The ionic nature of the eel intestinal response to hypotonic stress.

Cell volume undergoes dynamic changes in many cellular events like epithelial transport, metabolic processes, hormone secretion, cell migration, proliferation and apoptosis. To protect themselves from extensive cell volume perturbations, which could be dangerous for survival, animal cells developed ‘emergency’ systems of rapid cell volume autoregulation. The aim of the present work was to investigate the ion transport mechanisms involved in the regulatory volume decrease (RVD) after hypotonic swelling in the middle eel intestine, which represents a good experimental model for cell volume regulation research. The study was carried out on the native tissue using the short circuit current measurement technique. The eel intestinal epithelium responded to a hypotonic challenge with a biphasic decrease in the basal short circuit current (Isc) and transepithelial potential (Vt). The electrophysiological response correlated with a RVD response, measured by the morphometric analysis of the epithelium height. The change of Vt and Isc was dependent on the intensity of the hypotonic stress applied, showing the highest value for a 40% external osmolarity decrease. The transepithelial resistance showed a slightly (13%) but significant increase during the swelling phase, presumably due to the reduction of the intercellular space. By using specific ion channel blockers the ionic nature of the electrogenic transepithelial response was revealed: it mainly results from the activation of iberiotoxin (specific inhibitor of the Ca2+-activated maxi K+ channels) and apamin (highly selective inhibitor of low conductance Ca2+- activated K+ channels) sensitive K+ conductances on the basolateral membrane of the enterocytes and from the activation of DIDS sensitive anion conductance on the apical membrane. Basolateral apamin and iberiotoxin sensitive K+ channels showed different time-course of activation. Both swelling activated anion and cation conductances were sensitive to Ca2+ removal from the physiological bathing solution