Renal compensation of hypercapnia in prolonged hypoxia

We previously showed that rats made hypoxic for three weeks were able to regulate their plasma pH better than normoxic rats during acute hypercapnia. This improved pH regulation was abolished by nephrectomy, suggesting that it was due, at least in part, to a more effective renal compensation of hypercapnia in hypoxic rats. To test this possibility renal acid excretion was measured in conscious rats that had been kept at PB 370-380 Torr for three weeks. The rats were studied in a chamber where PIO2 was kept at 68-70 Torr at ambient PB (740-750 Torr). Controls were pair-fed normoxic rats. After a 2 h control period, inspired PCO2 was increased for 4 h. The apparent non-bicarbonate buffer value of arterial blood plasma was twice as high in the hypoxic than in the normoxic rats. Renal excretion of ammonium increased to a similar extent during hypercapnia in both normoxic and hypoxic rats. Titratable acid excretion of normoxic rats did not change significantly during hypercapnia. In the hypoxic rats, on the other hand, total excretion of titratable acid in the 2 h control period was 90.9 +/- 16.4 mumol/rat; and increased to 150.0 +/- 13.4 mumol/rat in the first 2 h and to 232.9 +/- 26.0 mumol/rat in the last 2 h of hypercapnia. In spite of this large increase in acid excretion, urine pH of hypoxic rats did not change significantly, indicating a higher buffer value of the urine of hypoxic rats. These results confirm our previous observations and support the idea that the improved pH regulation of hypoxic rats is due in part to a more effective renal compensation of hypercapnia.