Studies on Freshwater Osmoregulation in the Ammocoete Larva of Lampetra planeri (Bloch)

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Abstract
An investigation has been made of the factors which cause sodium loss from ammocoetes when they are immersed in de-ionized water at 1° and 10° C. Sodium influx ceases when animals are first immersed in de-ionized water, but can recommence when the animal loses sufficient sodium to the environment. The concentration of sodium required for influx to take place decreases with succeeding periods of immersion in de-ionized water at 10° C. and reaches minimum equilibrium concentrations as low as 0·005 mM-Na/l. Low temperature inhibits sodium influx and thus promotes net loss of sodium to de-ionized water. Low temperature also decreases the initial loss of sodium to de-ionized water and probably lowers the permeability of the external surfaces of the animal to ions. This effect is small compared with the inhibition of ion uptake so that the combined result is to increase the net loss of sodium from the animal. Since animals lose calcium to de-ionized water and show a decreased rate of sodium loss when calcium salts are added, it is believed that the high rates of sodium loss in de-ionized water are attributable to the effect of calcium on permeability. Lack of calcium may also explain why animals which have been depleted of sodium by low-temperature treatment take up sodium much faster at higher temperatures from dilute Ringer solutions than from pure sodium chloride solutions. When animals lose ions to de-ionized water at low temperature, sodium and chloride are lost from the extracellular space, whilst the muscle cells lose potassium. These ions are recovered into the extracellular space when animals are allowed to take up ions at 10° C. from diluted Ringer solution later.