Thyroid hormone stimulates the Na(+)-PO4 symporter but not the Na(+)-SO4 symporter in renal brush border.

In our previous studies we established that thyroid hormones [L-thyroxine (T4) or 3,5,3'-triiodothyronine (T3)] elicit an increase in Na(+)-Pi symport in rat and mouse renal brush-border membrane (BBM) vesicles (BBMV), but the Na(+)-coupled symports of other solutes were not influenced. However, a recent report [H. S. Tenenhouse, J. Lee, and N. Harvey. Am. J. Physiol. 261 (Renal Fluid Electrolyte Physiol. 30): F420-F426, 1991]claimed that T3 increases to a similar degree both Na(+)-Pi symport and Na(+)-SO4 symport in murine renal BBM. Adult male rats were fed either normal (0.7% Pi; NPD) or high-phosphate (1.4% Pi; HPD) diet and received T3 (0.2 mg/100 g body wt ip) for 3 days before the kidneys were removed, BBMV were prepared, and the transport rates were determined. Although the Na(+)-Pi symport significantly increased (delta = +35%) in both NPD and HPD rats treated with T3, the Na(+)-35SO4 symport and Na(+)-D-[3H]glucose symports were not influenced by T3. Furthermore, treatment of NPD-fed mice with T3 using a similar protocol as rats resulted in a significant increase (delta = +26%) of Na(+)-(Pi)4 symport, but did not alter Na(+)-SO4 symport or Na(+)-glucose symport. Our findings thus document that T3 regulates selectively the Na(+)-Pi symporter in BBM without having any effect on other major divalent anions, such as SO4.