Regulation of amino acid transport in chick embryo heart cells: I. Adaptive system of mediation for neutral amino acids

Abstract The regulation of amino acid transport systems in chick embryo heart cells has been studied. Experiments were designed to investigate activity and kinetics of transport for analogue and natural amino acids as a function of time under various in vitro conditions (active and inhibited protein and RNA synthesis, amino acid dependence, transinhibition). Conclusions based on the proposed experimental approach include the following: 1. 1. The transport activity of a group of neutral amino acids (corresponding to those ascribed to the A mediation by Christensen and co-workers, J. Biol. Chem. , 238(1963)3686; Adv. Enzymol. , 32(1969)1) increases with time when cells or intact hearts are incubated in the absence of added amino acids. This increase is abolished in the presence of puromycin, cycloheximide and actinomycin D. 2. 2. The increase in activity of the A transport system (investigated by aminoisobutyrate and proline as representative amino acids) is prevented by the addition to the incubation medium of amino acids assigned to the same system of mediation on the basis of competition studies. It is not substantially altered by the addition of amino acids pertaining to the L system of mediation or basic amino acids. 3. 3. Transinhibition does not appear of major importance in the regulation of the activity of the A transport system. 4. 4. Kinetic analyses of aminoisobutyrate uptake in which the transport of the analogue by the A and L systems of mediation could be separated stress further that a time-dependent regulation is effective for the sole A transport system (affecting the maximal velocity without substantial changes in K m ) and provide a comprehensive explanation for previously reported changes with time and intracellular substrate concentration of the kinetic parameters governing the uptake of the analogue 9 . 5. 5. Provisional mechanisms for the observed adaptive regulation of the activity of the A transport system for neutral amino acids have been considered.