Erythrocyte sodium fluxes, ouabain binding sites, and Na+,K(+)-ATPase activity in hyperthyroidism.

Erythrocyte sodium pump activity, in contrast to other tissues, is decreased in hyperthyroidism. In order to examine whether the effect of thyroid hormones on erythrocytes is part of a generalized effect on other transport pathways, we measured sodium pump activity, Na+,K(+)-adenosine triphosphatase (ATPase) activity, ouabain binding sites, bumetanide-sensitive sodium potassium cotransport (SPC), sodium lithium countertransport (SLC), and ouabain- and bumetanide-insensitive passive efflux of sodium (sodium "leak") in erythrocytes from 20 healthy subjects and 18 untreated hyperthyroid subjects. Sodium pump activity (ouabain-sensitive sodium efflux rate constant), Na+,K(+)-ATPase activity, and the number of ouabain binding sites were lower and the erythrocyte sodium content was higher in hyperthyroid subjects. The rate constants of erythrocyte SPC (P less than .05), SLC (P less than .001), and sodium "leak" (P less than .05) were also significantly lower in hyperthyroidism. In 11 of the hyperthyroid subjects, sodium flux measurements were repeated after 20 weeks of treatment. Sodium pump activity, the number of ouabain binding sites, and the rate constant for SLC increased. These results suggest that the effect of thyroid hormones on the erythrocyte sodium pump is part of a generalized effect on membrane proteins, rather than a specific effect.