Zum Problem der gegenseitigen Beeinflussung der Ionenfluxe am Myokard
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1. In non-beating frog atria the potassium exchange as a function of temperature and the sodium exchange as effected by replacing Li or sucrose for Na in the extracellular fluid were measured, using the isotopes K42 and Na24.
2. In addition to these measurements, the membrane potential (by intracellular electrodes), the intracellular concentrations of K and Na (by flame photometry), and the energy expenditure (from oxygen consumption) were determined.
3. For K influx a mean Q10 of 2,1 and for K efflux a mean Q10 of 1,5 was found. Assuming a passive movement of K ions in the efflux but a passive and an active component in the K influx, and further assuming a Q10 of 1,5 for the passive ion movement in efflux and influx and a Q10 of 4 for the active K inward transport, a ratio of 0,6:0,4 between the passive and the active component in the influx can be calculated. Since influx and efflux are equal under steady state conditions, the ratio of the passive influx to the efflux is p=0,6.
4. This result is compared to the equation of Ussing (1949a) where the ratio of the passive influx to the efflux is expressed as a function of membrane potential and the extracellular and intracellular K concentrations. A numerical value q=0,89 of this ratio results.
5. The discrepancy between p and q may suggest that the theoretical assumptions of the Ussing relation are not valid. The main assumption is the independence of ion movements in passive influx and efflux. It is proposed that an interaction between efflux and influx exists, in the sense of a mutual obstruction of the two fluxes, possibly by way of a single-file-mechanism as proposed by Hodgkin and Keynes (1955 b) in giant axons.
6. The Na24 efflux from the cells in freshly dissected preparations is reversibly reduced to about 2/3 when the Na in the bathing solution is replaced by Li or sucrose for short periods. Simultaneously the membrane potential is slightly decreased.
7. The reduction of Na efflux under Li or sucrose can be explained, if 1/3 of the Na exchange of resting frog atria is a Na exchange diffusion. This agrees qualitatively with the suggestion of Ussing (1949 b) and the findings of Keynes and Swan (1959) on frog muscle.
8. The energy available from metabolism in resting preparations under normal conditions amounts to 18 cal/kg myocardium (wet weight) · min. On the other hand, the energy required for the Na efflux would be 11,3 cal/kg · min, if all Na efflux is active transport. The energy needed for the Na efflux is markedly reduced, if part of the efflux uses exchange diffusion. This seems to support the hypothesis of an exchange diffusion.
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