The effect of triiodothyronine (T3) on Na+,K(+)-ATPase activity of K562 human 
erythroleukemic cell was studied to understand why the erythrocyte sodium pump 
activity is decreased in hyperthyroidism. Na+,K(+)-ATPase activity of K562 cell 
lysates was assayed by measuring the release of inorganic phosphate (Pi) from 
ATP. Na+,K(+)-ATPase activity of K562 cell grown in the presence of T3 for 48 
hours was significantly higher than that of control (0.98 +/- 0.05 mumol Pi h-1 
mg protein-1 vs 0.82 +/- 0.10 mumol Pi h-1 mg protein-1, p < 0.05). The 
Na+,K(+)-ATPase activity could be stimulated in a time- and 
concentration-dependent manner; maximum stimulatory effect of T3 was seen at a 
concentration of 10(-7) mol/L. When an inducer 
[cytosine-beta-D-arabino-furanoside (ARA-C)] was added to the culture medium, 
the K562 cells showed signs of differentiation and synthesised haemoglobin. At 
the same time, the Na+,K(+)-ATPase activity remained high. We conclude that T3 
stimulates Na+,K(+)-ATPase activity of K562 cells and in the presence of T3 
during differentiation, the enzyme activity remains high.