Is fatty acid uptake in cardiomyocytes determined by physicochemical fatty acid partition between albumin and membranes?
Palmitate uptake by isolated, calcium-resistant cardiomyocytes was measured by using a stimulation chamber in which cell contraction can be evoked electrically. Experiments were performed in a medium containing physiological interstitial concentration of albumin (2%) and palmitate/albumin (P/A) ratios ranging from 0.03 to 2.5, and were compared to experiments with fixed P/A ratio (= 1).
Initial rate of uptake (Vi) was calculated from fitted uptake vs. time curves as measured by accumulation of radioactivity in the cells from 14C-labelled palmitate. Vi-vs.-concentration curves exhibited a saturable component, if albumin concentration was kept constant. Almost no change in Vi was observed in experiments performed at constant P/A. This is in contrast to the albumin receptor hypothesis.
The 14C-palmitate content of the myocytes as estimated by thin-layer-chromatography did reach a plateau at ≤ 30 s and had the same value at 30 min after administration. The cellular content of labelled palmitate could be attributed to the membrane compartment as calculated from partition coefficient (Kc) of fatty acids (FA) between albumin and membranes. With electrical stimulation Vi-vs.-palmitate concentration kinetics showed a shift in apparent Km from 62µM (P/A = 0.22) to 23µM (P/A= 0.08), and presence of 2,4-dinitrophenol increases Vi.
Our results suggest that FA-transfer across the sarcolemmal membranes is determined by a physicochemical equilibrium between the compartments of extracellular FA-albumin complex, the membrane lipid phase, intracellular FA binding proteins and the respective aqueous phases. Consequently in cell suspensions the rate of palmitate uptake is controlled by a step of fatty acid metabolism possibly the formation of Fa CoA by the enzyme FA acyl CoA synthetase which is localized in membranes of endoplasmatic reticulum and mitochondria. This step is influenced by the metabolic state of the cells and by FA concentration in membranes.
Key wordsFatty-acid uptake albumin-receptor membrane-compartment
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