Influence of Duration of Experiment on Rate of Uptake of Radioactive Potassium in Smooth Muscle

Abstract

THE rates of uptake and loss of radioactive tracers by excitable tissues can often be described by a function involving one exponential term^1–3, but over relatively long periods deviations have been observed from such a simple course^4–7. The rates of exchange towards the end of such periods are commonly slower than would be predicted from the initial rates and two general causes for this behaviour have been suggested: either the rates are, in fact, diminishing during the course of an experiment or the tissue is not behaving uniformly. This latter possibility has been extensively investigated^1,8–15, because biological material shows an obvious lack of uniformity, and because deviations from single exponential behaviour are observed even when the stoichiometric composition of the tissue remains constant. However, a constant composition is not proof of constant exchange rates but might be maintained if all exchanges in the system slowed down, corresponding perhaps to the slow deterioration of the in vitro preparation, or indeed if movement stopped altogether. One might expect deviations due to the ageing of the tissue to be more marked the longer the duration of an experiment, and the uptake of potassium-42 by the smooth muscle of the guinea pig tænia coli has now been examined for such an effect since it only shows serious deviations from single exponential behaviour after 5 h¹².