Xanthine derivatives without PDE effect stimulate voltage-activated chloride conductance of toad skin

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.