Clearance of Surfactant Protein A from Rabbit Lungs

Surfactant protein A (SP-A) is a major surfactant protein with multiple biophysical, metabolic, and host defense functions. To further characterize its metabolism in vivo, we measured clearance of SP-A from adult rabbit lungs. Trace amounts of [125I]SP-A radiolabeled by the Bolton-Hunter method and mixed with [3H]dipalmitoylphosphatidylcholine (DPPC) were given intratracheally via a bronchoscope to rabbits. Groups of five to six animals were studied 10 min to 16 h after labeled surfactant administration. After collection of alveolar washes, lamellar bodies were isolated from lung tissue. Macrophages also were isolated from alveolar washes. [125I]SP-A was cleared more rapidly than DPPC from the airspaces. Both [125I]SP-A and [3H]DPPC were lost exponentially from the total lungs, with half-life values of 6.5 h for SP-A and 12 h for DPPC (P < 0.01). In macrophages, the highest radioactivities for SP-A and DPPC were at 10 to 45 min and the radiolabels subsequently disappeared similarly. In lamellar bodies, 125I and 3H radioactivities per mumol saturated phosphatidylcholine (Sat PC) increased in parallel and were highest at 2 h. Whereas radiolabeled lipids were recovered in lamellar bodies for up to 16 h, iodinated SP-A was lost, indicating less recycling of SP-A than DPPC. We previously showed independent pathways of SP-A and Sat PC secretion in rabbits. These results demonstrate the different clearance kinetics of these two principle components of surfactant.