Mechanism of long chain fatty acid permeation in the isolated adipocyte

The mechanism of permeation of [¹⁴C]oleate into isolated rat adipocytes has been examined. The initial rates of uptake of the fatty acid were determined at 23°C as a function of the concentration of unbound fatty acid in the medium. Under the conditions employed, the following observations were made. 1) The rates were maximal and linear for at least 15 s and were the same in the presence or absence of glucose. 2) In the absence of glucose, all fatty acid taken up was recovered in the cell as unesterified fatty acid, whereas about 75% was esterified in the presence of the sugar. Thus uptake appeared to be independent of fatty acid metabolism. 3) Influx of fatty acid was strongly inhibited by phloretin, particularly at low concentrations of unbound fatty acid. Efflux was also blocked. (Phloretin in an albumin-free medium at 0°C was a very effective stop solution for abruptly terminating fatty acid fluxes and for washing cells without loss of unesterified fatty acid.) 4) The fatty acid taken up was not bound to the cell surface but probably was largely bound within the cell. 5) Uptake was not limited by dissociation of fatty acid from albumin in the medium nor by an interaction between albumin and the cell surface. From these considerations, we concluded that the uptake measurements were valid estimates of the influx of fatty acid. Partial saturation was observed as a function of external concentration of unbound fatty acid with a k(0.5) of 6 x 10^-8 M. It appears that permeation by long chain fatty acids is facilitated by a saturable, phloretin-inhibitable mechanism that is presumably protein in nature. Permeation is almost entirely by this mechanism at physiological levels of unbound fatty acid. At high concentrations of unbound fatty acid, however, permeation by a mechanism having diffusion kinetics is detectable, which may indicate some limited passage of fatty acid directly through the phospholipid bilayer.