Phosphate excretion and reabsorption in the conscious dog

When normal conscious dogs were given small doses of (NH4)2HPO4 by stomach tube to increase their plasma PO4, the rate of excretion of PO4 increased without change in creatinine clearance. After eating meat, increased PO4 excretion was accompanied by increase in both creatinine clearance and plasma PO4. The calculated rate of tubular reabsorption of PO4 did not change significantly after (NH4)2HPO4 administration, but there was a significant increase in PO4 reabsorption after meat; in comparison there was no change in SO4 reabsorption. Similarly, administration of certain amino acids, which caused an increase in creatinine clearance, also caused a significant increase in PO4 reabsorption. Administration of SO4 with PO4 had no significant effect on PO4 reabsorption. It appears from the results, that PO4 has an apparent tubular maximum rate of reabsorption (Tm) when plasma PO4 alone is varied; but this Tm is not a real maximum because PO4 reabsorption increases above this value after the administration of meat or amino acids, both of which cause an increase in glomerular filtration rate. There is no evidence to suggest that there is any inhibition of PO4 reabsorption by SO4 or amino acids.