INTERACTION BETWEEN NA-DEPENDENT AMINO-ACID-TRANSPORT IN MIDGUT BRUSH-BORDER MEMBRANE-VESICLES FROM PHILOSAMIA-CYNTHIA LARVAE( AND THE K+)

Both sodium and potassium can drive leucine uptake into brush-border m embrane vesicles from Pholosamia cynthia midgut, but the effects of th ese cations are not additive. 2mM sodium reduces leucine uptake at sat urating potassium concentration, which indicates that these cations in teract with the same transporter. The mixed type inhibition of sodium was explained in terms of different kinetic parameters of the co-trans porter when this cation binds to the transport protein instead potassi um. At 0.2 mM leucine, the affinity of sodium for the transporter was about 18 times that of potassium, whereas leucine Vmax was 2.5 times h igher with potassium. Kinetic experiments performed to characterize Na +-dependent leucine uptake showed that the leucine kinetics at differe nt sodium concentrations did not follow Michaelis-Menten kinetics and that the effect of sodium was mainly to increase the affinity of the a mino acid for the co-transporter. Na+-activation curves, as fixed leuc ine concentrations, showed that the Vmax increased with leucine concen tration. Since both cations are present in the midgut lumen of P. cynt hia larva (200 mM potassium and 1mM sodium), the interaction between s odium and the K+-dependent co-transporter was observed by measuring le ucine uptake as a function of potassium concentration at different fix ed sodium concentrations. In accordance with the kinetic parameters th at characterize the co-transport in the presence of either Na+ or K+, sodium reduces leucine uptake at high potassium concentrations and inc reases leucine uptake at low potassium concentrations. Assuming that t he translocation is the rate limiting step of the process, we present a model for two alternative drivers (Na+ and K+) and for leucine trans location in the absence of any cation. The derived velocity equation a dequately describes the experiments reported here and previously. The physiological meaning of this transport mechanism, probably evolved fr om a more primative Na+-dependent co-transport, is discussed and its r ole in ensuring sodium intake, in an eqithelium which contains no conv entional sodium pump, is suggested.