Surface-active phospholipid: a Pandora's box of clinical applications. Part II. Barrier and lubricating properties.

In Part I, it was described how their configuration renders phospholipid molecules surface active and capable of acting at interfaces in addition to the liquid-air interface to which conventional theory has hitherto confined the study of 'surfactant' in the lung. Surface-active phospholipid (SAPL) appears no different to comparable surfactants studied in the physical sciences for the highly desirable properties that their adsorption (reversible binding) can impart to solid surfaces. In Part II, these properties are considered in sites where there is no air. Highly desirable properties include boundary lubrication (lubricity), release (antistick) and the ability of the strongly adsorbed and strongly cohesive SAPL linings to act as barriers against abrasion, corrosion and, possibly, against invasion by microorganisms. As the 'sealant', it could be the true barrier rather than the cells providing its mechanical support. Evidence is reviewed for SAPL providing the gastric mucosal barrier to acid in the stomach and preventing the digestion of Helicobacter pylori until that barrier is broken by bile in the duodenum, where H. pylori cause ulcers. The concept that SAPL provides effortless sliding of many tissues, including pleura, pericardium and peritoneum is reviewed. Particular attention is paid to the load-bearing joints, where a deficiency has been associated with osteoarthritis. The ability of the same SAPL lining to perform multiple roles is discussed in relation to the peritoneum, where it could provide the lubricant/release agent preventing surgical adhesions, while imparting semipermeability to 'the membrane' vital for peritoneal dialysis. In each site, the prophylactic use of exogenous SAPL is discussed for its potential clinical applications.