Effect of intraluminal pressure on the ultrastructure and protein transport in the proximal tubule.

Proteins filtered in the kidney are generally considered to be catabolized by the renal lysosomes, but it has also been suggested that some protein may pass the tubule wall intact via the intercellular spaces. To examine the latter possibility, electron-dense tracers were in one part of the study injected retrograde into the kidney of living flounders and in another part by micropuncture directly into the tubule lumen of isolated flounder kidney tubules incubated in vitro. In both cases disruptions in the tubule wall were demonstrated and this was shown to be associated with the passage of tracer into the lateral intercellular spaces. From the in vitro studies it was further shown that only when the intraluminal pressure is increased do tracers pass into the intercellular spaces and they do so via the disruptions in the tubule wall. It is concluded that no passage of tracer occurs at intraluminal pressures in the physiological range.