L-Lysine uptake in giant vesicles from cardiac ventricular sarcolemma: Two components of cationic amino acid transport

Cationic L-amino acids enter cardiac-muscle cells through carrier-mediated transport. To study this process in detail, L-[¹⁴C]lysine uptake experiments were conducted within a 10³-fold range of L-lysine concentrations in giant sarcolemmal vesicles prepared from rat cardiac ventricles. Vesicles had a surface-to-volume ratio comparable with that of an epithelial cell, thus representing a suitable system for initial uptake rate studies. Two Na⁺-independent, N-ethylmaleimide-sensitive uptake components were found, one with high apparent affinity (K_m =222 ± 71 μM) and low transport capacity (V_max =121±36 pmol/min per mg of vesicle protein) and the other with low apparent affinity (K_m =16± 4 mM) and high capacity (V_max =4.0 ± 0.4 nmol/min per mg of vesicle protein). L-Lysine uptake mediated by both components was stimulated by the presence of intravesicular L-lysine as well as by valinomycin-induced membrane hyperpolarization. Altogether, this behaviour is consistent with the functional properties of the CAT-1 and CAT-2A members of the system y⁺ family of cationic amino acid transporters. Furthermore, mRNA transcripts for these two carrier proteins were identified in freshly isolated rat cardiac myocytes, the amount of CAT-1 mRNA, relative to β-actin, being 33-fold larger than that of CAT-2A. These two transporters appear to function simultaneously as a homoeostatic device that supplies cardiac-muscle cells with cationic amino acids under a variety of metabolic conditions. Analysis of two carriers acting in parallel with such an array of kinetic parameters shows significant activity of the low-affinity component even at amino acid plasma levels far below its K_m.