SUMMARY

The relationship between sodium influx and external sodium concentration in Phoxinus is complex and unusual. In non-depleted fish the relationship is approxi-mately that given by the Michaelis—Menten equation of enzyme kinetics. However, the A."m value (a measure of the affinity of the sodium uptake mechanism for sodium) is very high (3mmoll~'), indicating a low affinity of the uptake mechanism for sodium. On sodium depletion, the relationship between sodium influx and external sodium concentration changes to produce a curve which has a stepped appearance, and is unusual in that the maximum influx is not increased above that in non-depleted fish. The overall Km alters very little; however, the A."m for the lower part of the curve is very low (005 mmol 1~'). A model is proposed to explain these results in the form of two sodium uptake mechanisms working in parallel across the gill. The second carrier is only active when the fish is sodium-depleted and kept in low external sodium concentrations. Neither the external sodium concentration nor the external calcium concentration has any direct effect on sodium efflux. However, fish depleted in 1 mmol I"1 calcium have a lower sodium efflux than fish depleted in distilled water. Calcium appears to reduce the permeability of the gill to ions such as sodium. Since calcium has no effect on sodium influx, changes in gill permeability do not involve the sodium influx mechanism