Fatty Acid Transport in Adipocytes Monitored by Imaging Intracellular Free Fatty Acid Levels

Transport of free fatty acids (FFA) across the adipocyte plasma membrane is critical for maintaining homeostasis. To determine the membrane's role in regulating transport we describe here the first measurements of the intracellular (unbound) FFA concentration (FFAi) and their use in monitoring transport of FFA across 3T3F442A adipocytes. FFAi was measured by microinjecting cells with ADIFAB, a fluorescently labeled fatty acid-binding protein that is used to measure unbound FFA levels. We used ratio fluorescence microscopy of intracellular ADIFAB to image unbound FFA levels and determined the time course of FFAi in response to changing the extracellular unbound FFA concentration (FFAo). FFAo was clamped at defined levels using complexes of FFA and bovine serum albumin. We show that FFA influx is slow, requiring about 300 s to reach steady state (rate constant ∼ 0.02 s-1) and saturable (Ko ∼ 200 nm). Efflux is twice as fast as influx, for zero FFAo, but decreases with increasing FFAo. Surprisingly, at steady state FFAi is 2–5-fold (average 2-fold) greater than FFAo and this FFAi/FFAo gradient is abolished by depleting cellular ATP. Our results indicate that FFA transport across adipocyte membranes is highly regulated, involving an ATP-driven pump and a mechanism for gating efflux that is sensitive to FFAo. These characteristics are well described by a membrane carrier model but are not consistent with FFA transport across the membrane's lipid phase. We suggest that these characteristics are important in regulating circulating FFA levels by the adipocyte. Transport of free fatty acids (FFA) across the adipocyte plasma membrane is critical for maintaining homeostasis. To determine the membrane's role in regulating transport we describe here the first measurements of the intracellular (unbound) FFA concentration (FFAi) and their use in monitoring transport of FFA across 3T3F442A adipocytes. FFAi was measured by microinjecting cells with ADIFAB, a fluorescently labeled fatty acid-binding protein that is used to measure unbound FFA levels. We used ratio fluorescence microscopy of intracellular ADIFAB to image unbound FFA levels and determined the time course of FFAi in response to changing the extracellular unbound FFA concentration (FFAo). FFAo was clamped at defined levels using complexes of FFA and bovine serum albumin. We show that FFA influx is slow, requiring about 300 s to reach steady state (rate constant ∼ 0.02 s-1) and saturable (Ko ∼ 200 nm). Efflux is twice as fast as influx, for zero FFAo, but decreases with increasing FFAo. Surprisingly, at steady state FFAi is 2–5-fold (average 2-fold) greater than FFAo and this FFAi/FFAo gradient is abolished by depleting cellular ATP. Our results indicate that FFA transport across adipocyte membranes is highly regulated, involving an ATP-driven pump and a mechanism for gating efflux that is sensitive to FFAo. These characteristics are well described by a membrane carrier model but are not consistent with FFA transport across the membrane's lipid phase. We suggest that these characteristics are important in regulating circulating FFA levels by the adipocyte. Transport of free fatty acids (FFA) 1The abbreviations used are: FFA, free fatty acid; ADIFAB, acrylodan labeled rat intestinal fatty acid-binding protein; FAFBSA, fatty acid-free BSA; FFAu, unbound FFA; FFAi, intracellular FFAu; FFAo, extracellular FFAu; kin, influx rate constant; kout, efflux rate constant; pHi, intracellular pH; DIDS, 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid; BSA, bovine serum albumin. into and out of adipocytes is essential for regulating circulating FFA levels (1Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. J. Clin. Investig. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar). Regulation of FFA levels is important because FFA provide a major portion of energy needs and elevated levels adversely affect human health (2Carlsson M. Wessman Y. Almgran P. Groop L. Arterioscler. Thromb. Vasc. Biol. 2000; 20: 1588-1594Crossref PubMed Scopus (89) Google Scholar, 3Boden G. Curr. Opin. Clin. Nutr. Metab. Care. 2002; 5: 545-549Crossref PubMed Scopus (178) Google Scholar, 4Jouven X. Charles M.A. Desnos M. Ducimetiere P. Circulation. 