Procaine has opposite effects on passive Na and K permeabilities in frog skin

Procaine has opposite effects on the active transport of Na+ when applied on the mucosal side of the frog skin [where it produces a stimulation of the short-circuit current (Isc)] or when added on the serosal side (where it produces an inhibition of Isc). In an attempt to reveal and localize the primary effect of procaine on either the apical or latero-basal membranes of the epithelial cells, we have tried to "chemically dissect" both membrane functions with inhibitors and ionophores. When applied on the apical side of the latero-basally depolarized epithelium, 25 mmol/l procaine increases Isc and Voc (transepithelial open-circuit potential), while decreasing the transepithelial resistance. The E1-E2 linearity domain of the I-V curves is narrowed. On the serosal side of the depolarized epithelium, the same concentration of procaine does not affect Isc and Voc (which are already inhibited) but it produces an increase in the transepithelial resistance (Rt). Procaine influence on the passive K+ permeability was studied by using the ionophore nystatin, which is assumed to form channels permeable to K+, when applied on the amiloride blocked apical membrane. In nystatin-treated epithelia, 25 mmol/l procaine on the apical side decreased Isc, Voc and Rt. In parallel experiments during Cl- substitution by SO2-(4), the procaine effects on Isc and Voc are no longer maintained, but transient.(ABSTRACT TRUNCATED AT 250 WORDS)