Osmotic reversal induces assembly of tight junction strands at the basal pole of toad bladder epithelial cells but does not reverse cell polarity

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Abstract
This paper reports the effect of reversing the osmotic environment between luminal and serosal compartments of a toad urinary bladder on the polarity of assembly of tight junction strands. Toad bladders were filled with Ringer's solution (220 mOsm) and were immersed in distilled water at room temperature or at 37°C. Within two minutes, new tight junction strands are assembled. The new tight junctional strands unite the basal pole of epithelial cells with the apical side of basal cells. Physiological studies show that oxytocin, a synthetic analog of antidiuretic hormone, is still capable of inducing increases in water transport in epithelia which were osmotically reversed. This capacity decreases significantly for longer periods of osmotic reversal. Osmotic reversal does not alter the original polarity of epithelial cells: 1) the apical tight junction belt, at the apical pole, is not displaced; 2) the freeze-fracture morphology typical of apical plasma membrane (particle-rich E faces; particle-poor P faces) is not altered; 3) oxytocin and cyclic AMP induce aggregates which are observed only at the apical plasma membrane. Massive assembly of junctional elements occurs even in epithelia preincubated in the presence of cycloheximide (an inhibitor of protein synthesis) or of cytoskeleton perturbers. Our experiments show that the polarity of assembly of tight junction strands depends on the vectorial orientation of the osmotic environment of the epithelium.