Na+-K+-2Cl- activation and the osmotic stress response in a model salt transport epithelium

Epithelia are physiologically exposed to osmotic stress resulting in alteration of cell volume in several aspects of their functioning; therefore the activation of “emergency” systems of rapid cell volume regulation is fundamental in their physiology. In the present study the physiological response to osmotic stress was investigated in a salt transporting epithelium, the intestine of the euryhaline teleost Anguilla anguilla. This epithelium, when symmetrically bathed with normal Ringer solution, develops a net Cl- current (giving rise to a negative transepithelial potential, Vte), based on the operation of the luminal absorptive Na+-K+-2Cl- cotransporter in parallel with a luminal K+ conductance and in series with a basolateral Cl- conductance. The eel intestine, because it is physiologically exposed to changes in extracellular osmolarity, constitutes a good model of cellular volume regulation, permitting the study of ion transport activation that occurs in cells or tissues in response to osmotic stress. Tissue responses were studied by using freshly isolated epithelial cell sheets with intact tight junctions. As determined by morphometrical analysis of epithelium height, the eel intestinal epithelium is able to perform either RVI response following hypertonic shrinkage or RVD response following hypotonic swelling. RVI response is sustained by the increased activity of the luminal absorptive Na+-K+-2Cl- cotransporter, detectable as a bumetanide-sensitive increase in the basal Isc and Vte. Changes in the cotransporter activity are not involved in the RVD response, since the cotransporter remains still active after hypotonic swelling. The hypertonic shrinkage activation of the luminal absorptive cotransporter is Ca2+-calmodulin dependent, requires the integrity of cytoskeleton, both microfilaments and microtubules, and it is also dependent on the activity of protein kinase C (PKC) and myosin light chain kinase (MLCK). Therefore, the hypertonicity induced activation of the absorptive Na+-K+-2Cl- cotrasporter is dependent on cellular transduction mechanisms involving an interaction between the cytoskeleton and phosphorylation events.