Distribution of surfactant protein A in rat lung

Although surfactant protein A (SP-A) is an integral component of alveolar surfactant, its relative abundance in lamellar bodies, regarded as the intracellular storage organelles for surfactant, remains contentious. We have previously shown that lamellar bodies, isolated from rat lung by upward flotation on a sucrose gradient, can be subfractionated into classic-appearing lamellar bodies (Lb-A) and a vesicular fraction (Lb-B), which we have speculated may be a second release form of surfactant. In the present study, we have used two-dimensional protein electrophoresis and immunochemical analysis to clarify the origin and the composition of these two subcellular fractions. In addition, we have examined the hypothesis that the secretion of SP-A and surfactant phospholipids occurs by independent pathways by examining the distribution of SP-A, total protein, and disaturated phospholipids (DSP) in the tubular myelin-rich (Alv-1) and tubular myelin-poor (Alv-2) fractions separated from lavaged material and in Lb-A and Lb-B isolated from both lung homogenate and purified alveolar type II cells. Our findings indicate that Lb-B is derived from type II cells, although they do not indicate whether it is a secretory form of surfactant, a reuptake vesicle, or a mixture of both. We found that the lung has a large tissue pool of immunoreactive SP-A. The %SP-A/DSP of total lamellar bodies isolated from type II cells was 0.96 +/- 0.1 (mean +/- SE), intermediate between that in Lb-A (1.67 +/- 0.13) and in Lb-B (0.65 +/- 0.04). In contrast, the %SP-A/DSP was 11.16 +/- 0.84 in whole lung homogenate and 13.14 +/- 1.71 in whole type II cells. In the alveolar compartment, the %SP-A/DSP was 17.38 +/- 3.40 in Alv-1, 6.34 +/- 0.31 in Alv-2, and 10.49 +/- 1.43 in macrophages, values an order of magnitude greater than found with the lamellar bodies. Our results indicate that only a relatively small portion of alveolar SP-A is derived from lamellar bodies, and we suggest that secretion of SP-A and DSP occurs via independent pathways.