Changes in membrane and surface potential explain the opposite effects of low ionic strength on the two lysine transporters of human erythrocytes.
The sucrose-induced stimulation of lysine influx in human erythrocytes has been attributed to the removal of a competitive inhibition exerted by Na+ on system y+ (Young, J. D., Fincham, D. A., and Harvey, C. M. (1991) Biochim. Biophys. Acta 1070, 111-118). We have reexamined this phenomenon separating the contribution of the two cationic amino acid transporters present in these cells (system y+ and system y+L). NaCl replacement with sucrose increased influx through system y+L, but decreased influx through system y+. We conclude that 1) the inhibition of system y+ is a response to the membrane depolarization that results from chloride removal, and 2) the stimulation of system y+L is due to the enhancement of the negative surface potential. Consistently, lysine influx through system y+L (in sucrose medium) was reduced by Na+, K+, Li+, and choline (K0.5 = 25-34 mM), the effect reaching a maximum at 35-40% of the original flux. Divalent cations (Ca2+ and Mg2+) were also inhibitory, but lower concentrations were required (K0.5 1.1-1.8 mM). The finding that sucrose stimulates uptake through changes in the surface potential explains similar effects observed in other cells with various cationic substrates.
Devés
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R
Department of Physiology and Biophysics, Faculty of Medicine, University of Chile, Santiago, Chile. rdeves@bitmed.med.uchile.cl
Angelo
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D001120:Arginine / Q000097:blood; D002352:Carrier Proteins / Q000097:blood*; D004912:Erythrocytes / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D008239:Lysine / Q000097:blood*; D008562:Membrane Glycoproteins; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000097:blood*; D009994:Osmolar Concentration; D011991:Receptors, Virus; D012965:Sodium Chloride / Q000494:pharmacology; D013395:Sucrose / Q000494:pharmacology
D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D011991:Receptors, Virus; C070506:ecotropic murine leukemia virus receptor; D012965:Sodium Chloride; D013395:Sucrose; D001120:Arginine; D008239:Lysine; D005033:Ethylmaleimide