The Absorption of Sodium Ions by the Crayfish Astacus Pallipes Lereboullet

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Abstract
1. The effect of various cations in the external solution on the sodium influx in the crayfish, Astacus pallipes, has been studied. 2. Potassium in concentrations up to 4 mM./l. has no significant effect on the sodium influx from 0.05 mM./l. NaCl solutions. 3. Calcium has no effect on the influx in concentrations up to 1 mM./l. At higher concentrations the influx may be reduced in some cases. 4. Magnesium generally increases the influx by about 30%. The effect is not related to the external concentration. 5. Ammonium ions reduce sodium influx. With an ammonium/sodium concentraton ratio of 20:1 the influx is reduced to about 20% of normal. Ammonium ions do not affect the sodium loss rate. 6. Simple substituted ammonium compounds have little effect on the influx. 7. The external hydrogen ion concentration reduces sodium influx if the pH is below 6. At pH 4 the influx is reduced to 20-30% of normal. A low pH does not decrease the rate of sodium loss. 8. The nature of the specific inhibition of sodium influx by ammonium ions is discussed. It is suggested that the ammonium ions interfere with a normal sodium ammonium exchange mechanism.