Phosphate excretion during 24 h of hypoxia in conscious rats

The objective of this study was to investigate renal phosphate excretion during 24 h of hypoxia in conscious rats fed by total parenteral nutrition. Wistar rats weighing 190 g were exposed to hypoxia (inspired oxygen fraction = 0.10) or normoxia (inspired oxygen fraction = 0.21) for 24 h in a normobaric chamber. Renal clearance and hormonal studies were performed. The results showed a greater fractional excretion of phosphate (5.37 +/- 0.07%, P < 0.05) and hypophosphataemia (7.40 +/- 0.12 mg dL-1, P < 0.01) in hypoxic rats (n = 10) than in normoxic rats (n = 13; 3.50 +/- 0.37% and 8.02 +/- 0.16 mg dL-1, respectively). In addition, during hypoxia there was a significant decrease in the excretion of urinary adenosine 3',5'-cyclic monophosphate per glomerular filtrate (2.97 +/- 1.27 nmol dL-1, P < 0.005), a parameter of the renal action of parathyroid hormone, and a stable level of serum parathyroid hormone (10.2 +/- 2.6 ng mL-1) (cf. normoxia: 8.57 +/- 0.70 nmol dL-1 and 8.0 +/- 1.7 ng mL-1, respectively). However, creatinine clearance and the renal adenosine triphosphate level, both of which affect adenosine 3',5'-cyclic monophosphate excretion, were not different between the two groups. These data suggest that exposure of conscious rats to 24 h of hypoxia causes renal hyporesponsiveness to physiological levels of parathyroid hormone, which is manifested as a decrease in adenosine 3',5'-cyclic monophosphate excretion. Phosphaturia is not a direct net effect of hypoxia and secondary hypocapnia on renal phosphate transport, which is known to be regulated by parathyroid hormone through adenosine 3',5'-cyclic monophosphate.