Elektrophysiologie / Electrophysiology (Leitung: Prof. Dr. Wolf-Michael Weber)
Influence of intracellular Na+ or Li+ concentration on the expression of the epithelial sodium channel (ENaC) in Xenopus laevis oocytes
(Project 2)
Apical membranes of tight epithelia are characterized by the presence of an epithelial Na+ channel (ENaC) that mediates the uptake of Na+ from
the luminal side into the cells using a gradient that is maintained by the basolateral Na+/K+-ATPase. Changes in structure or regulation of ENaC
cause severe human diseases including cystic fibrosis. We used the oocytes of Xenopus laevis to express ENaC derived from rat colon. Conventional
two-microelectrode voltage clamp in combination with special hard- and software enabled us to monitor continuously and simultaneously ENaC current (Im),
conductance (Gm) and capacitance (Cm). Cm is a measure of the membrane surface area and continuous monitoring of
Cm allows investigations of exo- and endocytotic processes. We found that the rate of expression and/or activity of ENaC is strongly dependent on the
intracellular Na+ concentration [Na+]i and follows a bell-shaped curve with maximum ENaC current and conductance between 10
and 15 mM intracellular Na+. Lower and higher [Na+]i caused decreased Im and Gm and
[Na+]i of 20 mM or more often led to dying of the oocytes. Li+ is well conducted by ENaC and could mimic the positive effects of
[Na+]i on ENaC expression and/or activity. Yet, with Li+ as the Na+ replacing anion we found that the maximal effect
of Li+ already occurred at a Li+ concentration of 5 mM. It could be that the oocyte's Na+/K+-ATPase does not accept
Li+ as a substrate and therefore Li+ accumulates faster in the oocytes than Na+. We conclude that, at least in Xenopus
oocytes, expression of ENaC is dependent on [Na+]i or [Li+]i. More
information: http://www.uni-muenster.de/Biologie.Zoophysiologie/electrophys/ElectroHome.htm
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