The Protein Concentration in the Proximal Tubule of the Dog*

The mammalian proximal tubular fluid is gen- erally considered to contain a low but definite pro- tein concentration (1-3). The degree of glomeru- lar capillary membrane permeability has been con- sidered to range from that of a perfect ultrafilter to one yielding a protein concentration of 30 mg per 100 ml or more in the immediate filtrate. The most obvious evidence that protein is present in the glomerular filtrate is the presence of plasma proteins in the urine of normal man, dog, and rat. Estimates of the albumin in normal human urine, based upon a number of different methods, range from 15 to 31 mg per 24 hours (4-8). This amount of albumin could easily be accounted for by a concentration less than 1 mg per 100 ml in the glomerular filtrate provided none was absorbed. The only direct estimate of protein in the mammalian nephron is to be found in the micropuncture studies of Walker, Bott, Oliver, and MacDowell (9). The protein concentration in 41 samples of glomerular and proximal tubular fluid from the guinea pig and the rat was found to range from less than 30 mg per 100 ml to less than 200 mg per 100 ml. Earlier, Wearn and Richards (10) had found no protein in 11 frog glomerular samples and concluded that the glomerular filtrate was protein free.