THE EXCRETION OF URINE IN THE DOG: VI. The Filtration and Secretion of Exogenous Creatinine

The excretion of ingested creatinine was recommended by Rehberg (1926) as a measure of glomerular filtration on the grounds that this substance is concentrated by the kidney to a greater extent than any other substance in the urine. Behre and Benedict (1922) had previously expressed doubts about the existence of creatinine in normal blood in sufficient quantities to account for the creatinine normally excreted, but Rehberg concluded that, since ingested creatinine is excreted by the kidney with the same readiness as the chromogenic substance or substances which are normally present in the blood, the main part of this chromogenic substance must be really creatinine. Behre and Benedict’s conclusion that part of the chromogenic subst,ance in the blood is not creatinine has been confirmed by Gaebler and Keltch (1928), by Gaebler (1930) and by experiments of ours reported in this paper. Meanwhile Marshall and Grafflin (1928, 1930, 1932) have shown that exogenous creatinine is secreted by the aglomerular fish kidney, and also by the glomerular fish kidney both when the glomeruli are functional and when they have been rendered non-functional by the irritant action of phlorizin; and Clark and Smith (1932) have shown that exogenous creat’inine is secreted by t*he elasmobranch kidney, although it is now well established that this substance is not normally present in significant amounts in the urine of t)hese or ot’her fishes. These facts render the use of creatinine as a measure of glomerula#r filtration in the mammal suspect,, and it is of interest, therefore, to compare the excretion of creatinine with the excretion of the non-metabolized sugars which Jolliffe, Shannon and Smith (1932) have recommended for this purpose. The sugar methods used here have been fully described in the paper referred to above, and only a brief comment will be added here. For the determination of creatinine at low plasma levels, we have endeavored to devise a method which will distinguish true creatinine from other chromogenic substances present in plasma and urine. This method is described