Protein excretion: micropuncture study of rat capsular and proximal tubule fluid.

VAN LIEW, JUDITH B., WOLF BUENTIG, HILMAR STOLTE, AND JOHN W. BOYLAN. Protein excretion: micropuncture study of rat capsular and proximal tubulefluid. Am. J. Physiol. 219(Z) : 299-305. 1970.Samples of capsular and proximal tubule fluid taken under free-flow conditions from the rat kidney were analyzed chemically and. immunologically for protein. An average concentration of 19.9 mg/lOO ml =t 6.8 SD was found in the proximal tubule. Similar concentrations were present in capsular fluid. The relative constancy of protein concentration in the proximal tubule, found in the presence of normal fluid reabsorption (normal TF/P inulin ratios) suggests that two-thirds of the filtered protein is being reabsorbed in the convoluted segment of the proximal tubule. The majority of the protein present was identified immunologically as serum protein. No serum protein could be detected in fluid collected from single proximal tubules microperfused with protein-free solutions. This is taken as evidence that proximal tubule protein is predominantly serum protein of glomerular origin. Estimates of the magnitude 24-hr renal protein turnover are discussed in terms of protein conservation and mechanisms of reabsorption.