Effect of transtubular osmotic gradients on the paracellular pathway in toad kidney proximal tubule: electron microscopic observations.

We have evaluated with the aid of electron microscopy changes in the width of the paracellular pathway and in the magnitude of the paracellular passage of lanthanum ions when a transtubular osmotic gradient was established by making the tubular lumen hyperosmotic with 50 mM urea or mannitol. Both transtubular gradients (with urea or mannitol) induce changes in the ultrastructure of the tight junction. The changes are characterized by widening of the tight junction and the "disappearance" of the substance forming the intermediate line. Urea has a larger effect than mannitol. The magnitude of lanthanum crossing extracellularly through the tubular epithelium also increases under the urea induced transtubular gradient.