ABSTRACT. Pulmonary surfactant proteins SP-B and SP-C were isolated from lavage fluids of bovine lungs and recombined (lipid/proteins, 911, wt/wt) with dipalmitoyl phosphatidylcholine for testing in vitro and in surfactantdeficient adult rats. Using a pulsating bubble surfactometer, we found that inflation pressures of bubbles at minimum radii in these mixtures were 0.34 f 0.05 cm H 2 0 (+ SD, n = 24) after 1 min. These values were not affected by increasing amounts of surfactant protein relative to dipalmitoyl phosphatidylcholine (DPPC). Minimum inflation pressures were similar to those of modified bovine surfactant, surfactant Tokyo Akita (TA) (0.33 f 0.05 cm H20, n = 7). I n vivo testing was carried out in adult rats made surfactant deficient by repeated lavage and ventilated with 100% oxygen. Rats received tracheal instillations of either air, DPPC, DPPCISP-B,C (9:1), or surfactant TA at 50 mg/kg body weight. Surfactant TA and DPPCISP-B, SP-C mixtures resulted in similar immediate and sustained improvements in arterial oxygenation (308 + 66 torr, n = 10 and 312 2 101 torr, n = 6 at 30 min posttreatment) that were significantly greater than those of sham (76 f 24 torr, n = 17) and DPPC-treated rats (64 f 32 torr, n = 7). Rats treated with either DPPC/SP-B,C mixtures or surfactant TA showed similar postmortem static lung compliances (2.3 f 0.8 ml/cm H20/kg, n = 8 and 1.9 2 0.4 ml/cm H20/kg, n = 5, respectively) that were significantly larger than sham (1.3 + 0.3 ml/cm H 2 0 / kg, n = 14) and DPPC-treated rats (1.2 f 0.2 ml/cm H20/ kg, n = 6). We conclude that simple mixtures of DPPC

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