Physiological activity of pulmonary surfactant with low protein content: effect of enrichment with synthetic phospholipids.

A natural surfactant with low protein content (1%) was prepared by a sequence of cold centrifugation, heating to 90 degrees C, sucrose-gradient centrifugation, and extraction with chloroform:methanol. Some of the material was enriched with dipalmitoylphosphatidylcholine (DPPC) and unsaturated phosphatidylglycerol (PG) to relative concentrations of 56% and 10%, respectively. The in vitro physical properties of these preparations were evaluated with pulsating bubble and Wilhelmy balance and their in vivo activity with experiments on artificially ventilated premature newborn rabbits, delivered on day 27 of gestation. The animals were kept in body plethysmographs at 37 degrees C and ventilated artificially with a standardized sequence of insufflation pressures: 25, 20, and 15 cm H2O. The lungs were fixed by vascular perfusion and the alveolar expansion evaluated morphometrically in histologic sections. Enrichment of surfactant with DPPC and PG resulted in lower minimal surface tension during surface compression but did not further improve lung compliance or the alveolar expansion pattern. Treatment with nonenriched surfactant at a phospholipid concentration of 100 mg/ml (individual dose = 200 mg/kg) caused a markedly increased lung compliance at all insufflation pressure levels (p vs. controls less than .01). Our data indicate that pulmonary surfactant remains physiologically active after removal of most of its protein components and that enrichment with DPPC and PG reduces the in vitro minimal surface tension without adding to the in vivo efficacy.