2001; 104: 756-761Crossref PubMed Scopus (219) Google Scholar). Whether the adipocyte plasma membrane plays an important role in regulating FFA is an intensely debated issue (5Hamilton J.A. J. Lipid Res. 1998; 39: 467-481Abstract Full Text Full Text PDF PubMed Google Scholar, 6Zakim D. J. Membr. Biol. 2000; 176: 101-109Crossref PubMed Scopus (35) Google Scholar, 7Abumrad N.A. Harmon C. Ibrahimi A. J. Lipid Res. 1998; 39: 2309-2318Abstract Full Text Full Text PDF PubMed Google Scholar, 8Berk P.D. Stump D.D. Mol. Cell Biochem. 1999; 192: 17-31Crossref PubMed Google Scholar). Evidence from a number of studies has yielded two essentially contradictory mechanisms of FFA transport. One view envisions FFA transport as a rapid diffusive process across the lipid bilayer phase of the membrane, in which the rate-limiting step is dissociation from rather than translocation across the membrane (6Zakim D. J. Membr. Biol. 2000; 176: 101-109Crossref PubMed Scopus (35) Google Scholar, 9Hamilton J.A. Curr. Opin. Lipidol. 2003; 13: 263-271Crossref Scopus (153) Google Scholar, 10Kleinfeld A.M. J. Memb. Biol. 2000; 175: 79-86Crossref PubMed Scopus (50) Google Scholar). The second view proposes that transport is facilitated by membrane transport proteins and for the adipocyte three such proteins have been identified (7Abumrad N.A. Harmon C. Ibrahimi A. J. Lipid Res. 1998; 39: 2309-2318Abstract Full Text Full Text PDF PubMed Google Scholar, 8Berk P.D. Stump D.D. Mol. Cell Biochem. 1999; 192: 17-31Crossref PubMed Google Scholar, 11Schaffer J.E. Lodish H.F. Cell. 1994; 79: 427-436Abstract Full Text PDF PubMed Scopus (744) Google Scholar). These different views reflect, in part, the different methods used to monitor transport. Reports of rapid transport rely on measurements of the change in intracellular pH upon adding extracellular FFA (12Civelek V.N. Hamilton J.A. Tornheim K. Kelly K.L. Corkey B.E. Proc. Natl. Acad. Sci. 1996; 93: 10139-10144Crossref PubMed Scopus (79) Google Scholar, 13Kamp F. Guo W. Souto R. Pilch P.F. Corkey B.E. Hamilton J.A. J. Biol. Chem. 2003; 278: 7988-7995Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar). Identification of FFA transport proteins derive from measurements of FFA uptake, in which the time course is determined for the association of radio- or fluorescently labeled FFA with adipocytes (11Schaffer J.E. Lodish H.F. Cell. 1994; 79: 427-436Abstract Full Text PDF PubMed Scopus (744) Google Scholar, 14Schwieterman W. Sorrentino D. Potter B.J. Rand J. Kiang C.L. Stump D. Berk P.D. Proc. Natl. Acad. Sci. U. S. A. 1988; 85: 359-363Crossref PubMed Scopus (155) Google Scholar, 15Abumrad N.A. el Maghrabi M.R. Amri E. Lopez E. Grimaldi P.A. J. Biol. Chem. 1993; 268: 17665-17668Abstract Full Text PDF PubMed Google Scholar). Transport across the plasma membrane involves the movement of FFA from the extracellular aqueous phase to the cytoplasmic aqueous phase, as well as the reverse process. Elucidation of the steps involved in the process would optimally be done by monitoring directly the time course of the intracellular aqueous phase concentration (FFAi) following changes in extracellular concentration (FFAo). Such measurements have not been reported for FFA transport in cells. We have previously developed methods for encapsulating ADIFAB in lipid vesicles and red cell ghosts and used these systems for studying FFA transport (16Kleinfeld A.M. Chu P. Romero C. Biochemistry. 1997; 36: 14146-14158Crossref PubMed Scopus (85) Google Scholar, 17Kleinfeld A.M. Storms S. Watts M. Biochemistry. 1998; 37: 8011-8019Crossref PubMed Scopus (43) Google Scholar, 18Cupp D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). In this report we have extended these methods by injecting ADIFAB into 3T3F442A adipocytes. We have used ratio fluorescence microscopy to image FFAi and thereby determined the characteristics of FFA transport across adipocytes. The results of these direct measurements of FFA transport indicate a much more complex and highly regulated transport mechanism in adipocytes than expected from previous studies. Buffering FFAo—FFAo was buffered by FFA:BSA complexes formed by titrating with sodium salts of the FFA (Nu-Chek Prep) solutions of 600 μm BSA (fatty acid free from Sigma) in a buffer consisting of 20 mm HEPES, 140 mm NaCl, 5.5 mm glucose, 5 mm KCl, 1 mm NaH2PO4, 1 mm CaCl2, and 1 mm MgSO4 at pH 7.4 (C-HEPES). The titration was done by adding multiple 40–70-μl aliquots of a 50 mm solution of FFA in 4 mm NaOH to a stirred BSA solution at 37 °C. The palmitate stock solution was maintained above 70 °C during preparation of complexes. The FFA:BSA complexes were stored at -20 °C. The unbound FFA concentration (FFAu) for each complex was determined using the fluorescent probe ADIFAB as described previously Kleinfeld A.