Objective Five studies are described to determine whether there is an outermost lining of surface-active phospholipid (SAPL) adsorbed to the peritoneum and to quantify its ability to act as a release (antistick) agent and boundary lubricant by standard tests. Methods Using a hydrophobic probe (phosphin E), epifluorescence microscopy was used to demonstrate an outermost lining of oligolamellar SAPL by spectral analysis of the emitted light, a finding consistent with the appreciable hydrophobicity demonstrated on canine peritoneal mesothelium and its virtual elimination by incubation with bile salt. Good release and excellent lubricating capabilities of human peritoneal SAPL have been quantified as the release factor and coefficient of friction, respectively, by standard tests from the physical sciences. Results A well-defined outermost layer was clearly visible on peritoneal mesothelium whose color spectrum was identical to that produced by pure phosphatidylcholine ultrasonicated into its oligolamellar state. Further evidence for a SAPL lining was demonstrated by a parietal contact angle of 43° (47° visceral) on this surface and its virtual elimination by incubation with dilute bile salt. Human SAPL from continuous ambulatory peritoneal dialysis (CAPD) effluent proved an effective release agent, reducing adhesion by 67%, and an excellent lubricant as quantified by a coefficient of friction of 0.091 under load (1.9 kg/cm2). Conclusions The good release and excellent lubricating properties of SAPL adsorbed to mesothelial surfaces are highly desirable in reducing wear and exfoliation of epithelial cells. In spanning epithelial cells, the same lining might also serve to render tight junctions tight and reduce macromolecular escape while compatible with many aspects of CAPD, including lipid permeability and conflicting results obtained from administering exogenous SAPL.
It was proposed some years ago that, in osteoarthritis, one source of joint stiffness arises from 'articular gelling', but, if so, why does this not occur in the normal joint? In a preliminary experiment using agar gels, it is shown how such fusion of gel surfaces can be inhibited by surface-active phospholipid (SAPL)--both synthetic and human--as quantified by the shear stress needed to cause cleavage between samples after prolonged contact. On the other hand, normal bovine articular cartilage (BAC) does not fuse to itself, but can be made to do so if rinsed with a powerful lipid solvent known to remove the outermost layer of adsorbed SAPL along with the hydrophobicity so characteristic of the normal 'waxy' surface it imparts. It is then shown how the inhibition of gel fusion can be restored by treating both bovine and human articular surfaces with exogenous SAPL derived from human AC and with synthetic SAPL. Samples of human articular cartilage excised from osteoarthritic hips and knees during total joint replacement showed a 55% greater tendency to fuse together than normal BAC. This was exacerbated by solvent rinsing and can be attributed to a deficiency in the outermost lining of SAPL previously studied as a load-bearing boundary lubricant capable of reducing friction and wear to the remarkably low levels observed physiologically. Hence, joint stiffness can be attributed, in part, to a deficiency in the lubricating layer of SAPL lining the normal articular surface where it can inhibit articular gelling/gel fusion, possibly imparting other desirable physiological functions. The possibility of clinical replenishment of SAPL in the osteoarthritic joint is discussed.
This study analyzed processes underlying osteoporosis and osteoarthrosis after short-term immobilization of the right hind limb of postadolescent (2.8 kg) and mature (4.0 kg) rabbits. After 3 weeks, the lateral posterior aspect of the lateral tibial plateau and the lateral femoral condyle of the immobilized limb exhibited prominent subchondral vascular eruptions. Femoral metaphyseal bone density decreased 27 and 18% in the immobilized limbs of postadolescent and mature rabbits, respectively. Calcein green fluorescence increased 1.9-fold (p less than 0.001) in the metaphyseal trabeculae of immobilized femurs. With immobilization, sulfate incorporation into femoral cartilage glycosaminoglycan increased, although total cartilage glycosaminoglycan and hydroxyproline levels were unchanged. Thymidine incorporation into DNA increased four- to fivefold in tibial and femoral cartilage of the immobilized limb. In this study, bone loss and remodeling preceded erosive cartilage degradation.
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