Linear arrays of intramembranous particles in pulmonary tubular myelin.

Abstract

Freeze-fracture preparations of tubular myelin in edematous rat lungs reveal the presence of linear arrays of intramembranous particles. The lines of particles are approximately 50 nm apart and appear to correspond to the intersections of sheets of bilayer membranes. Particles are not seen in lamellar bodies but become evident first in membranes transitional between lamellar bodies and tubular myelin. It is proposed that the particles represent a hydrophobic protein that plays a significant role in the organization of tubular myelin. Tubular myelin isa highly ordered array of phospholipids and proteins found in pulmonary alveoli. Interest in tubular myelin arises from its presence as a major alveolar form of surfaceactive material (1, 2). Details of its unique structure have been described by a number of investigators (3-7).Tubular myelin occurs in normal adult lung but is particularly prominent in unaerated fetal lung, in which its derivation from lamellar bodies has been reported by Williams (8). Increased amounts of tubular myelin have been described in association with lung injury (6, 9), and we have observed considerable tubular myelin in the edema fluid induced by epinephrine in adult rats. We report here the results of our studies on the ultrastructure features of such tubular myelin. The presence of orderly, linear arrays of intramembranous particles in freeze-fracture preparations adds to the morphologic basis for the detailed analysis of the molecular structure and function of tubular myelin.MATERIALS AND METHODS Pulmonary edema was induced in 2-to 3-mo-old male Sprague-Dawley rats (Tyler Laboratories, Bellevue, WA) by the intravenous administration of epinephrine. Rats were anesthetized lightly with ether, their abdomens were opened, and epinephrine was injected into the inferior vena cava. Doses ranged from 0.1 to 0.2 ml of saline containing 0.1 mg of epinephrine per ml. Pulmonary edema developed rapidly; the thorax was opened between 3 and 4 min after epinephrine administration. The lungs were removed and fixed by immersion in 2% (vol/vol) glutaraldehyde/1% (vol/vol) formaldehyde (freshly prepared from paraformaldehyde)/0

Correlated Atomic Force and Transmission Electron Microscopy of Nanotubular Structures in Pulmonary Surfactant

“…The cross hatches as shown in the TEM image in Fig. 2 (and possibly in the bilobe structures seen in AFM) were previously suggested by others to be surfactant protein (probably SP-A) associated with the lipid membranes of TM (Beckman and Dierichs, 1984;Voorhout et al, 1991), although freeze-fracture scanning EM has failed to reveal any internal structures in the core of the tubes (Chi and Lagunoff, 1978;VanGolde et al, 1994).…”

“…Limited threedimensional information of such structures is avail- able using scanning electron microscopy (Chi and Lagunoff, 1978;Williams, 1978;Manabe, 1979) and reconstructions based on computer modeling (Young et al, 1992). Part of the difficulty in obtaining information on TM is due to its relatively low abundance found in the natural surfactant or in reconstituted systems (5-10%) (Benson et al, 1984;Hawgood, 1997).…”

The pulmonary extracellular lining.

“…Also, lamellar bodies can be converted to tubular myelin in vitro (67). Freeze-etching studies indicate that lamellar body membranes are "smooth" (69), while membranes of tubular myelin contain particles at approximately the same periodicity as that of the tubular myelin lattice (66,69,70). Thus, it is considered that protein(s) have an important function in organizing the structure of tubular myelin.…”

“…It has been suggested by King and Macbeth (125) that the 34,000 mw apoprotein is necessary for fast adsorption of surfactant from the alveolar subphase onto the air-lipid interface but Metcalfe et al (126) reported in vitro evidence against this supposition. Another possible function for the putative surfactant apoprotein is that of organizing the tubular myelin, for which protein is thought to be important (69,70,127).…”

Quantitation of phosphatidylcholine secretion in lung slices and primary cultures of rat lung cells

“…Because PC is hydrophobic and because intra-alveolar surfactant (as tubular myelin) has a membranous structure (25), the mechanism by which surfactant is distributed around the alveolar surface may be quite different from the secretion of hydrophilic molecules such as proteins. The current view of surfactant as a discrete entity secreted by the lung should probably be re-evaluated in vivo and in vitro in order to provide a better understanding of the control of surfactant production, its distribution, and its role in lung disease.…”