Effects of parathyroid hormone on urinary acidification

Recollection micropuncture experiments were carried out in plasma-replete euvolemic thyroparathyroidectomized rats to examine the effects of a purified bovine parathyroid hormone (PTH) infusion on urinary acidification. After a 60-min equilibration period, PTH administration had the following effects: reabsorption in the proximal convoluted tubule was inhibited 13% for total CO2, 31% for chloride, and 28% for water; early distal delivery, however, remained unchanged for bicarbonate, chloride, and water principally as a result of stimulation of bicarbonate and chloride absorption in Henle's loop; the urinary bicarbonate and chloride excretion rates did not vary significantly, but the urinary pH decreased from 6.78 +/- 0.11 to 6.39 +/- 0.07 and titratable acid and ammonium excretion increased from 63 +/- 18 to 405 +/- 45 and from 422 +/- 30 to 647 +/- 44 nmol X min-1 X g kidney wt-1, respectively. In another group of rats, the bicarbonate urinary excretion rate increased more than twofold during the first 60 min of PTH infusion and then returned to control levels as was observed in the other groups; the transient increase in bicarbonaturia was attributable to a PTH-induced transient augmentation in glomerular filtration rate and bicarbonate filtered load. Finally, no change was noted in micropuncture or whole-kidney data in time-control rats. We conclude that PTH only transiently enhances the bicarbonate filtered load and urinary excretion rate during the first 60 min of administration secondary to an early hemodynamic action but that the steady effect is to stimulate urinary acidification and net acid excretion, which could generate metabolic alkalosis; and that the inhibition of the proximal bicarbonate and chloride reabsorption induced by PTH is counterbalanced by stimulation of reabsorption in Henle's loop.