A Furosemide-Sensitive Cotransport of Sodium plus Potassium in the Human Red Cell

Abstract

The influxes of Na⁺ and K⁺ into the human red cell appear to be interrelated. This relationship was investigated under conditions in which either Na⁺ or K⁺ concentration outside the cell was varied or one cation was replaced by Mg²⁺, choline⁺, or Li⁺. The effects of furosemide on Na⁺ and K⁺ movements were studied in the presence of ouabain. When ouabain was present, Na⁺ influx was higher with K⁺ ions externally than with other cations externally. Furosemide inhibited this K⁺-stimulated Na⁺ influx, but it had little effect when K⁺ was absent. Ouabain-insensitive K⁺ influx was stimulated two-fold by external Na⁺ compared with other cations. Furosemide also inhibited this stimulation, but it had little effect when Mg²⁺ or choline⁺ replaced external Na⁺. Thus it was confirmed that synergism exists between the ouabain-insensitive influxes of Na⁺ and K⁺ and it was demostrated that furosemide inhibits this cooperative effect. The ouabain-insensitive influx of both K⁺ and Na⁺ showed a hyperbolic “saturating” dependence on the external concentration of the transported cation. Furosemide therefore eliminates a saturable component of influx of each cation. The net uptake of Na⁺ in the presence of ouabain was stimulated by K⁺ ions. A similar effect was observed with red cells, in which Li⁺ replaced nearly all the internal Na⁺ plus K⁺ ions. In these cells, net Na⁺ uptake was stimulated by external K⁺, and net K⁺ uptake was stimulated by external Na⁺. Furosemide inhibited this mutual stimulation of net cation entries. The inhibitory action of furosemide was not limited to inward flux and net movement of Na⁺ and K⁺. Furosemide also inhibited the efflux of Na⁺ into Na⁺-free media and the efflux of K⁺ into K⁺-free media. It appeared, therefore, that the action of furosemide was not explained by inhibition of exchange diffusion. These data are consistent with an ouabain-insensitive transport process that facilitates the inward cotransport of Na⁺ plus K⁺-ions, and that can produce a net movement of both ions. Although this process under some conditions mediates an equal bidirectional flux of both Na⁺ and K⁺, it cannot be defined as exchange diffusion. The contransport process is inhibited by furosemide.