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Distribution and fluxes of sodium and hydrogen in crustacean muscle cells Menard, Michael Reald


A new technique was devised for measurement of the unidirectional sodium efflux from single striated muscle cells of the giant barnacle, Balanus nubilus. It involves the continuous measurement of the activity of sodium in the myoplasm with an intracellular sodium-specific microelectrode, during the collection of radiosodium from the same cell by a standard method. Changes in the specific activity inside the cell, which are larger than had been thought previously, can be calculated directly. Thus the sodium efflux can be calculated accurately. It is assumed in these calculations that the only pool of intracellular sodium of appreciable size which exchanges rapidly with the extracellular solution is the sodium in solution in the myoplasm. Several experiments which test this assumption, together with results from the literature, are consistent with the hypothesis that most of the sodium associated with the cell yet not detected by the intracellular sodium-specific microelectrode resides in the extracellular space in association with the glycocalyx. Intracellular microinjection was used to load the myoplasm of single cells with radiosodium. It was necessary to take into account the longitudinal diffusion of tracer inside the cell from injected to noninjected regions. Use of the new technique to measure the sodium efflux from intact single muscle cells revealed several new results. Saturation of the efflux into normal Ringer's solution was not apparent even in cells with very high sodium content. However, saturation of the efflux into potassium-free solution and into ouabain-containing solution occurred at relatively low levels of intracellular sodium. The efflux into sodium-free solution was similar to that into normal Ringer's solution. The decline in the sodium efflux reported by other workers to occur in this situation was found to be due to the rapid decline of the sodium content of the myoplasm which occurs. No 'sodium-sodium exchange' was found. Most of the sodium efflux under normal conditions appears to be due to a mechanism which is not sensitive to external ouabain or potassium. The sodium efflux in barnacle muscle was shown to be electrogenic. A correlation between the measured values of the active sodium efflux and the electrogenic portion of the membrane potential was found. The correlation was consistent with the predictions of a phenomenological extension of the leading model for the membrane potential, the Goldman-Hodgkin-Katz equation. The efflux of hydrogen ions from the cell can only be measured indirectly, from changes in the intracellular pH. Measurements of the intracellular pH with an intracellular pH-specific glass microelectrode revealed no 'pH transients' of the type reported by other workers in different preparations of barnacle muscle. Measurements of the intracellular pH made with the microelectrode and with an indicator method were in close agreement. However, the distribution of the indicator DMO (5,5-dimethyl-2,4-oxazolidinedione) exhibited unusual behavior not previously reported. A refinement of the DMO method which takes this behavior into account is described.

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