Surfactant subtypes of mice: metabolic relationships and conversion in vitro

Abstract

Mouse alveolar surfactant can be separated by equilibrium centrifugation on continuous sucrose gradients into three subtypes which we call "ultraheavy", "heavy", and "light" on the basis of their buoyant densities. We examined their metabolic relationship by in vivo labeling studies and by physical manipulation, cycling the surface area in vitro in an attempt to convert one subtype into another. Labeling studies indicated rapid quantitative progression of surfactant through ultraheavy, heavy, and light subtypes in sequence. To mimic the in vivo conversion of subtypes in vitro we "cycled" the surface area of surfactant in plastic tubes. Newly secreted surfactant obtained from incubated lungs, as well as surfactant obtained by alveolar lavage and lamellar bodies, exhibited conversion of material from heavier to lighter subtypes. The conversion between subtypes was quantal and was dependent on cycling, temperature, and time. We conclude that the three subtypes are discrete forms of alveolar surfactant that evolve from one into another. Cycling may provide a means to study the mechanisms of their interconversion in vitro.

LUNG SURFACTANT: A Personal Perspective

“…In freeze-fracture images (50), the alveolar liquid shows many small unilamellar vesicles that may represent material shed from the surface and on its way to removal by alveolar macrophages and by recycling into type II cells. Results of tracer labeling of surfactant components to measure their turnover (33,34,49) are compatible with this description.…”

“…The questions were easy to ask but difficult to answer, because the surfactant is not one molecule but rather a complex of many components, each of which has its own metabolic pathways, and because lung tissue contains dozens of different cell types that must be separated away before unambiguous experiments can be done with the cells that process the surfactant. The connection between the surfactant and the lamellar bodies of the type II alveolar epithelial cells was appreciated very early (11,42), but it was a decade before respectable lamellar body fractions (29) and type II cell preparations were achieved (40,51), and yet another decade before extracellular surfactant was resolved into meaningful subfractions (33,49,68). With these tools in hand, many studies have been done revealing the pathways and control points of synthesis of phospholipid components (summarized in 4).…”

Mechanisms responsible for surfactant changes in sepsis-induced lung injury

“…The LA were then labelled with 3 H-DPPC and the association of the radiolabel with the LA was verified with the use of sucrose density centrifugation, as previously described [17]. Radiolabelled SA were obtained via surface area cycling of aliquots of labelled LA [18]. Briefly, aliquots of labelled LA were isolated and cycled at 40 rev?min -1 , at 37uC, for 180 min.…”

Properties of modified natural surfactant after exposure to fibrinogen in vitro and in an animal model of respiratory distress syndrome

“…In this procedure, a tube containing a surfactant suspension is rotated end over end at 37 °C so that the surface area of the suspension changes twice each cycle. The method was originally developed by Gross and Narine (8) to study conversion of large functionally superior surfactant aggregates (LAs) into small functionally inferior surfactant aggregates (SAs). In vitro cycling in the presence of supernatant from bronchoalveolar lavage fluid of injured animals, containing inflammatory mediators and blood proteins, produced significantly less LA-to-SA conversion.…”