Evidence for a peritubular-to-luminal flux phosphate in the dog kidney

The possible presence of a direct influx of phosphate into the tubular lumen of the dog kidney was examined by two different techniques. A 0.3-ml bolus of 32PO4 and [3H]inulin was injected into the renal artery of dogs with and without phosphate loading. The mean urinary transit time (TT) for PO4 was consistently less than the TT for inulin in nonphosphate-loaded dogs. Following phosphate loading the TT for phosphate approached but did not equal the TT for inulin. In contrast, after microinjections of 32PO4 and [3H]inulin into proximal tubules, the urinary TT of PO4 was significantly longer than the TT of inulin. The application of 32PO4 and [3H]inulin to the surface of the left kidney was associated with a greater recovery of 32PO4 from the left kidney compared to that from the right kidney. The recovery of [3H]inulin was proportional to the glomerular filtration rate for both kidneys. Application of the 32PO4 and [3H]inulin to the surface of a muscle resulted in an equal recovery of phosphate and inulin from both kidneys. The results demonstrate the presence of a detectable tubular influx of phosphate.