Excretion of Inorganic Phosphate by the Aglomerular Kidney

Abstract

The urine of the winter toadfish, Opsanus tau (aglomerular kidney), contains only the faintest trace of inorganic phosphate when the fish are kept in an aquarium in the laboratory. The injection of large amounts of either monobasic or dibasic sodium phosphate intramuscularly or intravenously raises the plasma inorganic phosphate to high levels, but does not cause the excretion of any inorganic phosphate in the urine. The excretion of inorganic phosphate has not resulted from various procedures (feeding, injection of glucose, parathyroid extract, insulin). The injection of sodium glycerophosphate results in a marked rise in the inorganic phosphate of the plasma, but in no excretion of inorganic phosphate by the kidney. The above findings are interesting in view of the fact that the goosefish (another aglomerular marine teleost) secretes urine which may contain large amounts of inorganic phosphate. 1 Since we have been unable to obtain freshly caught or summer toadfish, further investigation of phosphate excretion has been carried out upon the goosefish (Lophius piscatorius).

Specimens of urine obtained from the bladders of 10 freshly caught summer goosefish contained from 0.7 to 45.0 millimols of phosphate∗ per liter. The plasma phosphate in the same fish varied from 4.2 to 7.7 millimols per liter. The high concentrations of phosphate observed initially in the urine of some of these fish decreased markedly when the fish were kept in live cars at the laboratory.

The injection of inorganic phosphate into a goosefish does not increase the phosphate excretion. The following protocol is typical of many experiments.

Goosefish 5. 2.6 kilos. 9:54—11:56 Urine, 10 cc. containing 3.0 mM phosphate per liter (excretion 0.015 mM per hour).

11:58 Plasma phosphate, 7.7 mM per liter.

12:00 Inject intramuscularly 17 mM disodium phosphate (40 cc. of 6%).

1:52 Plasma phosphate, 19.2 mM per liter.

Summary. Injected inorganic phosphate is not secreted by the aglomerular kidney but the urine of the aglomerular fish may contain large amounts of inorganic phosphate. This suggests that the inorganic phosphate in the urine of these fish is formed in the kidney from some precursor. From the above results it would appear that injected inorganic phosphate should be excreted only by glomerular filtration in the glomerular kidney. In support of this hypothesis it has been found that the clearances of inorganic phosphate and xylose agree fairly closely in frogs.