Phosphaturic effect of PTH during hypoxia and hypocapnia in rats

Abstract

This study examined the effect of acute hypoxia or hypocapnia on renal phosphate excretion in thyroparathyroidectomized rats. Hypoxia is usually accompanied by a secondary hypocapnia due to hypoxic hyperventilation. Respiratory alkalosis has been described as blunting the phosphaturic effect of parathyroid hormone (PTH). In the present study, to know the effect of hypoxia on renal phosphate excretion in the absence of hypocapnia, the rats were ventilated mechanically, and arterial PCO2 levels were controlled. The rats were divided into three groups depending on the arterial PO2 and PCO2 levels: 1) hypoxic normocapnic group; 2) normoxic normocapnic group; 3) normoxic hypocapnic group. Hypoxia was achieved by ventilating with 10% oxygen, and hypocapnia by hyperventilating with 25-30% oxygen. PTH infusion significantly increased fractional excretion of phosphate (FEPi) from 4.1 +/- 0.9 (mean +/- SE) to 37.7 +/- 2.6% in the hypoxic group (n = 7), from 1.4 +/- 0.3 to 27.4 +/- 2.5% in the normoxic group (n = 8), and from 1.5 +/- 0.4 to 19.5 +/- 1.2% in the hypocapnic group (n = 10). The change of FEPi (delta FEPi) after PTH infusion during hypoxia was significantly greater (33.6 +/- 2.1%) than that during normoxia (26.1 +/- 2.4%, p < 0.05). In contrast to this, hypocapnia blunted the phosphaturic response to PTH (18.0 +/- 1.1% delta FEPi, p < 0.05). Urinary adenosine 3', 5'-cyclic monophosphate (cAMP) increased similarly after PTH infusion in all three groups. To test whether the enhanced phosphaturic effect of PTH during hypoxia and the blunted phosphaturic effect of PTH during hypocapnia are due to steps beyond the production of cAMP, cAMP was administered to the three groups. Cyclic AMP infusion displayed greater phosphaturia in the hypoxic group (n = 6, 30.0 +/- 1.4%) and less phosphaturia in the hypocapnic group (n = 7, 11.3 +/- 1.8%) as compared the the normoxic group (n = 6, 24.1 +/- 1.0%). In conclusion, acute hypoxia enhances the phosphaturic effect of PTH, whereas acute hypocapnia attenuates the phosphaturic effect of PTH.