Correlations between (Na + K)-ATPase, Ca-ATPase and cellular potassium concentration in cattle red cells

Abstract

THE existence in erythrocytes of active, ATP-dependent Na-K exchange on one hand and of active, ATP-dependent outward transport of Ca^1–5 on the other raises the question whether two independent or one common system account for these functions. Inhibition of the Na-K system by Ca, although easily demonstrated, does not lend particularly strong support to the hypothesis of a common pathway for Na and Ca in active transport, because full activation of the Ca system^6–9 is achieved by lower Ca concentrations than those required to inhibit Na+K-ATPase⁸ and because Epstein and Whittam¹⁰ have pointed out that the effect is due to competition between Ca-ATP and Mg-ATP for the substrate site and that competition between Ca and Na for the transport site is unlikely. The Na-K system is inhibited by cardiac glycosides whereas Ca transport and Ca-activated ATPase are not^2,3,11 which seems to exclude a common site, but not necessarily a common protein. If alkali cation and Ca movements were mediated by the same membrane protein independent measurements of Na+K-activated ATPase and Ca-stimulated ATPase activity should positively correlate. K concentration in cattle red cells varies widely between animals^12,15, there is agreement as to the ability of these cells to pump Na and K^13–17, and indications exist that .different intensity of pumping accounts for the different K (and Na) content of the cells^12–14. Cattle cells, therefore, seem suitable objects to study phenomena which might correlate with NA+K-ATPase.