(1) A comparative study of the renal plasma flow was carried out with the Fick principle, employing PAH and sodium thiosulfate at the same time and firstly, keeping the the plasma PAH concentration low, i.e. between 2.01 and 11.6 mg/dl. in normal dogs whose renal vein has been catheterized. Thus C
PAH/E
PAH and C
T/E
T were determined in a total of 31 measurements on 11 dogs and the results were compared. It was found that C
PAH/E
PAH did not agree with C/
TE
T; while C
PAH/E
PAH per unit body weight gave a mean value of 9.60 cc/min/kg., C
T/E
T per unit body weight gave a mean value of 8.02 cc/min/kg., and the two values were significantly different on statistical basis. The ratio of C
PAH/E
PAH to C
T/E
T was 1.25 on the average.
(2) A similar experiment was performed with the same plasma concentration of thiosulfate but with higher PAH concentrations, i.e. 32.5-94.5 mg/dl. A total of 21 determinations of C
PAH/E
PAH and C
T/E
T were done on 7 dogs. C
PAH/E
PAH per unit body weight gave a mean value of 10.00 cc/min/kg. and C
T/E
T per unit body weight gave a mean value of 10.30 cc/min/kg.; two values were not significantly different on statistical basis. In this case C
PAH/E
PAH bore a ratio of 0.98 in respect to C
T/E
T on the average.
(3) The two ratios were significantly different on statistical basis, i.e. 1.25 obtained at low plasma concentrations of PAH on one hand, and 0.98 obtained at higher plasma concentrations of PAH.
(4) From these observations the author infers that a considerable fraction of the administered thiosulfate is carried out of the kidneys other than by way of the glomerulus into urine, i.e. postglomerular loss by way of the renal lymph. According to this view, the observed C
T/E
T does not represent the real renal plasma flow; it should give a value lower than C
PAH/E
PAH determined at low plasma PAH concentration.
This view seems to be supported by the fact that the ratio of C
PAH/E
PAH to C
T/E
T approaches unity, when plasma concentration of PAH is high, i.e. where extraction ratio for PAH approaches the extraction ratio for thiosulfate.
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