Renal Function in Man 25 



the glucose reabsorptive mechanism is loaded to capacity, 

 xylose is excluded from tubular reabsorption, and the xylose 

 clearance rises to identity with the inulin clearance. (The 

 behavior of sucrose and rafiinose under these conditions has 

 not been examined.) Consequently, as illustrated in figure 

 4, during hyperglycemia we may equate the clearances of 

 xylose, mannitol, sorbitol and inulin. 



It has long been known that the drug phlorizin induces 

 glycosuria at all plasma levels, and in our first studies we at- 

 tributed this to the blocking of the reabsorption of glucose 

 in the tubules. ^^ When phlorizin is administered to dog or 

 man the process of sugar reabsorption by the tubules is com- 

 pletely abolished so that not only the glucose but also the 

 xylose and sucrose clearances are nearly or quite identical 

 with the inulin clearance,""^" and prior to direct examination, 

 we may anticipate that after phlorizin this identity will in- 

 clude sorbitol and mannitol. 



Concerning the excretion of exogenous creatinine in man 

 there is not, in my opinion, the slightest doubt of Shannon's^* 

 original evidence in favor of tubular participation. And at 

 high plasma levels of creatinine, the creatinine clearance ap- 

 proaches to within a few per cent of the inulin clearance, and 

 this fact itself was advanced by Shannon to support the be- 

 lief that the inulin clearance was at the level of filtration. On 

 the other side, Ralli, Friedman and Rubin's"^ demonstration 

 that the vitamin C clearance rises from zero and approaches 

 to within 10 per cent of the inulin clearance at high vitamin 

 C levels is additional evidence in this direction. 



Phlorizin depresses the tubular excretion of creatinine in 

 lower animals where this process is highly developed ;^^ and 

 in man it entirely abolishes the normal difference between the 

 creatinine and inulin clearances, a result which, in view of 