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Kleinfeld A.M. Mol. Cell Biochem. 1999; 192: PubMed Google Scholar). Cell adipocytes were as described by and Cell. Full Text PDF PubMed Scopus Google Scholar). adipocytes were using with and at cells on to mm The were in with serum and for were using a fluorescence with 20 and a with a were with a were in a to the cells and 37 °C and were in this cells were with μm ADIFAB in and fluorescence were at and upon at was used to and ratio were with of and of of ADIFAB an in which was by time and in the The for each in the image was by from the and ADIFAB the cell a from cells. was than and from cells ADIFAB was than and was in the of the cell were used for of transport The were by in the of these rather cells and the of the cells were not in a and the in these are not and were not used in transport with were at and and was at for was at and were at Transport to cells in the were with were with fatty acid-free BSA to FFA, with to BSA, and FFA:BSA was to and FFAi steady the FFA:BSA μm was to FAFBSA, which be more than on the cells with consistent The with FFA:BSA used in each transport was and FFAo was determined with ADIFAB Kleinfeld A.M. Mol. Cell Biochem. 1999; 192: PubMed Google Scholar). the highly buffered of this μm FFAo was during transport. The ADIFAB response measured with the fluorescence was by for 4 μm ADIFAB with complexes free from to was to the FFAi was determined for FFA using the above and the of ADIFAB for FFA Kleinfeld A.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar). The of ADIFAB from cell to cell by as much as as by the fluorescence at not FFAi determined from the ratio of ADIFAB with fluorescence in or cells. The time course of FFA dissociation from plasma membranes was determined by monitoring the in BSA fluorescence with plasma membranes D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). membranes were from 3T3F442A cells as described previously J. Kleinfeld A.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar). and was by cells in and 37 mm in buffer for than was for adipocytes in with the in intracellular were for adipocytes by in A. L. M. M.R. J. 1996; PubMed Scopus Google with were in 5 of the cell for and in for has a than for in are consistent with a in intracellular ATP. The as in the upon was We in in cells that were with ADIFAB to determine levels in different cells were consistent with their levels. is to with a ratio probe such as ADIFAB, because reflect, in to the the intracellular concentration of the probe and the of the in the pH changes in adipocytes were using adipocytes were in μm for and with The intracellular response was by the fluorescence ratio cells were to of pH in the of for 3T3F442A were to by with μm for to to transport studies of the of 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid cells were for and in 200 μm to transport but transport characteristics were was in with and complexes or was used at or μm for transport and studies of with BSA and of were by cells in 1 for in the cells to but on transport. have a carrier model of FFA transport by the state model of to that is In this model the rate of change of the intracellular unbound FFA is as the influx and efflux in The influx and efflux are and and are of the at the extracellular and intracellular and to the fast and of the efflux carrier and the of these are determined by FFAo and the dissociation constant for to the efflux by The carrier translocation rate are and and and are the dissociation for FFA to the at the and intracellular the translocation steps are in this model because the constant is about than the influx or efflux rate the of this in which FFAo is the rate constant for is than D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). The were determined by a to the of the influx, efflux and FFAi/FFAo FFA Transport by have a fluorescence ratio microscopy to image using methods to used to image intracellular free M. Biochem. Sci. Full Text PDF Scopus Google Scholar). 