Interaction between Na+ and the K+-dependent amino acid transport in midgut brush-border membrane vesicles from Philosamia cynthia larvae

Both sodium and potassium can drive leucine uptake into brush-border membrane vesicles from Philosamia cynthia midgut, but the effects of these cations are not additive. 2 mM sodium reduces leucine uptake at saturating potassium concentrations, which indicates that these cations interact with the same transporter. The mixed type inhibition of sodium was explained in terms of different kinetic parameters of the co-transporter when this cation binds to the transport protein instead potassium. 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 higher with potassium. Kinetic experiments performed to characterize Na+-dependent leucine uptake showed that the leucine kinetics at different sodium concentrations did not follow Michaelis-Menten kinetics and that the effect of sodium was mainly to increase the affinity of the amino acid for the co-transporter. Na+-activation curves, at fixed leucine concentrations, showed that the Vmax increased with leucine concentration. Since both cations are present in the midgut lumen of P. cynthia larva (200 mM potassium and 1 mM sodium), the interaction between sodium and the K+-dependent co-transporter was observed by measuring leucine uptake as a function of potassium concentration at different fixed sodium concentrations. In accordance with the kinetic parameters that characterize the co-transport in the presence of either Na+ or K+, sodium reduces leucine uptake at high potassium concentrations and increases leucine uptake at low potassium concentrations. Assuming that the translocation is the rate limiting step of the process, we present a model for two alternative drivers (Na+ and K+) and for leucine translocation in the absence of any cation. The derived velocity equation adequately describes the experiments reported here and previously. The physiological meaning of this transport mechanism, probably evolved from a more primitive Na+-dependent co-transport, is discussed and its role in ensuring sodium intake, in an epithelium which contains no conventional sodium pump, is suggested.