ROLE OF THYROID-HORMONES IN RENAL TUBULE ACIDIFICATION

Renal tubule acidification was studied in thyroparathyroidectomized ra ts which had the parathyroids reimplanted into cervical muscle tissue, by stopped-flow microperfusion using ion-exchange resin microelectrod es. Hypothyroid rats had decreased rates of proximal and late distal b icarbonate reabsorption. This reduction occurred in the absence of cha nges in pH gradients, and was due mostly to decreases in acidification half-times, that is, of the rate of bicarbonate exit from the tubule lumen. H+ back-flux from the lumen measured during luminal perfusion w ith solutions at pH 6 (below stationary pH) was decreased in proximal tubule of hypothyroid rats, showing that the acidification defect was not due to an increased H+ shunt across the epithelium. These data ind icate that in hypothyroid rats the proximal tubule luminal density of Na+/H+ exchangers or their turnover is decreased in the absence of alt erations in the driving force (H+ and Na+ gradients across the luminal membrane) for H+ secretion. The effect observed in distal tubule may be due to action on Na+/H+ exchangers that are present also on this si te, or to an impairment of the action of other H+ transporters such as H+-ATPases, including the provision of energy for them.