L-leucine transport in rat heart under normal conditions and effects of a simulated hypoxia.

L-leucine plays a central role in the regulation of protein metabolism in heart and has been implicated in myocardial protection, but little is known about the relationship between these phenomena and leucine transport across the cardiac sarcolemma. In this study we used sarcolemmal vesicles and ventricular myocytes isolated from rat heart to characterise L-leucine transport under normal conditions and to investigate the effect of simulated hypoxia or inhibition of protein synthesis. The Km and Vmax of leucine uptake were 5.24+/-0.65 mM and 1.43+/-1.84 nmol min(-1) mg(-1) protein in vesicles compared to 2.17+/-0.13 mM and 1.7+/-0.76 nmol min(-1) microl(-1) intracellular space in cells. Transport was not dependent on Na+ or H+ gradients. In vesicles L-leucine uptake was increased by trans-stimulation, whilst inhibition was observed with classical system L substrates including 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH) suggesting that this system mediated L-leucine transport in heart. L-Leucine uptake into isolated cardiac myocytes was inhibited after 20, 30 and 60 min of simulated hypoxia. This was not caused by reduced cell viability, although the cells underwent a rigor contracture. Inhibition of protein synthesis did not affect L-leucine transport.