Adipose Tissue Free Fatty Acid Storage In Vivo: Effects of Insulin Versus Niacin as a Control for Suppression of Lipolysis

Abstract

Insulin stimulates the translocation fatty acid transport protein 1 (FATP1) to plasma membrane, and thus greater free fatty acid (FFA) uptake, in adipocyte cell models. Whether insulin stimulates greater FFA clearance into adipose tissue in vivo is unknown. We tested this hypothesis by comparing direct FFA storage in subcutaneous adipose tissue during insulin versus niacin-medicated suppression of lipolysis. We measured direct FFA storage in abdominal and femoral subcutaneous fat in 10 and 11 adults, respectively, during euglycemic hyperinsulinemia or after oral niacin to suppress FFA compared with 11 saline control experiments. Direct palmitate storage was assessed using a [U-(13)C]palmitate infusion to measure palmitate kinetics and an intravenous palmitate radiotracer bolus/timed biopsy. Plasma palmitate concentrations and flux were suppressed to 23 ± 3 and 26 ± 5 µmol ⋅ L(-1) (P = 0.91) and 44 ± 4 and 39 ± 5 µmol ⋅ min(-1) (P = 0.41) in the insulin and niacin groups, respectively, much less (P < 0.001) than the saline control group (102 ± 8 and 104 ± 12 µmol ⋅ min(-1), respectively). In the insulin, niacin, and saline groups, abdominal palmitate storage rates were 0.25 ± 0.05 vs. 0.25 ± 0.07 vs. 0.32 ± 0.05 µmol ⋅ kg adipose lipid(-1) ⋅ min(-1), respectively (P = NS), and femoral adipose storage rates were 0.19 ± 0.06 vs. 0.20 ± 0.05 vs. 0.31 ± 0.05 µmol ⋅ kg adipose lipid(-1) ⋅ min(-1), respectively (P = NS). In conclusion, insulin does not increase FFA storage in adipose tissue compared with niacin, which suppresses lipolysis via a different pathway.

Figure 1

Time course of plasma palmitate concentrations before (time −90 min) insulin or niacin administration and during the time of the radiotracer palmitate bolus (time 0 min) and adipose biopsies (time 30 min).

Figure 2

Direct palmitate storage rates in abdominal subcutaneous and femoral adipose tissue in male and female participants in the saline control group and in those with suppressed FFA via a hyperinsulinemic-euglycemic clamp vs. oral niacin administration. There were no significant differences between saline, insulin, and niacin or between abdomen and thigh for the combined male/female group.

Figure 3

The relationship (r = 0.63, P = 0.003) between DGAT activity and direct palmitate storage rates in femoral adipose tissue for the niacin and insulin groups is shown in A. These same data from the saline control group are also provided. The regression line is for the insulin and niacin groups only. The relationship (r = 0.53, P = 0.02) between ACS activity and direct palmitate storage rates in femoral adipose tissue for the combined groups (same symbols) is shown in B; the regression line is for the insulin and niacin groups only. The data from the saline control group are also depicted.