Relationship between Phosphaturia and Acute Hypercapnia in the Rat

Abstract

A B S T R A C T Standard clearance studies were performed in mechanically ventilated intact and acutely thyroparathyroidectomized (TPTX) rats to document and characterize the effect of hypercapnia (HC) onl urinary phosphorus excretion (UpV). HC as compared to normocapnia (NC) was associated with an increase in UpV in intact (62.5 vs. 7.93 gg/min) and TPTX (30.5 vs. 0.59 ,ug/min) rats, an increase in filtered load of phosphorus in intact (218 vs. 191 ug/min) and TPTX (243 vs. 146 ,ug/min) rats, an increase in blood bicarbonate concentration in intact (27.8 vs. 26.0 meq/ liter) and TPTX (24.5 vs. 22.3 meq/liter) animals, and a (lecrease in blood pH in intact (7.15 vs. 7.42) and TPTX (7.07 vs. 7.39) rats. Additional TPTX rats with NC and HC were studied during phosphorus infusion at a comparable filtered load of phosphorus (NC = 307 ,ug/min and HC = 328 ,glmin). UpV was 18.5 ,ug/min in NC and 85.2 gg/min in HC animals. Intact NC ani- mals infused with NaHCO3 achieved a blood bicar- bonate of 45.9 meq/liter compared to 26.0 meq/liter in intact NC NaCl-infused rats. UpV was 10.0 ,ug0min in the NaHCO3 and 7.93 ,ug/min in NaCl-infused animals. In intact HC animals infused with NaHCO3, blood pH was 7.36 compared to 7.42 in NC intact NaCl- infused animals. UpV was 83.2,u0gmin in the HC bicar- bonate-infused and 7.93 ug/min in the NC NaCl-in- fused rats. These experiments demonstrate that ele- vated blood carbon dioxide tension per se increases UpV. Increases in filtered load of phosphorus and blood bicarbonate which are associated with HC con- tribute to the phosphaturia as does parathyroid hor- mone. The phosphaturia is not dependent upon reduc- tion of extracellular pH.