Relationship between plasma free fatty acid, intramyocellular triglycerides and long-chain acylcarnitines in resting humans

Abstract

We hypothesized that plasma non-esterified fatty acids (NEFA) are trafficked directly to intramyocellular long-chain acylcarnitines (imLCAC) rather than transiting intramyocellular triglycerides (imTG) on the way to resting muscle fatty acid oxidation. Overnight fasted adults (n = 61) received intravenous infusions of [U-(13)C]palmitate (0400-0830 h) and [U-(13)C]oleate (0800-1400 h) labelling plasma NEFA, imTG, imLCAC and im-non-esterified FA (imNEFA). Two muscle biopsies (0830 and 1400 h) were performed following 6 h, overlapping, sequential palmitate/oleate tracer infusions. Enrichment of plasma palmitate was approximately 15 times greater than enrichment of imTG, imNEFA-palmitate and im-palmitoyl-carnitine. Fatty acid enrichment in LCAC was correlated with imTG and imNEFA; there was a significant correlation between imTG concentrations and imLCAC concentrations in women (r = 0.51, P = 0.005), but not men (r = 0.30, P = 0.11). We estimated that approximately 11% of NEFA were stored in imTG. imTG NEFA storage was correlated only with NEFA concentrations (r = 0.52, P = 0.004) in women and with V(O(2),peak) (r = 0.45, P = 0.02) in men. At rest, plasma NEFA are trafficked largely to imTG before they enter LCAC oxidative pools; thus, imTG are an important, central pool that regulates the delivery of fatty acids to the intracellular environment. Factors relating to plasma NEFA storage into imTG differ in men and women.

Figure 1. Schematic of the experimental design

Time is presented as clock time. RMR, resting metabolic rate using indirect calorimetry; FFM, fat free mass.

Figure 2. Relationship between palmitate enrichments of intramyocellular fatty acid pools

A, relationship between intramyocellular triglyceride (imTG) palmitate enrichment (MPE) and intramyocellular non-esterified fatty acid (imNEFA) palmitate enrichment for all participants. B, relationship between imNEFA palmitate MPE and intramyocellular palmitoyl-carnitine (PC) MPE for all participants. C, relationship between imTG palmitate MPE and PC MPE for all participants.

Figure 4. Relationship between aerobic fitness as measured by peak and the % of NEFA calculated to be stored in skeletal muscle in men (left panel) and women (right panel)

There was no significant relationship in women.

Figure 3. Relationship between plasma free fatty acid (NEFA) concentrations and the % of NEFA calculated to be stored in skeletal muscle in women (left panel) and men (right panel)

There was no significant relationship in men.

Figure 5. Proposed trafficking of plasma NEFA in skeletal muscle

The observed isotopic enrichment ratio of 15 : 1 between plasma non-esterified fatty acids (NEFA) and intramyocellular fatty acid pools – intramyocellular triglyceride (imTG) and long-chain acyl-carnitines (LCAC) – suggests that once inside the cell, plasma NEFA probably are shunted into complex lipid synthesis, including imTG, which then equilibrates with the NEFA pool. This pool appears to be both the recipient of fatty acids from the hydrolysis of complex lipids, such as imTG, and may be a precursor pool for DG and TG synthesis. The hydrolysis of imTG (lipolysis) releases long-chain fatty acids that can enter the NEFA pool and/or be esterified to long-chain acyl-CoA and LCAC. LCAC are committed to enter the mitochondria for β-oxidation. These observations suggest that imTG is a central intracellular pool that plays a central role in regulating plasma NEFA trafficking and utilization by skeletal muscle.