The effect of serum on the secretion of radiolabeled mucous macromolecules into the lumen of the cat trachea.

We studied the effect of placing serum within a segment of trachea on secretion into its lumen in the cat. A segment of cervical trachea was isolated from the rest of the airway in situ. Secretions were radiolabeled biosynthetically by the administration of two radiolabeled precursors: [35S]sodium sulphate and [3H]glucose. Autologous serum placed in the segment at a dilution of 1 in 8 increased the output of radiolabeled macromolecules: [35S] by 80% and [3H] by 159% (p less than 0.001). At a dilution of 1 in 24, serum still increased the output of both isotopes. At dilutions of 1 in 48 and 1 in 80 the increases were significant for [35S] but not for [3H]. Heating the serum to 90 degrees C diminished its effects. Fractionating the serum by dialysis and gel filtration showed that the components of molecular weight less than about 13,000 daltons had no effect on secretion, whereas three higher molecular weight fractions all increased secretion. Two alien proteins (horseradish peroxidase and bovine serum albumin) stimulated secretion but a large molecular weight carbohydrate (carboxymethyl cellulose) did not. Atropine and propranolol, at doses that greatly reduced the effect of parasympathetic and sympathetic nerve activity, did not diminish the effects of serum, which therefore appeared to be independent of nerve activity. Gel filtration of the secretions elicited by serum showed that the predominant component was excluded even by Sepharose CL-2B and thus had a high molecular weight. We conclude that there are several components of serum that promote the secretion of mucus glycoproteins into the cat trachea. The relevance of these findings to diseases of human airways is considered.