Activation of the Sodium Uptake System at High Blood Concentrations in the Amphipod Gammarus Duebeni

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Abstract
Some factors responsible for eliciting an increase in the rate of active uptake of sodium by Gammarus duebeni have been studied. Animals previously acclimatized to high salinities (100-161% sea water) had their blood concentration suddenly lowered by treatment with deionized water to a level similar to, but a little above, that of animals kept in 50-66% sea water. Both groups were placed in the same tracer medium, i.e. 5% sea water labelled with 22Na and with sucrose added. The animals treated with deionized water showed an influx, on average, of 4 times that of the controls from 50 to 66% sea water. No increase in influx followed treatment of animals from 161% sea water with 50% sea water or with sucrose solution isosmotic with 50% sea water, despite the fact that the osmotic gradient between 161 and 50% sea water is greater than the gradient between 100% sea water and deionized water. It is concluded that in these experiments the rate of uptake is not influenced primarily by the absolute concentration of the blood, the rate of change of blood concentration, the rate of swelling of the tissues or the extent of the blood volume. The possibility is considered that both the concentration of the urine and the rate of uptake of sodium may, in some circumstances, be controlled by an exteroreceptor which monitors the concentration of the medium and mediates its effect via a humoral system.