Proximal tubular stop flow pressure: an index of glomerular capillary pressure?

Central to the assumption that glomerular capillary pressure (Pgc) can be equated with the sum of arterial oncotic pressure (pi art) and the pressure in a blocked proximal tubule ("stop flow" pressure, Psf) is that filtration ceases in the blocked nephron. Should filtration not cease, but continue at a rate equal to tubular reabsorption between the block and the glomerulus, Psf, for a given Pgc, will depend on the distance between block and glomerulus. This would have serious consequences for the interpretation of Psf, particularly in respect to its frequent use in analysis of the tubuloglomerular feedback (TGF) mechanism. Experiments were performed in anaesthetized Wistar rats to examine whether a length dependency of Psf exists and, if so, to what extent this relationship alters during maximal TGF stimulation by loop of Henle perfusion. A length dependency of Psf existed both in the absence and presence of loop flow. The regression coefficients were significantly different from 0 and from each other. Pgc cannot thus be equated with the sum of Psf and pi art. The length dependent error in Psf makes it unsuitable for the quantitative analysis of TGF and glomerular haemodynamics.