High conductance Ca2+-activated K+ channels in the eel intestinal epithelium and their role in Regulatory volume Decrease (RVD).

Aim. The high conductance Ca2+-dependent K+ channels (BKCa) are ubiquitously expressed in both excitatory and non-excitatory cells where they integrate changes in intracellular Ca2+ concentrations with membrane potential, thus contributing to various and diverse physiological outcomes. The aim of the present work was to investigate the functional expression and the physiological role of BKCa in a native salt transport epithelium, the intestine of the teleost fish Anguilla anguilla, which is a useful model system for functional studies of epithelia that perform near-isosmotic fluid absorption. Methods: The study was carried out by short circuit current (Isc) measurements, confocal microscopy immunolocalization, and morphometrical analysis of the epithelium height by optical microscopy. Results: BKCa channels appeared localized along all the plasma membrane of the enterocytes; the apical part of the villi showed the most intense immunostaining. These channels were silent in basal condition, but were activated on both membranes (apical and basolateral) by increasing intracellular Ca2+ concentration with the Ca2+ ionophore ionomycin (1 μm). BKCa channels were also activated on both membranes by hypotonic swelling of the epithelium and their inhibition by 100 nM iberiotoxin (specific BKCa inhibitor) abolished the Regulatory Volume Decrease (RVD) of the intestinal cells after hypotonic swelling. Conclusions: The eel intestinal epithelium expresses BKCa channels, which are localized either on the apical or the basolateral membrane. These channels are volume-sensitive and their activation represents the physiological volume recovery mechanism after hypotonic swelling in this tissue.