Potassium turnover was studied in rainbow trout, Salmo gairdneri Richardson, adapted to fresh water or 22% sea water using 42K and 86Rb. Potassium space of the whole body increased with time and was about 5 mmol kg−1 after 20h, 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 % sea water; the values for efflux 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.

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