3T3F442A adipocytes were with ADIFAB, the fluorescent probe of Kleinfeld A.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Kleinfeld A.M. Mol. Cell Biochem. 1999; 192: PubMed Google Scholar). microscopy the lipid of 3T3F442A cells were at and The ratio of the image at and were used to determine FFAi and are using a not in these ratio the fluorescence at or that ADIFAB fluorescence is the but from the lipid The of are during the course of a transport FFAi the major portion of the intracellular for the transport are in To measure cells were first with FAFBSA, which clamped FFAo at the of this cells were with to and from about FFAi constant at about was by buffer BSA for buffer complexes for which FFAo was and influx, FFAi to steady state in about 300 an influx rate constant of about 0.02 and a steady state concentration of steady state was efflux was by buffer for buffer with and this FFAi to an efflux rate constant of about about twice of influx and efflux be more than on the cells with consistent These transport measurements are of more than such a with FFAi these measurements that steady state FFAi are greater than FFAo and for different cells in the in the FFAi/FFAo the 4 cells of FFAi/FFAo that 1 and In we have as as the is about and that the FFAi/FFAo gradient as well as the cell to cell is abolished by influx or efflux rate from in which FFAi was measured in cells labeled with and FFAi not These results are the of this with a cell to cell in FFAi with these results suggest that an ATP-driven pump is involved in FFA influx and that of FFAi at steady state is to Efflux by an FFAo of FFAi FFAo and efflux rate that are about greater than influx would to be contradictory because for these rate FFAi be than these rate were determined using elevated FFAo for influx and zero FFAo for efflux We the efflux rate constant be a of FFAo that would with increasing FFAo. Such measurements indicate that decreases by about for FFAo above for FFAo greater than plasma levels Kleinfeld A.M. J. Lipid Res. 36: Full Text PDF PubMed Google Scholar, Kleinfeld A.M. J.E. Clin. 2004; Google the rate of efflux from greater than to about to the influx which with the of FFAi FFAo. FFA pump and efflux suggest that a membrane carrier protein FFA transport. In this the rate of transport with increasing FFAo. We measured the FFAo of the influx rate of and the results indicate with a carrier constant of about 200 carrier model that at an FFAo of about a of the efflux the FFAi/FFAo gradient with increasing FFAo because the number of is The results in this Transport of the of to influx, and efflux time were measured using palmitate and these FFA about influx and efflux rate and for FFA was about twice In the FFA steady state FFAi FFAo with a for the FFA The of influx and efflux rate for the different FFA with the in of transport dissociation for these FFA in lipid membranes (16Kleinfeld A.M. Chu P. Romero C. Biochemistry. 1997; 36: 14146-14158Crossref PubMed Scopus (85) Google Scholar, F. F. Hamilton J.A. Biochemistry. 1996; PubMed Scopus Google Scholar, J.B. J. 1997; Full Text PDF PubMed Scopus Google Scholar). These results indicate a different FFA transport mechanism in adipocytes than for lipid the for and efflux measurements not indicate which step in transport is dissociation from BSA, of FFA from the aqueous phase to the membrane, translocation across the membrane, and dissociation from the membrane into the aqueous phase. In transport measurements FFAo is buffered by the of BSA used in the FFA:BSA complexes. these of FFA from to the membrane in than s and dissociation from BSA is not rate-limiting D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). To determine the membrane dissociation step is rate-limiting we measured from 3T3F442A plasma membranes to BSA, monitoring the in BSA fluorescence by D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). The results a fast with a rate constant of that of the The fast rate is to dissociation from lipid vesicles and red cell ghosts A.M. Storms S. Watts M. Biochemistry. 