Abstract
Secreted lung surfactant phospholipid has been shown to be recycled by alveolar type II cells. We hypothesize that the 35 kDa surfactant apoprotein (SAP) participates in surfactant reutilization. SAP was purified from rabbit lung lavage, radiolabeled with [125I], and instilled, along with carrier surfactant, into the lungs of neonatal rabbits. After intervals of 0, 5, 15, 30 or 60 minutes, the rabbits were sacrificed, their lungs lavaged, then the lung tissue homogenized. Uptake of [125I]-SAP by the lung tissue was significant and time-dependent, reaching 16.9 ± 1.6% of the amount originally instilled by 1 hour. Less than 1% of the instilled [125I]SAP was detected in blood, liver and kidney. These data are suggestive that instilled [125I]SAP does not cross the alveolar wall. The proportion of trichloracetic acid-precipitable [125I]SAP in the lung tissue or lavage did not change as a function of time. These data are evidence that the instilled [125I]SAP is not degraded. To control for possible non-specific uptake of protein, lungs of neonatal rabbits were instilled with [125I]-rabbit serum albumin, a protein abundant in alveolar fluid. Less than 3% of the [125I] albumin was taken up by the lung tissue after 1 hr. We have shown that the surfactant apoprotein is taken up by the lung, and thus may be involved in the process of surfactant reutilization.
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