Uptake and Loss of Potassium by Rainbow Trout (Salmo Gairdneri) in Fresh Water and Dilute Sea Water

Potassium turnover was studied in rainbow trout, Salmo gairdneri Richardson, adapted to fresh water or 22 %o sea water using 42K and 86Rb. Potassium space of the whole body increased with time and was about 5 mmol kg−1 after 20 h, while Rb+ space under the same conditions was only about 0·5 mmol kg−1, indicating slow penetration of body K+ by Rb +, especially in muscle and red blood cells. Potassium influx, measured by decrease in specific activity of the medium, was 0·07 mmol kg 1 h−1 in fresh water and 0·48 mmol kg−1 h−1 in 22 %o sea water; the values for eflflux were comparable, indicating that unfed fish are able to maintain K + balance. In both fresh water and dilute sea water, K+ fluxes are 5 % or less of the simultaneous Na+ and Cl− fluxes. The mechanism for K + fluxes is discussed in terms in K + -ATPases.