Lipolysis and fatty acid transport in rat heart: electron microscopic study

Lamellar structures with a periodicity of 50 A developed in myocytes of glutaraldehyde-fixed heart tissues from young and adult rats when the tissues were incubated with tannic acid (pH 7.4) at 25 degrees C. The increase in lamellar structures (P less than 0.025) was accompanied by a significant decrease in intracellular lipid droplets (P less than 0.025), indicating that tissue lipase was active in fixed tissue and that the lamellar structures were probably composed of fatty acids formed by lipolysis. The lamellar structures in myocytes were located in the lumen of intracellular channels near lipid droplets and mitochondria and in the outer compartment of mitochondria. Lamellar structures were found at the periphery of chylomicrons, in intraendothelial channels, and in extracellular space of incubated fixed tissues from chylomicron-injected young rats. Chylomicron-lipid disappeared from capillaries (P less than 0.025) and lamellar structures with wide interlamellar spacings (80-1,000 A) developed in the extracellular space surrounding capillaries (P less than 0.025) in unfixed heart tissue from chylomicron-injected fasted young rats when the tissue was incubated without tannic acid; lamellar structures did not develop in similarly treated tissue from uninjected rats. Thus the lamellar structures found in extracellular space represent fatty acids derived from lipolyzed chylomicrons. We conclude that fatty acids produced by lipolysis in incubated heart accumulated and spread in an interfacial continuum of external leaflets of cell membranes extending from the capillary lumen to extracellular space and from intracellular lipid droplets to the interior of mitochondria in myocytes. When fatty acids overcrowded the continuum, they formed lamellar extensions of the continuum at different sites along its course through the tissue.