The comparative transport of K+ and Rb+ in normal and malignant rat tissuesin vivo and in liver slices, diaphragm, and tumor slicesin vitro

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Abstract
Summary The selectivity in the steady state uptakes of Rb+ and K+ has been studied in a number of normal and malignant rat tissues. The selectivity is minimal in erythrocytes and the two fastest-growing of four transplantable tumors, in which there is little discrimination between the two ions, and ranges upwards to a maximum Rb+ uptake in liver. In each tissue, the selectivity is independent of Rb+ concentration or of K+ deficiency (except in skeletal muscle). In liver slicesin vitro, reduction of energy metabolism by lowering the temperature or by the addition of metabolic inhibitors reduces the Rb+∶K+ discrimination proportionately much more than K+ transport. Diaphragm and slices of a transplantable tumor give similar results. With temperature reduction, there is a logarithmic relation between the Rb+∶K+ discrimination ratio and the respiration rate of liver slices. The results are quantitatively accounted for by simultaneous diffusion and metabolically coupled transport across a homogeneous membrane in which Rb+ transport is more closely coupled than that of K+ to a metabolic flux across the membrane. There is evidence that the tissue differences in Rb+∶K+ selectivity originate in the different levels of the coupling metabolic flux in different cell types and thus of the energy expenditure on ion transport. In contrast to the differences in steady state selectivity between Rb+ and K+, the initial ratio of uptakes of trace43K and86Rb, in otherwise steady state conditions, is close to unity in both liver and tumor slices, in agreement with theoretical calculations.