THYROID-HORMONE STIMULATION OF NA P-I-COTRANSPORT IN OPOSSUM KIDNEY-CELLS/

Thyroid hormone (T-3), a known stimulator of renal proximal tubular br ush border membrane Na-dependent phosphate (P-i) uptake (Na/P-i-cotran sport), stimulated Na-dependent P-i transport in opossum kidney (OK) c ells. Na/P-i-cotransport was stimulated in a time- and dose-dependent manner with maximal effects (57%) at 24 h and 10(-10) M T-3. This stim ulation was related to an increase in the apparent capacity (V-max) of Na/P-i-cotransport. Treatment with T-3 had no effect on Na-independen t transport of P-i or of L-arginine. The stimulation of Na/P-i-cotrans port was paralleled by an increase in the messenger ribonucleic acid ( mRNA) encoding the OK cell apical Na/P-i-cotransporter (termed NaPi-4) ; the mRNA levels related to the activity of Na-independent L-arginine transport (rBAT) were unaffected by T-3. Actinomycin D (10(-7)M) comp letely prevented the stimulatory effect of T-3 On OK cell Na/P-i-cotra nsport and on NaPi-4 mRNA content. In conclusion, T-3 stimulates apica l Na/P-i-cotransport in OK cells most likely by enhancing its transcri ption.