Triacylglycerol hyrolysis in the isolated, perfused rat lung

We have studied the mechanism of hydrolysis of labeled long-chain triacylglycerols in the isolated, ventilated, perfused rat lung. Hydrolysis of emulsified tri[3H]oleate or doubly labeled [3H]glyceryl, tri[14C]oleate was measured by quantitation of [3H]oleate or of [14C]oleate and [3H]glycerol released into the perfusate. Hydrolysis was directly proportional to the initial triacylglycerol concentration of the perfusate in the range of 0.30 to 2.2 mM. The release of free fatty acids was linear after an initial lag period, the length of which was inversely proportional to the triacylglycerol concentration. Studies with doubly labeled [3H]glyceryl, tri[14C]oleate showed that, during triacylglycerol hydrolysis, the molar ratio of free fatty acid to glycerol released is close to 1, suggesting that about one-third of the fatty acids hydrolyzed is released into the pulmonary circulation. The earlier appearance of free fatty acid than glycerol in the venous effluent indicates that the first step in triacylglycerol hydrolysis occurs at the endothelial surface. In order to investigate the role of lipoprotein lipase in this process, we administered heparin, which leads to immediate release of lipoprotein lipase from the endothelium to the circulation in vivo, 10 min and 4 h before isolation and perfusion of the lungs. Heparin administration 10 min prior to perfusion led to marked release of lipoprotein lipase from the lungs and completely abolished the subsequent hydrolysis of circulating triacylglycerols. Perfusions carried out 4 h after heparin administration show that in the lung, endothelial lipoprotein lipase levels did not return to normal within 4 h after heparin administration. The data show that circulating triacylglycerols are hydrolyzed by endothelial lipoprotein lipase during passage through the lung.