Modified natural surfactant preparations, used for treatment of respiratory distress syndrome in premature infants, contain phospholipids and the hydrophobic surfactant protein (SP)-B and SP-C. Herein, the individual and combined effects of SP-B and SP-C were evaluated in premature rabbit fetuses treated with airway instillation of surfactant and ventilated without positive end-expiratory pressure. Artificial surfactant preparations composed of synthetic phospholipids mixed with either 2% (wt/wt) of porcine SP-B, SP-C, or a synthetic poly-Leu analog of SP-C (SP-C33) did not stabilize the alveoli at the end of expiration, as measured by low lung gas volumes of approximately 5 ml/kg after 30 min of ventilation. However, treatment with phospholipids containing both SP-B and SP-C/SP-C33 approximately doubled lung gas volumes. Doubling the SP-C33 content did not affect lung gas volumes. The tidal volumes were similar in all groups receiving surfactant. This shows that SP-B and SP-C exert different physiological effects, since both proteins are needed to establish alveolar stability at end expiration in this animal model of respiratory distress syndrome, and that an optimal synthetic surfactant probably requires the presence of mimics of both SP-B and SP-C.

The secondary structure, orientation and hydrogen/deuterium exchange of SP-C33, a surfactant protein C analog, in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/egg phosphatidylglycerol (8:2, wt./wt.) bilayers, was studied by attenuated total reflection Fourier transform infrared spectroscopy. This showed a transmembrane α-helix, in which about 55% of the amide hydrogens do not exchange for up to 20 h. Moreover, C-terminally modified SP-C33, either truncated after position 30, or having the methionine at position 31 exchanged for either lysine or isoleucine, showed the same secondary structure and orientation. The different peptides, suspended in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol 68:31 (wt./wt.), were tested for surfactant activity in vitro in a captive bubble surfactometer and in vivo in an animal model of respiratory distress syndrome using premature rabbit fetuses. All preparations showed similar surface activity in the captive bubble surfactometer. Also, in the rabbit model, all preparations performed equally well and significantly better than non-treated controls, both regarding tidal volumes and lung gas volumes. Thus, truncation or residue replacements in the C-terminal part of SP-C33 do not seem to affect membrane association or surfactant activity.

Background: The development of synthetic surfactants for the treatment of lung pulmonary diseases has been going on for many years. Objectives: To investigate the effects of phospholipid mixtures combined with SP-B and SP-C analogues on lung functions in an animal model of respiratory distress syndrome. Methods: Natural and synthetic phospholipid mixtures with/without SP-B and/or SP-C analogues were instilled in ventilated premature newborn rabbits. Lung functions were evaluated. Results: Treatment with Curosurf® or phospholipids from Curosurf combined with SP-B and SP-C analogues gave similar results. Treatment with phospholipids from adult rabbit lungs or liver combined with dipalmitoylphosphatidylcholine (DPPC) and palmitoyloleoylphosphatidylglycerol (POPG) gave tidal volumes (V_T) well above physiological levels, but alveolar stability at end-expiration was only achieved when these phospholipids were combined with analogues of SP-B and SP-C. Treatment with egg yolk-PC mixed with DPPC with and without POPG gave small V_T, but after addition of both analogues V_T was only somewhat lower and lung gas volumes (LGV) similar to those obtained with Curosurf. Substitution of egg yolk-PC (≥99% PC) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine, and combining them with DPPC, POPG and 2% each of the SP-B and SP-C analogue gave a completely synthetic surfactant with similar effects on V_T and LGV as Curosurf. Conclusions: Phospholipid composition is important for V_T while the SP-B and SP-C analogues increase alveolar stability at end-expiration. Synthetic surfactant consisting of unsaturated and saturated phosphatidylcholines, POPG and the analogues of SP-B and SP-C has similar activity as Curosurf regarding V_T and LGV in an animal model using preterm newborn rabbits ventilated without positive end-expiratory pressure.

Background: Plasma proteins are known to interfere with pulmonary surfactant. studies have proven the hypothesis that fibrinogen preserves exogenous surfactant subjected to longterm surface area cycling. Methods: The exogenous surfactant curosurf was subjected to long-term surface area cycling without or with fibrinogen (ratio 2:1 w/w) and was tested by captive bubble surfactometer and on newborn premature rabbits. results: surface tension increased in curosurf (80 mg/ml) samples without fibrinogen after 6-12 d of cycling. In samples with fibrinogen the cycling time had no effect on surface tension. addition of fibrinogen to surfactant prevented lipid peroxidation. Lung gas volumes of animals with noncycled curosurf or curosurf cycled with fibrinogen for 6 d were comparable and higher than in rabbits with curosurf cycled without fibrinogen. alveolar volume density was higher in groups with noncycled curosurf or curosurf cycled with fibrinogen than in curosurf cycled without fibrinogen (both P < 0.001). conclusion: The effect of fibrinogen on pulmonary surfactant cycled at 37 °c depends both on surfactant concentration and cycling time. at high phospholipid concentration used in clinical practice fibrinogen has a protective effect on biophysical and physiological properties of natural modified surfactant subjected to surface area cycling. This effect is partially mediated by reduction in lipid peroxidation.