1998; 37: 8011-8019Crossref PubMed Scopus (43) Google Scholar, 18Cupp D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar, J.B. J. 1997; Full Text PDF PubMed Scopus Google and the is consistent with dissociation from these second about of the in BSA fluorescence and a rate constant of about translocation across the plasma membrane of a of with the much rate in the rapid dissociation from the membranes is consistent with translocation as the rate-limiting step for transport across 3T3F442A cells. for translocation was by monitoring the in intracellular pH upon of to 3T3F442A cells the pH The measurements were using the as for influx in cells The rate constant determined from the in in response to extracellular was to that determined with ADIFAB the dissociation and measurements indicate that translocation is rate-limiting for FFA transport across 3T3F442A adipocytes. These results indicate that the is the is consistent with that the FFA, is not not ADIFAB well to and intracellular ADIFAB not to the of extracellular of the ADIFAB of is critical for of the ADIFAB response that is in the intracellular of the 3T3F442A cells. for the of FFAi from the ADIFAB response is the of FFAi and FFAo in for To the of the ADIFAB response we intracellular pH or the ADIFAB was reported to dissociation from adipocyte membranes Biochem. J. 1996; PubMed Scopus Google and because the for ADIFAB with pH Kleinfeld A.M. Biochem. PubMed Scopus Google ADIFAB rather than The influx rate constant determined by monitoring which not FFA and that determined by ADIFAB are that the pHi, with the to FFA not the dissociation rate In rate and the FFAi FFAo were by cells with which clamped to not We that changes in the of transport measurements have a on transport or steady state FFAi levels. was reported to ADIFAB F. Biochem. 1997; PubMed Scopus Google and in the with the FFAi results indicate of on the intracellular response of ADIFAB in 3T3F442A cells. the rapid of the ADIFAB response by and and the of membrane of indicate that not to the the P. Biochem. J. 1997; PubMed Scopus Google to not affect FFAi levels the of not These results are consistent with in these cells not and 13Kamp F. Guo W. Souto R. Pilch P.F. Corkey B.E. Hamilton J.A. J. Biol. Chem. 2003; 278: 7988-7995Abstract Full Text Full Text PDF PubMed Scopus (104) Google and levels of in these cells during the than transport time course and studies indicate that or FFA We that of by which FFA, results in a preparation that of the for We that the for is greater than the for is of of results of suggest that FFA transport across 3T3F442A adipocyte membranes is studies of adipocytes have reported that FFA efflux DIDS, and (7Abumrad N.A. Harmon C. Ibrahimi A. J. Lipid Res. 1998; 39: 2309-2318Abstract Full Text Full Text PDF PubMed Google Scholar, J. C. Kleinfeld A.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar, D.D. X. Berk P.D. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar). Our to on transport with these were not In studies suggest that the of efflux by N.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar, W. Berk P.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google is a of of to BSA rather than an on cellular efflux of We using ADIFAB to monitor FFAu, that FFA to BSA is and thereby upon of at about of at the is N.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar, W. Berk P.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google we FFA by for FFA In measurements of influx and efflux we of on rate we an in FFAi in the of because of the of FFA from is the first to image intracellular and to use these of this concentration to determine the time course for FFA transport across adipocytes. The results of these measurements the following of FFA transport across 3T3F442A an pump that FFAi FFAo, in influx and efflux rate an efflux that is regulated by FFAo, influx that is transport characteristics that are of FFA a rate-limiting step to translocation across the membrane, and transport rate that are more than than for lipid These results are to with a lipid phase transport Lipid phase FFA transport rate that are sensitive to the FFA at of influx and rate that are at than for transport across adipocytes A.M. Storms S. Watts M. Biochemistry. 1998; 37: 8011-8019Crossref PubMed Scopus (43) Google Scholar, 18Cupp D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar, F. F. Hamilton J.A. Biochemistry. 