine inhibits the pilocarpine effect. Of the radioactive protein in the total surface-active fraction (tissue plus lavage returns), a greater percent appears in the lavage surface-active fraction at 2 and 4 h, after a pulsed injection [U-14C] leucine, in the mouse than in the rat, which in turn has a greater amount than the rabbit. There is also a difference in the amount of nonradioactive protein per square meter of alveolar surface area in the lavage surface-active fraction of different species: mouse greater than rabbit greater than cat greater than dog. The amount of nonradioactive protein per square meter of alveolar surface area in the lavage surface-active fraction is directly proportional to the species respiratory rate; the log of the nonradioactive protein in the lavage surface-active fraction is inversely proportional to the log of the species alveolar diameter. I conclude that the lung actively secretes protein into the lavage surface-active fraction, that this secretion is under neurohumoral regulation, and that respiratory rate and alveolar size may influence this secretory activity and the amount of protein in this surface-active fraction.