Modulatory Effect of Thyroid Hormones on Uptake of Phosphate and Other Solutes across Luminal Brush Border Membrane of Kidney Cortex*

Abstract

The mechanism whereby thyroid hormones modulate the transport properties of luminal brush border membrane (BBM) or renal proximal tubules was studied in thyroparathyroidectomized rats. Administration of both T4 [thyroxine] and T3 [triiodothyroinen] increased BBM capacity of Na+ gradient-dependent uptake of phosphate (Pi) by BBM vesicles (BBMV). This effect of thyroid hormones was present in thyroparathyroidectomized and hypophysectomized rats, and it was not blocked by a saturating dose of propranolol. The stimulatory effect of T3 and T4 on BBM transport of Pi was dose dependent in the range of 5.2-520 nmol/100 g BW. Pretreatment of rats with inhibitors of 5''-monodeiodinase (5''-DI), iopanoic acid or ipodate, prevented the increase in serum T3 in rats injected with T4, but it did not diminish the increase in BBM transport of Pi. Administration of iopanoic acid and ipodate also prevented a 5-fold increase in 5''-DI activity in renal cortical tissue elicited by T4 administration. Treatment with T3 resulted in an increase of Pi transport across BBM from kidneys of rats subjected to dietary Pi deprivation due either to total fasting or to feeding of a lower Pi diet. Further, T3 administration enhanced amiloride-sensitive Na+ H+ countertransport across BBM, but the uptake of 22Na+ but BBMV in the absence of pH gradient was not changed. The Na+ gradient-dependent uptake of L-[3H]proline by BBMV was slightly decreased, but the uptake of [14C]citrate was not changed in response to T3. Administration of T3 increased Pi transport in BBMV prepared from juxtamedullary cortex, but not in BBMV from superficial cortex. Conversely, the rate of Na+-H+ countertransport was enhanced, and the enzymatic activity of alkaline phosphatase was decreased in BBMV from superficial cortex; no changes in these parameters were found in BBMV from juxtamedullary cortex. Results indicate that the stimulatory effect of administered T3 and/or T4 on renal BBM transport of Pi is distinct from the modulatory effect of dietary Pi intake and is not secondary to the .beta.-effect of catecholamines or to altered secretion of GH. Both T3 and T4 elicit the increase in BBM transport of Pi in a similar manner, T4 is effective even after profound inhibition of in vivo conversion of T4 to T3 by potent 5''-DI inhibitors. Administration of T3 stimulates Na+ gradient-dependent Pi uptake only in BBMV prepared from the juxtamedullary zone, and it stimulates the H+-Na+ countertransport only in BBMV from the outer cortical zone. These findings indicate that thyroid hormones modulate the 2 distinct transport functions of BBM derived from 2 different populations of renal proximal tubules.