Effect of mucosal halides on Ca(2+)-blockable currents through the skin of Rana ridibunda

The present study deals with the interaction of mucosal anions with apical Ca(2+)-blockable cation channels of the skin of Rana ridibunda. The intracellular potential was depolarized by exposing the basolateral membranes to K2SO4 Ringer solution. The apical bathing medium consisted of nominal Ca(2+)-free K+ or Na+ solutions with SO4(2-), Cl-, Br-, or I- as the major anion. The effects of mucosal anion substitutions were studied by analyzing 1) the fluctuation in K+ current across the apical membrane driven by imposed transepithelial clamping potentials and 2) alterations of the transepithelial current (It) and conductance (Gt) as well as the Lorentzian parameters in response to anion substitution in the mucosal bathing solution. It and current noise spectra were recorded at different transepithelial potentials (Vt). A Lorentzian component was present in the power density spectrum when Vt was clamped at mucosa-positive voltages. Such noise components were never observed with mucosa-negative potentials. These findings suggest a rectifying behavior of the transepithelial cation currents. The Lorentzian noise component and the inward-oriented cation currents were depressed by the addition of micromolar concentrations of Ca2+ to the apical solutions as well as by replacing mucosal K+ or Na+ by N-methyl-D-glucamine. The Ca(2+)-blockable current and Lorentzian noise plateau (So) were gradually increased by raising Vt. Both parameters, as well as the corner frequency (fc), depended strongly on the major anion species in the apical solution; replacing mucosal SO4(2-) by one of the halides tested reduced fc and elevated So, It, and Gt considerably.