Regulation of surfactant phosphatidylcholine secretion from alveolar type II cells during Pneumocystis carinii pneumonia in the rat

Abstract

We used an immunosuppressed rat model to test the hypothesis that normal mechanisms regulating surfactant phosphatidylcholine synthesis and secretion in alveolar type II cells are aberrant in Pneumocystis carinii pneumonia. Animal groups included: group 1, healthy controls; group 2, immunosuppressed, without pneumocystosis; group 3, immunosuppressed with pneumocystosis; group 4, immunosuppressed with well-established pneumocystosis treated with trimethoprim-sulfamethoxazole (TMP-SMX). Type II cells were isolated from rats in each group and compared for [3H]choline incorporation into phospholipid and response of the type II cells to secretagogues. Incorporation of [3H]choline into phospholipid subclasses exhibited significant differences. Incorporation into phosphatidylcholine fell from 89.3 +/- 2.2% of total incorporation in group 1 control rats to 79.6 +/- 3.1% in group 3 rats with P. carinii pneumonia, while incorporation into sphingomyelin rose from 5.6 +/- 1.2% in group 1 animals to 15.2 +/- 2.7% in group 3 rats. Incorporation of [3H]choline into phospholipid subclasses in cells from group 2 and group 4 animals was not different from incorporation for group 1 animals. Type II cells from group 1 and group 2 (immunosuppressed control) rats responded appropriately to the secretagogues ATP, TPA, and terbutaline with a marked increase in surfactant phosphatidylcholine secretion; the effect of ATP was also blocked by the lectin, concanavalin A. In contrast, type II cells from group 3 rats failed to respond to the secretagogues with a significant increase in phospholipid secretion. Although treatment of group 4 rats with TMP-SMX markedly reduced the P. carinii organism burden, type II cells from these animals also responded poorly to the secretagogues. The depressed type II cell function described here provides a mechanism for the observed decrease in surfactant phospholipids from bronchoalveolar lavage fluid of experimental animals and patients with P. carinii pneumonia. The data also suggest this defect may become irreversible with advanced disease.