Uptake and Metabolism of Fatty Acids and Analogues by Cultured Cardiac Cells from Chick Embryo

Abstract

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 β-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, K_m from 4.5 to 16 μM. The V is from 0.03 to 0.3 nmol of fatty acid incorporated min⁻¹ (10⁶ 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 °C show that fatty acids accumulate up to 28-fold in the cells. Efflux rates can be measured after loading the cells at 20 °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. 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.