Effect of acute hypercapnia on PTH-stimulated phosphaturia in dietary Pi-deprived rat

Abstract

The effects of respiratory acidosis on renal inorganic phosphate (Pi) handling are controversial. Clearance experiments, therefore, were performed in fasted, chronically parathyroidectomized (PTX), dietary Pi-deprived rats. The objectives were twofold: to study the effects of compensated and uncompensated hypercapnia per se on renal Pi excretion and to examine the interaction between acute hypercapnia, dietary Pi, and parathyroid hormone (PTH) on the renal handling of Pi. Acute hypercapnia increased the plasma Pi (delta 2.82 +/- 0.65 mg/dl, P less than 0.05) without altering the glomerular filtration rate (GFR). The FEPi increased (delta 7.26 +/- 0.48%, P less than 0.001) but the TRPi/GFR also increased. PTH (3 U X kg-1 X h-1) superimposed on hypercapnia resulted in a plasma Pi comparable to hypercapnia alone. The FEPi (7.56 +/- 0.78 vs. 24.43 +/- 2.20%; P less than 0.001) was higher and the TRPi/GFR (117 +/- 4 vs. 80 +/- 2 micrograms/min, P less than 0.01) lower, in the former group. PTH infusion during normocapnia resulted in a lower FEPi (0.20 +/- 0.10 vs. 24.43 +/- 2.20%, P less than 0.001) and a higher TRPi/GFR (106 +/- 2 vs. 80 +/- 2 micrograms/min, P less than 0.01) compared with PTH infusion during hypercapnia. Urinary adenosine 3',5'-cyclic monophosphate (cAMP) excretion was similar between the groups. During hypercapnia, when the extracellular acidemia was neutralized, the phosphaturic action of PTH persisted. These studies offer direct evidence that in chronically PTX, dietary Pi-deprived rats, the phosphaturic action of PTH is restored by hypercapnia per se. This effect appears to be independent of extracellular acidemia, changes in the plasma Pi and calcium, urinary pH and Na and cAMP excretion.