Uptake of arachidonic acid and glucose into isolated human adipocytes

Abstract

Both plasma glucose concentration and glucose uptake are deranged in insulin resistance. A high free fatty acid plasma level is a potential cause of insulin resistance, and therefore of type 2 diabetes mellitus animals and humans. The mechanism behind this is still unclear. The objectives of the present study were: (i) to research the effect of arachidonic acid (AA) as fatty acid representative, on glucose uptake into human isolated adipocytes, (ii) to investigate the uptake of AA into adipocyte membranes and nuclei, as a step to identify the mechanism whereby AA affects glucose uptake, and (iii) to verify the influence of insulin on AA uptake in adipocytes. The first objective was achieved by exposing adipocytes to AA and measuring the effect on deoxyglucose uptakt. To achieve the second objective, adipocytes were exposed to 14C-AA; radioactive uptake in membranes and nuclei was determined. The AA uptake into membranes was also determinate by membranes fatty acid profile using gas chromatography; the results of the two methods were compared. Finally, the third objective was achieved by exposing adipocytes to different concentrations of insulin and testing the effect by measuring arachidonic acid uptake by the entire cell. The results of this study shown that, acute (30 min) exposure of AA significantly stimulates glucose uptake by adipocytes (4.56 ± 0.6 nmole glucose /mg protein /min) compared to the control (3.12 ± 0.25 nmole glucose /mg protein /min). Secondly, 14C-AA was significantly taken up by the membranes between 20 and 30 minutes of exposure. The uptake into membranes was increased by 49.57 ± 29% and 123 ± 73% compared to the control 100% (1.77 ± 0.06 nmole AA /mg protein) respectively for 20 and 30 min exposure). AA significantly rose in the nuclei after 30 minutes (147 ± 19% increase) compared to the control 100% (2.25 ± 0.10 nmole AA /mg protein). The determination of AA uptake by gas chromatography analysis of the membrane fatty acid profile showed that the content of AA increased after 30 min exposure (0.57% AA of total membrane fatty acids) compared to the 10 min exposure (0.29% AA of total membrane fatty acid). Insulin was shown to stimulate 10 and 30 min AA uptake by adipocytes from a non-obese subject. The increases of AA uptake measured for 30 minutes were 20 ±8%, 21 ± 25% and 31 ± 4% compared to the control (0.58nmole AA / mg protein / min) respectively for the actions of 10nM, 20nM and 40 nM insulin. A similar tendency was observed when the AA uptake was measured for 10 min (81 ± 31% and 208 ± 36% respectively for the action of 10nM and 40nM insulin compared to the control 100% (0.06nmole AA/mg protein/min). In contrast to this finding, insulin depressed AA uptake by adipocytes from an obese subject (depression of 15 ± 5%, 14 ± 8% and 21 ± 5% respectively for 10nM, 20nM and 40nM insulin, compared to the control 100% (0.74 nmole AA/mg protein/min). In both situations the effect of insulin seemed dose dependent. The study demonstrated that AA acid positively modulates glucose uptake into adipocytes exposed for short periods (< 30 min). This was attributed to the probable this FA in the cell membrane, rather than its eventual effect on the DNA. The best method to measure membranes AA over short period of exposure when small amounts of adipocytes (2- 6 ml) are used was by radioactive means. It also suggested that insulin effect’s on AA acid uptake into adipocytes was dose dependent. This varies with the body mass index (BMI) of the patient, probably as a result of their cell’s insulin resistant state.