Systemic and forearm triglyceride metabolism - Fate of lipoprotein lipase-generated glycerol and free fatty acids

Little is known about the fate of the lipolytic products produced by the action of lipoprotein lipase (LPL) on circulating triglyceride-rich lipoproteins in humans. We studied eight lean, healthy male subjects after an overnight fast. Subjects received infusions of lipid emulsions containing triolein labeled with H-3 on both the glycerol backbone and the fatty acid portion of the molecule; C-14 glycerol and C-14 oleate were coinfused to quantify the systemic and forearm release of H-3 glycerol and H-3 oleate resulting from LPL action. There was significant forearm uptake of both whole plasma triglyceride (presumed to represent primarily VLDL; extraction fraction 2.6 +/- 0.6%, P < 0.005 vs. zero) and radiolabeled triglyceride derived from the lipid emulsion (a surrogate for chylomicrons; extraction fraction 31 +/- 4%, P < 0.005 vs zero). Systemic clearance and forearm fractional extraction of glycerol was greater than that of oleate (P < 0.001 and P < 0.02, respectively). The systemic and forearm fractional release of LPL-generated glycerol were similar at 51 +/- 4 and 59 1%, respectively (NS). In contrast, the forearm fractional release of LPL-generated oleate was less than systemic fractional release (14 +/- 2 vs. 36 4%, P < 0.0001). These results indicate that there is escape, or spillover, of the lipolytic products of LPL action on triglyceride-rich lipoproteins in humans. They further suggest that LPL-mediated fatty acid uptake is an inefficient process, but may be more efficient in muscle than in adipose tissue.