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-[(14)C]lysine uptake experiments were conducted within a 10(3)-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 microM) 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 beta-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).
Lu
Xiaodong
X
Department of Pharmacology and Physiology, University of Medicine and Dentistry of New Jersey - New Jersey Medical School, Newark, NJ 07101-1709, USA.
Zheng
Ruifang
R
Gonzalez
Jorge
J
Gaspers
Lawrence
L
Kuzhikandathil
Eldo
E
Peluffo
R Daniel
RD
D026923:Amino Acid Transport Systems, Basic / Q000378:metabolism*; D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002250:Carbon Radioisotopes / Q000378:metabolism; D002412:Cations / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005033:Ethylmaleimide / Q000494:pharmacology; D006352:Heart Ventricles / Q000378:metabolism; D007700:Kinetics; D008239:Lysine / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012508:Sarcolemma / Q000378:metabolism*
D026923:Amino Acid Transport Systems, Basic; D000596:Amino Acids; D002250:Carbon Radioisotopes; D002412:Cations; D008239:Lysine; D005033:Ethylmaleimide