No abstract

When a muscle is exposed to a solution containing Rb or Cs ions there is a gradual replacement of its K by the foreign ion (Lubin & Schneider, 1957;Sjodin, 1959). The aim of this paper is to present the kinetic results for the interchange of Rb or Cs and K, using a model proposed to explain the observed dependence of K-for-K exchange on external concentration (Harris & Sjodin, 1961). By 'interchange' is meant the equivalent-forequivalent replacement of an ion by a chemically unlike ion; this process can be followed by analytical methods. By 'exchange' is meant the equivalent-for-equivalent replacement of an ion by a chemically identical ion; this can only be followed by the use of isotopically labelled material. Net movement of a cation can take place either as a consequence of interchange with a different cation, or along with a mobile anion. The model used allows uptakes from K-free solutions and from mixtures to be treated consistently. Values for the relative exchangeabilities of K with itself and with Rb and Cs are obtained. METHODSThe uptakes of 42K or 89Rb or ordinary Rb were measured, using the sartorii of Rana temporaria. The Cs results are taken in part from Sjodin (1959), who used Rana pipien8. The tracer experiments were made by the technique described by Harris & Sjodin (1961); a series of readings of the radioactivity of the tissue was obtained after timed exposures to the radioactive solution. Before the measurements the muscles were blotted on filter paper moistened with an inactive solution chemically similar to the soak solution. At the end of each experiment the muscle was extracted overnight in dilute nitric acid and the extract used for analysis by flame photometry. In making most of the experiments depending on Rb or Cs analysis after timed exposure to a given solution, it was found convenient to apply a 1 min wash in Na methyl sulphate solution (0-11 M) at 0°C before extraction of the cations; this wash would remove much of the extracellular solution and so render less necessary a correction for the trapped Rb or Cs.The analyses were made with a flame spectrophotometer using a prism monochromator, Beckman oxy-hydrogen burner and a photomultiplier tube with sensitivity extending beyond 8500 A (E.M.I. type 9553B Some measurements of the space accessible to the dye naphthol green were made at the same time as certain analytical experiments by adding 0.5 % of the dye to the solution. The muscle extracts were then made for 30 min in distilled water without added acid and the dye contents were read on a colorimeter. The extracts were then returned to the vessels containing the muscles and a drop of nitric acid added to assist cation release. The space measured in relatively fresh muscles (up to 30 min in isolation) was 0 11 + 0-02 ml./g (S.D. for 11 results); after longer isolation values tended to be higher, for example, 0-14 ml./g after 2 hr. RESULTSMethod of presentation Penetration of the cations into the cell is assumed to occur through an outer compartment which has ion-exchange propertie...