1996; PubMed Scopus Google Scholar). of of adipocyte transport with a lipid phase mechanism the of a carrier process. previous studies have reported that the lipid phase, as lipid to rapid and is of or FFA F. D. F. Hamilton J.A. Biochemistry. PubMed Scopus Google Scholar, A. M. G. Biochemistry. 2002; PubMed Scopus Google Scholar). these results to transport across the lipid phase of the adipocyte membrane, a protein mechanism would be for rapid across the lipid phase has been in measurements of influx across adipocyte plasma membranes and cells F. Guo W. Souto R. Pilch P.F. Corkey B.E. Hamilton J.A. J. Biol. Chem. 2003; 278: 7988-7995Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar). These measurements were by monitoring the in adding to the adipocyte membranes and and the reported rate were than the of the We have the of FFA transport across lipid vesicles and results indicate that the previous of rapid were on an of the measurements D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). In measurements of influx with FFA not with BSA provide about FFA to the vesicles rather than Such measurements fast transport because of of the lipid In about be by influx with FFA with BSA D. Kampf J.P. Kleinfeld A.M. Bichemistry. 2004; 43: 4473-4481Crossref PubMed Scopus (58) Google Scholar). these is the rate-limiting step for transport of FFA across as well as lipid vesicles and is sensitive to the of the We suggest that the fast rate by F. Guo W. Souto R. Pilch P.F. Corkey B.E. Hamilton J.A. J. Biol. Chem. 2003; 278: 7988-7995Abstract Full Text Full Text PDF PubMed Scopus (104) Google for at than we using in the are a of the of by as we in lipid these F. V.N. M. Corkey B.E. Hamilton J.A. J. Metab. 2001; PubMed Google reported that measurements of influx in rat and human determined by but using of rate to we report for 3T3F442A cells in the We suggest that FFA transport across the adipocyte membrane is by a membrane protein previous studies have reported for protein mechanisms for FFA transport in the adipocyte (11Schaffer J.E. Lodish H.F. Cell. 1994; 79: 427-436Abstract Full Text PDF PubMed Scopus (744) Google Scholar, P. N.A. J. Biol. PubMed Scopus Google Scholar, Stump D. Sorrentino D. Potter B.J. Berk P.D. J. Biol. Chem. Full Text PDF PubMed Google the results of the suggest that mechanisms not identified previously be involved in the transport of FFA across adipocytes. In to the FFAi FFAo FFAi and the efflux which have not been reported results influx are different than previous of reported by studies (7Abumrad N.A. Harmon C. Ibrahimi A. J. Lipid Res. 1998; 39: 2309-2318Abstract Full Text Full Text PDF PubMed Google Scholar, D.D. X. Berk P.D. J. Lipid Res. 2001; Full Text Full Text PDF PubMed Google Scholar). The in studies have saturable and influx a and a Our results a saturable for is the lipid phase. that rather than rapid the lipid phase of the adipocyte plasma membrane be highly to FFA transport and transport is The transport characteristics we in this are well described by the carrier model were by to the of influx and efflux and the FFAi/FFAo and the model are in to this model the FFAi FFAo gradient is of FFA at the as with the intracellular of the carrier (Ko 4 for cells was used to these model to a for studies in cells that or the translocation rate upon The model an efflux by efflux to a or fast state of the with the of these regulated by the of to the with The change in FFAi/FFAo with increasing FFAo is by the model to a at an FFAo about to because the of to the efflux the of and the rate of in FFAo the and at FFAo FFAi/FFAo to The and model of are consistent with these The of this model and with the provide for a carrier and the model that studies. The results of this to the role by the adipocyte in regulating circulating FFA levels. of FFA are from to (1Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. J. Clin. Investig. 2003; 111: 981-988Crossref PubMed Scopus (110) Google serum FFAo is Kleinfeld A.M. J. Lipid Res. 36: Full Text PDF PubMed Google Scholar, Kleinfeld A.M. J.E. Clin. 2004; Google Scholar). is consistent with the rapid efflux we FFAo is FFAi FFAo. In influx FFAo We from the that levels reach 200 We not that steady state FFAi were by that at in 3T3F442A be than transport. The of the FFAi FFAo be to to and thereby the to more the in to and FFA be regulated by a membrane transport for FFA that is and sensitive to FFAo levels.