| Abstract: | The temperature-dependence of steady-state exchanges of K and Na were determined under conditions in which cell viability, ATP, water, Ca, and Mg were not confounding variables. Steady-state ion contents are near-normal between 37° and 10° C, Below 10° C K is replaced by Na in a mole-for-mole fashion with significant net K retention and Na exclusion occurring even below 3°C. The rates of steady-state Kand Na exchanges have markedly different temperature-dependences; between 37° and 10° C, for example, that of K decreases markedly while that of Na remains near-normal, and there is no consistent correlation between the steady-state exchanges and the ionic contents. Ouabain increases steady-state Na flux at 37° C and induces a more marked temperature-dependence over the entire temperature range. This effect is not due simply to inhibition of some processes and unmasking others; it mirrors a pronounced effect of ouabain on the intrinsic properties of Na self-exchange. These results are compatible with a model based on two simple concepts: (1) partial ionic exclusion from cellular water that is ordered by interaction with proteins; and (2) ionic accumulation mediated by adsorption onto and desorption from fixed macromolecular anionic sites, the majority of which interact with one another in a cooperative fashion. In this view, the sharp temperature transition in the net replacement of K by Na below 10° C is due to a critical transition in the selectivity of the cooperatively interacting adsorption sites. The rates of steady-state self-exchanges of K and Na are determined by parameters of ion-site interaction, and the major set of ion-adsorbing sites that interact cooperatively have a steep thermal activation energy of ionic self-exchange. When they are in the K-preferring state above 10° C, exchange of K has a steep temperature-dependence. When they are in the Na-preferring state below 10° C, exchange of Na has a steep temperature-dependence. When these sites are forced into a Na-preferring state at all temperatures by treatment with ouabain, exchange of Na acquires a steep temperature-dependence over the entire temperature range 37° to 0° C. |
