To investigate the selectivity of the reduction in Na,K,
ATPase activity in erythrocytes from patients with
hyperthyroidism, we have assessed
cytochalasin B-sensitive
galactose uptake and
insulin receptors as well as Na,K,
ATPase-mediated
rubidium uptake in erythrocytes from 11 patients with
hyperthyroidism, 5 patients
after treatment and 11 normal controls. There was a significant reduction in the Vmax values for
rubidium uptake and the number of
insulin receptors (23% and 20% below control, respectively), whereas there was a significant increase in the Vmax and Km values for
galactose uptake (49% and 30% above control) and also in the average affinity of
insulin receptors. The alterations both in
rubidium and
galactose transport activities were reversible with effective treatment of
hyperthyroidism. The magnitude of alterations in the Vmax for
rubidium uptake correlated inversely (r = -0.537 P less than 0.01, n = 16) and the Vmax and Km for
galactose uptake correlated positively (r = 0.597 P less than 0.02 and r = 0.553 P less than 0.05, respectively) with serum T4 level. No correlation was found between the number of
insulin receptors and serum IRI or T4 levels. These results suggest that the reduction in Na,K,
ATPase activity observed in erythrocytes from
hyperthyroid patients is not selective to this
enzyme, but rather a reflection of widespread alterations of erythrocyte
cell-surface proteins.