Surfactant protein A labeling kinetics in newborn and adult rabbits.

Abstract

Surfactant protein A (SP-A), the major hydrophilic protein specifically associated with surfactant, has multiple metabolic and host defense functions as well as primary surfactant biophysical functions in association with the other surfactant proteins and lipids. To characterize its kinetics of secretion and clearance from the airspace, we measured specific activity-time curves for alveolar and lamellar body associated SP-A following the intravascular and/or intratracheal administration of the radiolabeled precursors Tran 35S-label containing primarily methionine and cysteine or [3H]methionine to newborn and adult rabbits. Alveolar SP-A specific activity peaked 30 min after precursor injection in both newborn and adult rabbits, and labeled SP-A was not detected in lamellar bodies until after 2 h. In newborns, a second peak of labeled SP-A appeared at 15 h. In both newborns and adult rabbits, lamellar body specific activity-time curves were most consistent with SP-A entering lamellar bodies via a recycling pathway from the airspaces. The airspace clearance of SP-A in adult rabbits had a biologic half-life of about 4.5 h. There was very little decrease in SP-A specific activity in the newborn rabbits, indicating minimal catabolism. These studies demonstrate secretion of endogenously synthesized SP-A by a pathway separate from lamellar bodies. The kinetics of secretion of SP-A and the surfactant phospholipid in newborn and adult rabbits indicate separate metabolic pathways.

Pathway to lamellar bodies for surfactant protein A

“…This latter study postulated the rapid recycling (resecretion) of endocytosed SP-A through an early endosome. On the other hand, studies of the intact lung following endotracheal instillation of labeled SP-A have demonstrated its uptake from the alveolar space and subsequent appearance in lamellar bodies (15,18,19), as confirmed in the present study. One possibility for these different results is that the isolated type II cells had lost the mechanism for normal SP-A trafficking to lamellar bodies, consistent with their differentiation in culture and loss of other phenotypic characteristics.…”

“…However, there is less agreement concerning the intracellular trafficking of SP-A. Like SP-B and SP-C, SP-A is synthesized by type II cells and is secreted into the alveolar space, where it associates with the lung surfactant phospholipids (19,22,37). Extracellular SP-A appears to play crucial roles in the formation of tubular myelin, in the regulation of lamellar body exocytosis, and in the uptake of phospholipids by endocytosis and their subsequent remodeling (20,23,30,40).…”

“…These latter publications have suggested an alternative pathway in which newly synthesized SP-A is secreted constitutively, i.e., by a nonregulated secretory pathway. In this scenario, the presence of SP-A in lamellar bodies is accounted for entirely by recycling following endocytosis of extracellular 19,28,29,36). Because of its cellular complexity, it has been difficult to evaluate the relative roles for these two potential pathways (regulated vs. constitutive secretion) in the intact lung, and the majority of recent studies related to intracellular SP-A trafficking have been carried out with isolated type II cells (28,29,31).…”

“…These studies were interpreted to indicate transfer of newly synthesized SP-A from the endoplasmic reticulum to storage organelles (lamellar bodies) prior to exocytosis, similar to the pathways for processing of lung surfactant phospholipid. However, more recent studies using the intact lung (11,19) or isolated type II cells (28,31) have indicated possible differences in kinetics for the appearance of SP-A in surfactant and lamellar bodies. These latter publications have suggested an alternative pathway in which newly synthesized SP-A is secreted constitutively, i.e., by a nonregulated secretory pathway.…”

Effects of aging on surfactant forms in rats

“…After opening the thorax, a short, blunt needle (ranging 14±22 gauge, depending on age) was inserted into the trachea. Five aliquots of saline at 48C were used to visibly fill the lungs as reported previously [10,15,16]. Each aliquot was flushed into and out of the airways three times.…”