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First published online February 4, 2005
Journal of Experimental Biology 208, 749-760 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01440

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Hypotonicity induced K⁺ and anion conductive pathways activation in eel intestinal epithelium

M. G. Lionetto¹, M. E. Giordano¹, F. De Nuccio¹, G. Nicolardi¹, E. K. Hoffmann² and T. Schettino^1,*

¹ Department of Biological and Environmental Sciences and Technologies, University of Lecce, Italy
² Biochemistry Department, August Krogh Institute, 13 Universitetsparken, Copenhagen, Denmark

^* Author for correspondence (e-mail: trifone.schettino{at}unile.it)

Accepted 9 December 2004

Control of cell volume is a fundamental and highly conserved physiological mechanism, essential for survival under varying environmental and metabolic conditions. Epithelia (such as intestine, renal tubule, gallbladder and gills) are tissues physiologically exposed to osmotic stress. Therefore, the activation of `emergency' systems of rapid cell volume regulation is fundamental in their physiology. The aim of the present work was to study the physiological response to hypotonic stress in a salt-transporting epithelium, the intestine of the euryhaline teleost Anguilla anguilla. Eel intestinal epithelium, when symmetrically bathed with Ringer solution, develops a net Cl^- current giving rise to a negative transepithelial potential at the basolateral side of the epithelium.

The eel intestinal epithelium responded to a hypotonic challenge with a biphasic decrease in the transepithelial voltage (V_te) and the short circuit current (I_sc). This electrophysiological response correlated with a regulatory volume decrease (RVD) response, recorded by morphometrical measurement of the epithelium height. Changes in the transepithelial resistance were also observed following the hypotonicity exposure.

The electrogenic V_te and I_sc responses to hypotonicity resulted from the activation of different K⁺ and anion conductive pathways on the apical and basolateral membranes of the epithelium: (a) iberiotoxin-sensitive K⁺ channels on the apical and basolateral membrane, (b) apamin-sensitive K⁺ channels mainly on the basolateral membrane, (c) DIDS-sensitive anion channels on the apical membrane. The functional integrity of the basal Cl^- conductive pathway on the basolateral membrane is also required.

The electrophysiological response to hypotonic stress was completely abolished by Ca²⁺ removal from the Ringer perfusing solution, but was not affected by depletion of intracellular Ca²⁺ stores by thapsigargin.

Key words: eel, Anguilla anguilla, intestine, RVD, Ussing chamber, K⁺ channel, anion channel

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