Uptake and metabolism of fatty acids and analogues by cultured cardiac cells from chick embryo.

Cultured heart cells from chick embryo, present as a confluent monolayer in a minimal medium, take up labelled fatty acids (from C6 to C22) bound to serum albumin with a rapid incorporation into neutral and complex lipids (visible at 15 s), while beta-oxidation is delayed (visible at 15 min). Fatty acids enter the cardiac cells through two mechanisms, one involving a readily saturable process, the other resembling passive diffusion. The saturable transport system recognizes fatty acids with a high affinity, Km from 4.5 to 16 muM. The V is from 0.03 to 0.3 nmol of fatty acid incorporated min-1 (10(6) cells)-1. Competitive inhibition occurs between fatty acids while glucose, aminoacids and lactate do not compete with palmitate for the entry into the cells. The analysis of unesterified fatty acids from the usual culture medium and from the cells show a gradient of concentration up to 54-fold; moreover experiments performed in minimal medium at 20 degrees C show that fatty acids accumulate up to 28-fold in the cells. Efflux rates can be measured after loading the cells at 20 degrees C; the curves are bimodal and the pseudo first-order rate constants thus determined are in the order of 0.5 min-1, as opposed to 2.5 to 5.0 min-1 for the passive-diffusion component calculated for the influx rates. Studies on the role of serum albumin at constant and at variable fatty acid-albumin molar ratios indicate that the rate of uptake is controlled by the total fatty acid concentration and not solely by the concentration of unbound fatty acids. No labelled fatty acid was found to be bound to the cytoplasmic fraction supposed to contain the fatty acids-binding protein; our results are in favour of the presence of a specific transport system for fatty acids either at the level of subcellular organelles or more likely at the surface of cardiac cells.