Uptake and metabolism of fatty acids by dispersed adult rat heart myocytes. I. Kinetics of homologous fatty acids.

An adult rat heart myocyte preparation was used to study the uptake and metabolism of the 1-(14)C-labeled free fatty acids decanoate, laurate, myristate, palmitate, and oleate at 37 degrees C in the absence of serum albumin. The rate of total uptake consisted of both a nonsaturable and a saturable component. The nonsaturable component corresponded to the fatty acid accumulating in the free fatty acid fraction, and the rate of this accumulation increased logarithmically as a function of chain length. The saturable component corresponded to that portion of fatty acid converted to the only detectable metabolic products: CO2, triglyceride, and polar lipid. The Km for this process was about 1 microM and was independent of chain length. The Vmax for the saturable component varied only slightly with chain length, from 20 +/- 1 nmol/h-mg of cell protein for decanoate to 47 +/- 18 nmol/h-mg of cell protein for palmitate. The relative product distribution did vary with chain length, however, ranging from primarily carbon dioxide for decanoate to approximately equal quantities of carbon dioxide, triglyceride, and polar lipid for palmitate. Two internal pools of free fatty acid are postulated: a minor pool that equilibrates rapidly with external fatty acid and serves as the precursor for fatty acid activation, and a major pool containing most of the accumulated free acid. These two pools are interconvertible. The data support a simple diffusion or membrane-partitioning process for the accumulation of fatty acid in the second pool. The data presented in this paper are not sufficient to distinguish between a simple diffusion or a carrier-mediated process for uptake into the first pool. The saturation kinetics observed appear to represent a metabolic step such as fatty acid activation, rather than a transport carrier. Evidence of toxicity at a higher concentration of the longer chain fatty acids limits the concentration range that can be studied in the absence of albumin. Decanoate did not appear to be toxic at concentrations up to 300 microM, but laurate at 10 microM and myristate at 5 microM appeared to uncouple respiratory control.