Transport of albumin by the rabbit trachea in vitro

The rabbit whole trachea was mounted in vitro in an organ bath containing Krebs-Henseleit (KH) solution. When the trachea was air filled there was no resting secretion and none was induced by methacholine (0.02 mM). Histology showed that the trachea has very few submucosal glands. When the trachea was filled with KH, with fluorescent bovine serum albumin in the surrounding KH solution, the rate of transport of albumin into the lumen was measured. Methacholine (0.02 mM) and phenylephrine (0.1 mM) more than doubled the output of albumin, and albuterol (0.1 mM) increased it more than fourfold. Cooling the preparation to 4 degrees C decreased the spontaneous output of albumin to less than one-half control and abolished the increase in output due to albuterol. Addition of sodium cyanide (1 mM) to the preparation abolished the increase in albumin transport due to albuterol. Serosal-to-mucosal transport of fluorescent dextran (mol wt 70,000) was less than one-third that of albumin and was not enhanced by methacholine, phenylephrine, or albuterol. Lysozyme output, an index of serous cell secretion, was barely detectable in controls and was not enhanced by any of the drugs. We conclude that the rabbit trachea has no measurable submucosal gland secretion and that it can actively transport albumin into the lumen via the epithelium. The transport rate is enhanced by methacholine, phenylephrine, and especially by albuterol.