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

In an earlier paper (Lockwood, 1961) it was shown that the brackish-water amphipod, Gammarus duebeni, begins to form urine hypotonic to the blood about 2 hr. after the animal is transferred from a highly concentrated medium (100-175% sea water) to fresh water. The osmotic concentration of the blood at the time the urine first becomes hypotonic may be as much as 1 M./l. NaCl, a value twice as high as that at which hypotonic urine would normally be produced by an animal in a steady state with its medium. It was suggested in the same paper that the capacity to produce hypotonic urine in these circumstances might be of adaptive significance to the species. G. duebeni is usually found in small streams and pools near the top of salt marshes, rock pools above the neap high tide mark and similar brackish-water sites (Kinne, 1959). One feature of such habitats is that they are liable to sudden and possibly large changes in salinity as a result of inundation by fresh water or sea water. The loss of sodium in the urine accounts for a major part of the total loss from the body. Hence the switch from isotonic to hypotonic urine production shortly after the medium is diluted will have the effect of slowing the rate of salt loss. This slowing of the loss will in turn allow more time for osmotic adjustments at the cellular level to be made, and there will consequently be less liability to cellular swelling or circulatory crisis as the blood concentration falls. An additional factor which could further slow the drop in blood concentration when the medium is suddenly diluted would be an increase in the rate of active uptake of ions. Activation of the ion transport system of animals has been studied mainly on freshwater species: Krogh (1938) on Eriocheir, Shaw (1959a) on Austropotamobius, Bryan (i960) on Austropotamobius and Lockwood (i960) on Asellus; and in these forms an increase in the rate of active transport is correlated with a fall in the concentration of the blood below its ' normal' level. However, it seems likely that in the invertebrates, as well as in the vertebrates, the systems responsible for the active transport of sodium are influenced by other factors in addition to the concentration of the blood. The work described in this paper shows that an increase in the rate of uptake of sodium can be elicited at high blood concentrations if the concentration of the medium is suddenly lowered.