Metabolism of individual fatty acids during infusion of a triacylglycerol emulsion.

The triacylglycerol emulsion Intralipid was infused into six normal subjects to investigate the metabolism of individual fatty acids in subcutaneous adipose tissue and forearm muscle, by measurement of arteriovenous differences. The composition of plasma nonesterified fatty acids changed steadily after passage through adipose tissue and became similar to that of the emulsion, reflecting hydrolysis of the Intralipidtriacylglycerol by lipoprotein lipase, since endogenous lipolysis (hormone-sensitive lipase activity plus lipoprotein lipase hydrolysis of very low density lipoprotein triacylglycerol) was decreased. There was no significant net release of total or individual fatty acids from forearm muscle although there was a tendency for the composition of the fatty acids in forearm venous plasma to change during passage through the tissue to reflect the composition of the emulsion. This may reflect hydrolysis of emulsion particles by lipoprotein lipase situated in capillaries which drain into the forearm vein. The behavior of stearic acid in the plasma nonesterified fatty acid pool was consistently aberrant, with arterialized concentrations considerably higher than predicted from adipose tissue release, both before and during Intralipid infusion. We conclude that there are no significant differences in the metabolism of specific fatty acids, with the exception of stearic acid.

Interstitial glycerol concentration in human skeletal muscle and adipose tissue is close to the concentration in blood.

1. The suggestion that the interstitial glycerol concentration in both adipose tissue and skeletal muscle is around 3 mmol/I (Maggs DG, Jacob R, Rife F, et al. J Clin Invest 1995; 96: 370-7), rather than close to the blood concentration as previously supposed, was tested by independent methods. 2. Free glycerol was infused, as part of a triacylglycerol emulsion, into six normal subjects and the arteriovenous difference for glycerol across the forearm was measured. In addition the relative interstitial glycerol concentration in subcutaneous adipose tissue was assessed simultaneously in four of the subjects by microdialysis. 3. During glycerol infusion the arterialized glycerol concentration rose from 52 +/- 5 mumol/I to 250-300 mumol/I (P < 0.001) in a square wave fashion. The net arteriovenous difference for glycerol across the forearm changed from negative (output) to positive (uptake) (P < 0.01). In subcutaneous adipose tissue the interstitial glycerol concentration rose during glycerol infusion (P < 0.001). 4. These observations are most easily explained by the movement of glycerol from plasma to interstitial fluid down a concentration gradient. We conclude that the interstitial glycerol concentration in skeletal muscle and adipose tissue is closer to the arterial concentration than to 3 mmol/I.