L-thyroxine inhibits intestinal phosphate absorption in developing rabbits

Abstract

Little is known about effects of l-thyroxine on phosphate absorption. To characterize effects of l-thyroxine on intestinal phosphate transport, four week old suckling rabbits were injected with l-thyroxine. 24 hours following injection, phosphate transport was studied in brush border membrane vesicles. Vesicles from l-thyroxine-treated and control animals demonstrated sodium-phosphate cotransport however uptake was significantly greater in controls than in treated animals. Controls demonstrated an “overshoot” in that peak phosphate uptake was nearly three-fold the equilibrium value whereas minimal overshoot was observed in treated animals. V max and K m for sodium-dependent phosphate transport were 230.4±6.4 pmol·mg protein −1·10s −1 and 0.034±0.05 mmol/L respectively for controls, and 97.7±4.4 pmol·mg protein −1·10s −1 and 0.023±0.006 mmol/L for treated animals. While phosphate uptake was greater in controls than in treated animals, maximal stimulation of phosphate uptake occurred between 75 and 100 mmol/L sodium in both groups. Uptake was half-maximally stimulated by 36.3±13.0 mmol/L sodium in controls, and 41.4±10.0 mmol/L sodium in treated animals. These results indicate that within 24 hours, l-thyroxine suppresses intestinal sodium-coupled phosphate transport in four week old suckling rabbits. This is due to decreased maximal transport capacity without changes in transporter affinity for phosphate or sodium.