A comparison of the distribution of potassium and exchangeable rubidium in the organs of the dog, using rubidium.

From determinations of radioactivity and potassium content at intervals after the injection of Rb, it was inferred that most organs of the dog contained exchangeable rubidium in a concentration paralleling but exceeding that of potassium, as compared with their relative concentrations in the plasma. A "lag phenomenon" resulting in higher rubidium specific activity in the myocardium than plasma was present. Analysis indicated that this was dependent on the relationship of myocardial rubidium exchange rate and the rate of decline of plasma rubidium specific activity. In experiments requiring prolonged observation, Rb (half life, 19.5 days) is a useful tracer of potassium.