Phosphate and cyclic AMP excretion decreases during less than 12 hours of hypoxia in conscious rats

Hypocapnia is known to have an antiphosphaturic effect that overcomes the phosphaturic effect of hypoxia. The objective of this study was to examine whether conscious rats exposed to acute hypoxia show a decrease in phosphate excretion due to the concomitant hypocapnia. Wistar rats weighing 200 g were exposed to hypoxia (inspired oxygen fraction = 0.10) or normoxia (inspired oxygen fraction = 0.21) for 6 h; and rats were alternately exposed to hypoxia or normoxia every 12 h for a total 36 h. Renal clearance and hormone studies were performed. Rats exposed to 6 h of hypoxia (n = 11) showed significant hypophosphaturia and decreases in absolute and fractional excretion of phosphate (0.38 +/- 0.10 microgram min-1, mean +/- SE, P < 0.0001 and 0.59 +/- 0.15%, P < 0.0001) as compared with normoxic rats (n = 11, 3.91 +/- 0.68 micrograms min-1 and 5.62 +/- 0.85%). In addition, nephrogenous adenosine 3',5'-cyclic monophosphate level per glomerular filtrate was significantly decreased (-0.87 +/- 0.64 nmol dL GF-1, P < 0.05) and plasma parathyroid hormone level was unchanged (45.2 +/- 9.5 pg mL-1) after 6 h of hypoxia as compared with normoxic rats (4.03 +/- 1.83 nmol dL GF-1 and 54.3 +/- 10.4 pg mL-1). A parallel increase in urinary noradrenaline and a decrease in dopamine excretion was observed in rats after 6 h of hypoxia. The decreased phosphate and adenosine 3',5'-cyclic monophosphate excretion during acute hypoxia were restored to normoxic levels by reoxygenation with 21% oxygen in the study of 12-h intermittent hypoxia. In summary, (1) hypoxia produced by inhalation of 10% oxygen for 12 h or less causes reduced phosphate and adenosine 3',5'-cyclic monophosphate (cAMP) excretion by spontaneously breathing rats; (2) these effects are reversed by reoxygenation and (3) hypoxia elicits a parallel increase in noradrenaline excretion and a decrease in dopamine excretion. These data suggest that renal adrenergic and dopaminergic systems play important roles in hypophosphaturia during acute hypoxia in conscious rats.