Thyroid hormone 3,3',5-triiodothyronine (T3) is an important regulator of Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity in a variety of mammalian target tissues. In the present study, we examined the expression of Na(+)-K(+)-ATPase alpha- and beta-subunit mRNAs by T3 in cultured rat mesangial cells. Northern blot analysis of total RNA isolated from cultured rat mesangial cells revealed the expression of mRNAs encoding Na(+)-K(+)-ATPase alpha 1- and beta 1-subunits; mRNAs encoding alpha 2- and alpha 3-subunits were undetectable. Exposure of mesangial cells to T3 (10(-8) M) caused a threefold increase in the alpha 1-mRNA expression, which was first detected at 6 h and sustained for at least 48 h. The beta 1-mRNA expression was gradually increased by T3, with a maximum fourfold elevation at 48 h. The 50% effective concentration (EC50) for the alpha 1- and beta 1-mRNA induction by T3 was approximately 10(-10) M. The half-life of alpha 1-mRNA analyzed by actinomycin D chase was approximately 3 h and was not affected by T3. The augmented alpha 1-mRNA expression by T3 was associated with a 2.4-fold increase in the alpha 1-subunit protein accumulation and a 1.6-fold increase in Na(+)-K(+)-ATPase activity. These data suggest that thyroid hormone stimulates Na(+)-K(+)-ATPase gene expression, protein accumulation, and enzyme activity in rat mesangial cells.