Pulmonary Surfactant Report

Pulmonary surfactant

Introduction

Pulmonary surfactant is a complex mixture of phospholipids (PL) and proteins (SP) that reduce surface tension at the air-liquid interface of the alveolus, thus preventing its collapse during end-exhalation (Daniels, 2003; Malloy et al., 2005). It also participates in innate host defense against inhaled pathogens (Malloy et al., 2005).

Surfactant is synthesized and secreted by Type II alveolar epithelial cells, also called pneumocytes, which differentiate between 24 and 34 weeks of gestation in the human. It is made up of 70% to 80% phospholipids, approximately 10% protein and 10% neutral lipids, mainly cholesterol (Jobe, 2002). The primary surface-active material found in surfactant is the phospholipid, dipalmitoylphosphatidylcholine

This theory evolved from studies of peptides synthesized according to sequences of SP-B amino acids or mimicking these sequences which showed that SP-B provided cohesiveness to molecules of phospholipids (Cochrane, 2005; Cochrane and Revak, 1991). The peptides and SP-B are hydrophobic and are positioned in the acyl side chains of the phospholipid monolayer, with strong electrostatic interactions between the positively charged amino acids and the negatively charged phospholipids. This bonding of SP-B, peptide and phospholipid molecules confers lateral stability to the phospholipid molecules in the monolayer of the alveolus and by virtue of this; the cohesive monolayer is able to prevent collapse of the alveolus (Cochrane, 2005; Mazela et al.,

It is then recycled in a highly complex and regulated mechanism. This process is slower in newborns (especially those born prematurely) than in adults or those with lung injury.

Fig. : Pulmonary surfactant metabolism (Hawgood and Poulain, 2001).

The rate of synthesis and the half-life of surfactant are influenced by many factors. Surfactant synthesis and turnover in preterm infants using stable isotopes of glucose, acetate and palmitic acid demonstrates that synthesis from glucose to surfactant phosphatidylcholine (PC) takes approximately 19 hours and reaches a peak at 70 hours after labeling. The absolute production rate of PC is 4.2 mg/kg/day while the half-life is 113 (± 25) hours (Bunt et al., 1998). The fractional synthesis rate of surfactant PC from plasma palmitate was significantly higher than that from palmitate synthesized de novo from acetate or glucose, but only accounted for half of the total surfactant production in preterm infants (Bohlin et al.,