A DIRECT EVALUATION OF GERTZ HYPOTHESIS ON SINGLE RAT PROXIMAL TUBULES IN VIVO - FAILURE OF TUBULAR VOLUME TO BE SOLE DETERMINANT OF REABSORPTIVE RATE

Studies have been undertaken to directly test the predictions of the Gertz hypothesis on single proximal tubules of the rat kidney in vivo. By altering the resistance to flow at the end of single proximal tubules, either by means of external obstruction by a glass device or by decompression secondary to fenestration of the last segment, marked changes in transit time and tubular radius were induced. In decompressed nephrons the reabsorptive rate did not decrease in spite of significant reductions in transit time and radius, while in obstructed proximal tubules a fall in reabsorptive rate occurred in the face of large increases in radius and transit time. Accordingly, this direct evaluation of single proximal tubular function supports the conclusion that geometric factors cannot determine the reabsorptive rate in the manner suggested by Gertz. Further, the prolongation of contact time during furosemide diuresis does not appear to significantly contribute to the constancy of fractional reabsorption. An alternate mechanism, which may determine reabsorptive rate, is suggested. © 1969 Springer-Verlag.