The effect of enternal sodium concentration upon sodium fluxes in Chironomus dorsalis (Meig.) and Camptochironomus tentans (Fabr.), and the effect of other ions on sodium influx in C. tentans.

In comparison with other freshwater animals, the sodium uptake mechanism in fourth instar larvae of both C. tentans and C. dorsalis has a moderate affinity for sodium. In both species half maximum influx (Km) occurs at about 0.57 mM-Na+ and is unaltered by salt depletion. Maximum influx is achieved in steady-state C. tentans at 1.9 mM-Na+, and in steady-state C. dorsalis at 3.0 mM-Na+. Both of these values increase on depletion. Efflux also appears to be saturable at higher external sodium concentrations. In C. tentans, sodium may be transported independently of chloride, although it seems likely that sodium movement is enhanced by chloride. Sulphate strongly inhibits sodium influx. Nitrate apparently inhibits sodium influx at low concentrations, but this inhibition is progressively overcome at external sodium concentrations approaching 4 mM. A number of cations interfere with sodium influx in depleted C. tentans, notably H+, Li+ and, to a lesser extent NH4+. It is suggested that these ions compete with sodium for carrier sites. Potassium is apparently transported independently of sodium.