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1 Department of Physiology, University of Munich, 80336 Munich, Germany; and 2 Department of Biology, Technion, Haifa, Israel
The effect of xanthine derivatives
on the voltage-activated Cl
conductance
(GCl) of amphibian skin was analyzed.
3-Isobutyl-1-methylxanthine (IBMX) and the recently synthesized
xanthine derivatives 3,7-dimethyl-1-propyl xanthine (X-32) and
3,7-dimethyl-1-isobutyl xanthine (X-33), which lack inhibitory effects
on phosphodiesterases in CHO and Calu-3 cells, increased
voltage-activated GCl without effect on baseline conductance at inactivating voltage. Half-maximal stimulation of
GCl occurred at 108 ± 9 µM for X-32 and
X-33 after apical or basolateral application. The stimulation of
GCl, which occurs only in the presence of
Cl
in the mucosal solution, is caused by a shift of the
voltage sensitivity to lower clamp potentials and an increase of the
maximally activated level. Furosemide reversed both the shift of
sensitivity and the increase in magnitude. These patterns are
fundamentally different from those seen after application of
membrane-permeant, nonmetabolized analogs of cAMP, and they indicate
that the xanthines stimulate GCl directly. This
notion is strengthened by the lack of influence on intracellular cAMP
content, which is consistent with the observations in CHO and Calu-3
cells. We propose that the xanthine derivatives increase the voltage
sensitivity of a regulative component in the conductive
Cl
pathway across amphibian skin.
Bufo viridis; furosemide; 3-isobutyl-1-methylxanthine; mitochondria-rich cells
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