Guo, ZengKui

This study examined the plasma FFA, intramuscular long chain acylcarnitine (LCAC) fatty acids and imTG stores during basal and hyperinsulinemic conditions. Methods: Endurance trained (TR, n=8) and sedentary individuals (SED, n=15) were studied under overnight postabsorptive and euglycemic hyperinsulinemic clamp (1.0mU·kg FFM −1 ·min −1 ) conditions. FFA kinetics were measured using a 5‐h infusion of [U‐ 13 C]oleate, for labeling of imTG and LCAC pools. Muscle biopsies were performed to measure imTG and imLCAC enrichment at 1400 h after 5‐h of infusion. imTG concentrations were greater in TR than SED individuals (P<0.01). Table SED Trained Saline** Clamp Saline** Clamp plasma oleate FFA (mpe) 0.210±0.018* 0.271±0.036* 0.189±0.026* 0.361±0.050* imLCAC (mpe) 0.025±0.003 0.005±0.002 0.016±0.005 0.002±0.002 imTG (mpe) 0.018±0.003† 0.011±0.001† 0.024±0.004† 0.013±0.001† P<0.001 saline vs. clamp; *P<0.000 between compartment (plasma vs. imLCAC vs. imTG); †P<0.000 imLCAC and imTG Methods: Endurance trained (TR, n=8) and sedentary individuals (SED, n=15) were studied under overnight postabsorptive and euglycemic hyperinsulinemic clamp (1.0mU·kg FFM −1 ·min −1 ) conditions. FFA kinetics were measured using a 5‐h infusion of [U‐ 13 C]oleate, for labeling of imTG and LCAC pools. Muscle biopsies were performed to measure imTG and imLCAC enrichment at 1400 h after 5‐h of infusion. Conclusion: The imTG contributes more to the imLCAC pools than plasma FFA both in the basal and hyperinsulinemic state, and hyperinsulinemia suppressed oleate FFA entry into the imLCAC pools, more so in trained than untrained adults. Muscle fatty acid oxidation is largely via obligatory cycling of fatty acids through imTG or other intracellular complex lipid pools.

A pulse ([14C]palmitate)-chase ([3H]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at ∼45% peak O2consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise;3H2O production, breath14CO2excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise. Results: during exercise, 8.2 ± 1.2 and 8.4 ± 0.7 μmol · kg−1· min−1of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different ( P = 0.6) from lipid oxidation by indirect calorimetry (15.4 ± 1.6 μmol · kg−1· min−1); the isotopic and indirect calorimetry values were correlated ( r = 0.79, P < 0.005). During exercise, imTG turnover rate was 0.32 ± 0.07%/min (6.0 ± 2.0 μmol of imTG · kg wet muscle−1· min−1) and plasma FFA were incorporated into imTG at a rate of 0.7 ± 0.1 μmol · kg wet muscle−1· min−1. The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.

We evaluated the applicability of stable isotopic tracers to the study of intramuscular fatty acid metabolism by infusing both [U-13C]palmitate and [1-13C]oleate intravenously for 4 h into fasted conscious rats. Skeletal muscles were sequentially biopsied, and the concentration and13C enrichment of fatty acids were measured by gas chromatography/combustion/isotope ratio mass spectrometry. Throughout the study, the13C enrichment of plasma palmitate and oleate remained substantially greater than intramuscular nonesterified palmitate and oleate enrichment, which in turn was greater than intramuscular triglyceride palmitate and oleate enrichment. Fractional synthesis rates of intramuscular triglycerides in gastrocnemius and soleus were 0.267 ± 0.075 and 0.100 ± 0.030/h ( P = 0.04), respectively, as determined by using [U-13C]palmitate, and were 0.278 ± 0.049 and 0.075 ± 0.013/h ( P = 0.02), respectively, by using [1-13C]oleate. We conclude that plasma free fatty acids are a source for intramuscular triglycerides and nonesterified fatty acids; the latter are likely the synthetic precursors of the former. Uniformly and singly labeled [13C]fatty acid tracers will provide an important tool to study intramuscular fatty acid and triglyceride metabolism.

We previously reported that epinephrine stimulates leg free fatty acid (FFA) release in men but not in women. The present studies were conducted to determine whether the same is true during exercise. Six men and six women bicycled for 90 min at 45% of peak O2 consumption, during which time systemic and leg FFA kinetics ([9,10-3H]palmitate) were measured. The catecholamine and hormonal responses to exercise were not different in men and women. The baseline systemic and leg palmitate release was 94 ± 15 vs. 114 ± 5 μmol/min and 16 ± 2 and 20 ± 3 μmol/min, respectively, in men and women [ P = nonsignificant (NS)]. Systemic and leg palmitate release increased (both P < 0.001) to 251 ± 18 vs. 212 ± 16 μmol/min and 73 ± 19 vs. 80 ± 12 μmol/min in men and women, respectively, during the last 30 min of exercise ( P = NS, men vs. women). We conclude that the systemic and leg adipose tissue lipolytic response to exercise is not different in nonobese men and women.

We hypothesized that insulin alters plasma free fatty acid (FFA) trafficking into intramyocellular (im) long-chain acylcarnitines (imLCAC) and triglycerides (imTG). Overnight-fasted adults ( n = 41) received intravenous infusions of [U-13C]palmitate (0400–0900 h) and [U-13C]oleate (0800–1400 h) to label imTG and imLCAC. A euglycemic-hyperinsulinemic (1.0 mU·kg fat-free mass−1·min−1) clamp (0800–1400 h) and two muscle biopsies (0900 h, 1400 h) were performed. The patterns of [U-13C]palmitate incorporation into imTG-palmitate and palmitoylcarnitine were similar to those we reported in overnight postabsorptive adults (saline control); the intramyocellular palmitoylcarnitine enrichment was not different from and correlated with imTG-palmitate enrichment for both the morning ( r = 0.38, P = 0.02) and afternoon ( r = 0.44, P = 0.006) biopsy samples. Plasma FFA concentrations, flux, and the incorporation of plasma oleate into imTG-oleate during hyperinsulinemia were ∼1/10th of that observed in the previous saline control studies ( P < 0.001). At the time of the second biopsy, the enrichment in oleoylcarnitine was <25% of that in imTG-oleate and was not correlated with imTG-oleate enrichment. The intramyocellular nonesterified fatty acid-palmitate-to-imTG-palmitate enrichment ratio was greater ( P < 0.05) in women than men, suggesting that sex differences in intramyocellular palmitate trafficking may occur under hyperinsulinemic conditions. We conclude that plasma FFA trafficking into imTG during hyperinsulinemia is markedly suppressed, and these newly incorporated FFA fatty acids do not readily enter the LCAC preoxidative pools. Hyperinsulinemia does not seem to inhibit the entry of fatty acids from imTG pools that were labeled under fasting conditions, possibly reflecting the presence of two distinct imTG pools that are differentially regulated by insulin.