Pflfigers Arch-Eur J Physiol (1995) 431:266-271 9 Springer-Verlag 1995 Victor Sorribas 9 Daniel Markovich 9 Tiziano Verri Jiirg Biber 9 Heini Murer Thyroid hormone stimulation of Na/Pi-cotransport in opossum kidney cells Received: 14 June 1995/Received after revision: 7 August 1995/Accepted: 16 August 1995 Abstract Thyroid hormone (T3), a known stimulator of renal proximal tubular brush border membrane Na- dependent phosphate (P0 uptake (Na/Pi-cotransport), stimulated Na-dependent Pi transport in opossum kidney (OK) cells. Na/P~-cotransport was stimulated in a time- and dose-dependent manner with maxi- mal effects (57%) at 24 h and 10 -l~ M T3. This stim- ulation was related to an increase in the apparent capacity (Vmax)of Na/Pi-cotransport. Treatment with T3 had no effect on Na-independent transport of Pi or of L-arginine. The stimulation of Na/Pi-cotrans- port was paralleled by an increase in the messenger ribonucleic acid (mRNA) encoding the OK cell apical Na/Pi-cotransporter (termed NaP~- 4); the mRNA levels related to the activity of Na- independent L-arginine transport (rBAT) were unaffected by T3. Actinomycin D (10-TM) com- pletely prevented the stimulatory effect of T3 on OK cell Na/P~-cotrransport and on NaP~-4 mRNA con- tent. In conclusion, T3 stimulates apical Na/Pi- cotransport in OK cells most likely by enhancing its transcription. Key words Renal proximal tubule 9 Na/P~-cotransport 9 Brush border membrane 9 mRNA. Transcription Introduction Regulation of renal proximal tubular phosphate (Pi) reabsorption is an important element in overall Pi V. Sorribas - D. Markovich 9 T. Verri J. Biber 9 Heini Murer ([]) University of Zfirich, Institute of Physiology, Winterthurerstrasse 190, CH-8057 Zfirich, Switzerland V. Sorribas Department of Toxicology, Veterinary Faculty, University of Zaragoza, E-500t3 Zaragoza, Spain homeostasis. Brush border membrane Na-dependent P~ transport (Na/Pi-cotransport) is the rate-limiting step in proximal tubular Pi reabsorption and is thus the target mechanism in physiological regulation [3, 16, 17]. The thyroid hormones (triiodothyronine, T3 and thyroxine, T4) are potent stimulatory factors of proximal tubular Pi reabsorption and thus of brush border membrane Na/P~-cotransport [2, 6, 10, 19, 26, 27]. The effects of T3/T4 seem to be "indepen- dent" of other hormonal factors, such as parathy- roid hormone (PTH), calcitonin, growth hormone and 1,25-dihydroxy vitamin D3; they can also be observed in thyroparathyroidectomized and in hypophysectomized rats [26]. Furthermore, the effects can also be observed in primary cultures of proximal tubular cells [19]. The molecular/cellular mechanisms involved in the effects on Na/P~-cotransport induced by thyroid hor- mones are not understood. Based on studies on rat brush border membranes isolated from T3-treated, thy- roparathyroidectomized rats it was concluded that the stimulation involves transcriptional and translational processes and that the increased Na/P~-cotransport rate is related to an increased abundance of Na/Pi-cotrans- porters at the brush border site. These conclusions were based on the use of actinomycin D and cycloheximide, as well as the binding of phosphonoformic acid (PFA), as a potential ligand, to the Na/Pi-cotransporter [19, 27]. In parallel studies, it was shown by the same authors that the stimulatory effect of a low P~ diet was related to a different cellular mechanism; although it was related to an involvement of protein synthesis, it did not lead to an increase in binding of PFA [27]. Furthermore, the studies on brush border membrane vesicles isolated from T3-treated rats have also sug- gested that the thyroid hormone effect is "specific" for the Na/P~-cotransport mechanisms(s); Na/D-glucose- cotransport and Na/L-proline-cotransport were found to be unaltered (e.g. [6, 26, 27]). In addition, "sec- ondary" effects (driving forces), e.g. related to altered