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+-P(i) 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+-P(i) symport and Na+-SO4 symport in murine renal BBM. Adult male rats were fed either normal (0.7% P(i); NPD) or high-phosphate (1.4% P(i); 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+-P(i) symport significantly increased (DELTA = +35%) in both NPD and HPD rats treated with T3, the Na+-(SO4)-S-35 symport and Na+-D-[H-3]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+-(P(i))4 symport, but did not alter Na+-SO4 Symport or Na+-glucose symport. Our findings thus document that T3 regulates selectively the Na+-P(i) symporter in BBM without having any effect on other major divalent anions, such as SO4.