Clearance of surfactant protein B from rabbit lungs.

To characterize the metabolism of surfactant protein B (SP-B) in vivo, we measured the clearance of SP-B from adult rabbit lungs. Purified rabbit SP-B was radiolabeled with 125I by the Bolton-Hunter method. Trace amounts of 125I-labeled SP-B mixed with [14C]dipalmitoylphosphatidylcholine (DPPC) were given intratracheally via a bronchoscope to rabbits 0-16 h before collection of alveolar washes (AW). Macrophages were recovered from AW, and lamellar bodies (LB) were isolated from lung tissue by differential centrifugation. 125I-SP-B was cleared more rapidly from the airspaces and the total lung (half-life 7 h) than was DPPC (half-life 11 h in the total lung). There was an approximately threefold accumulation of SP-B relative to saturated phosphatidylcholine in macrophages at all times. The proportion of 125I and 14C radioactivities in lamellar bodies was similar at 2 and 4 h, but there was 14-fold less 125I-SP-B than [14C]DPPC in lamellar bodies by 16 h. This loss of SP-B from the lamellar body fraction is consistent with less recycling of SP-B. The results demonstrate different clearance kinetics of these two components of surfactant and indicate a significant role of macrophages in the clearance of SP-B.