The Metabolism of Isolated Fat Cells

Ghosts of isolated fat cells take up oc-aminoisobutyric acid (AIB) by a saturable and partially energy-dependent transport process.Of several amino acids tested, methionine and alanine were the most effective in inhibiting influx and stimulating efflux of AIB, indicating that the transport of these amino acids and AIB is shared by a common carriermediated system in the plasma membrane of ghosts.The uptake of AIB was stimulated by sodium ion, inhibited by ouabain, and diminished in the absence of potassium ion in the incubation medium.The initial rate of AIB influx was not correlated to the initial rate of potassium influx.The results suggest that the capacity of ghosts to accumulate and maintain a steady state content of AIB is a function of the potassium and sodium gradients across the plasma membrane but is not directly linked to active sodium-potassium transport.Sodium-potassium transport is probably the major energy-requiring process involved in the accumulation of AIB.Insulin, adrenocorticotropin, and epinephrine did not alter the rate of uptake or the steady state levels of AIB in preparations of ghosts that are sensitive, with respect to glucose transport or metabolism, to the effects of these hormones.A number of studies have provided indirect evidence that the plasma or surface membrane of fat cells is the initial and primary site of action of hormones such as insulin and the fast acting lipolytic hormones (adrenocorticotropin, glucagon, catecholamines).Some of the evidence obtained from studies with fat cells or adipose tissue (for a recent review, see Reference 1) suggests that insulin alters the activities of transport systems