Regulation of amino acid transport in chick embryo heart cells III. Formal identification of the A mediation as an adaptative transport system

Abstract An amino acid transport system, subject to regulation involving a repression-derepression mechanism by its substrate molecules, is operative in chick embryo heart cells (Gazzola G.C.; Franchi R.; Saibene V.; Ronchi P.; Guidotti G.G. (1972) Biochim. Biophys. Acta 266, 407–421). The formal identification of this system with the A mediation has been achieved by studies yielding the following results. 1. 1. The system which exhibits adaptive control is Na + dependent, whereas its regulation (derepression upon incubation of cardiac cells in amino acid-free medium, repression in the presence of pertinent amino acid) is largely Na + independent. 2. 2. Among the four transport systems (A, ASC, L, Ly + ) individually discriminated from interfering systems by relatively specific model substrates, only the A mediation showed derepression when cardiac cells were incubated in amino acid-free medium. 3. 3. Among the four model amino acids tested ( i.e. α-(methylamino)isobutyric acid, b (±)-2-aminobicyclo(2,2,1)heptane-2-carboxylic acid, 4-amino-1-guanylpiperidine-4-carboxylic acid, α,α-diethylglycine) only α-(methylamino)isobutyric acid, developed as specific substrate of System A, inhibited the increase in transport activity of cardiac cells incubated under conditions otherwise derepressive. 4. 4. The addition of transport-specific amino acids during incubation under conditions of inhibited protein synthesis increased the rate of degradation of protein components of System A (as estimated by measurements of transport activity). This had no effect on the rate for System ASC and apparently decreased it for Systems L and Ly + . 5. 5. Experiments, showing that adaptive control of transport activity by the A mediation is operative in such mammalian tissues as newborn mouse and rat heart cells, suggest that this regulation is an intrinsic property of muscle tissue.